Structure Of The Atom

Hello students, welcome to today's science lesson. I'm so happy to see you all here, ready to learn about one of the most fascinating topics in chemistry - the structure of the atom. Today, we are going to explore Chapter 4, and I promise you, by the end of this lesson, you will have a complete understanding of how atoms are built. So let's begin our journey into the tiny world inside matter.

Now students, in the previous chapter, you learned about atoms and molecules being the fundamental building blocks of matter. You learned that different kinds of matter exist because different atoms constitute them. But have you ever wondered what makes an atom of one element different from an atom of another element? And more importantly, are atoms really indivisible, as John Dalton proposed, or are there smaller particles inside them? These are exactly the questions we will answer in this chapter.

You know, students, at the end of the 19th century, scientists faced a major challenge - to reveal the structure of the atom and explain its important properties. The journey to understand the atom began with some simple observations about electricity. Let me take you through this step by step.

Let's start with a very simple activity that all of you can try at home. Have you ever noticed that when you comb your dry hair, the comb can attract small pieces of paper? Or when you rub a glass rod with a silk cloth and bring it near an inflated balloon, the balloon gets attracted to the rod? This happens because objects become electrically charged when rubbed together. But where does this charge come from? The answer lies in the fact that an atom is not indivisible - it contains charged particles inside it.

Now students, many scientists contributed to revealing the presence of charged particles in an atom. By 1900, scientists knew that although the atom was considered indivisible, it contained at least one sub-atomic particle - the electron, which was identified by J.J. Thomson. Even before the electron was identified, in 1886, a scientist named E. Goldstein discovered new radiations in a gas discharge tube and called them canal rays. These rays were positively charged radiations, and they led to the discovery of another sub-atomic particle. This particle had a charge equal in magnitude but opposite in sign to that of the electron, and its mass was approximately 2000 times that of the electron. This particle was named the proton.

Let me make this clearer. An electron is represented as 'e⁻' and carries a negative charge. A proton is represented as 'p⁺' and carries a positive charge. The mass of a proton is taken as one unit and its charge as plus one. The mass of an electron is considered to be negligible compared to a proton, and its charge is minus one. So students, remember this - protons are positively charged, electrons are negatively charged, and they balance each other in a neutral atom.

Now, let's answer the questions from the chapter. The first question asks - what are canal rays? Canal rays are positively charged radiations discovered by E. Goldstein in a gas discharge tube. These rays led to the discovery of the proton.

The second question asks - if an atom contains one electron and one proton, will it carry any charge or not? Students, the answer is no. The atom will be electrically neutral because the positive charge of the proton and the negative charge of the electron cancel each other out. The atom as a whole has no net charge.

Now, let's move on to the next section - the structure of an atom. We have learned that Dalton's atomic theory suggested that the atom was indivisible and indestructible. But the discovery of electrons and protons inside the atom showed that this part of Dalton's theory was incorrect. Scientists now wanted to know how electrons and protons are arranged within an atom. Many scientists proposed various atomic models, and the first one to do so was J.J. Thomson.

Students, let me explain Thomson's model of an atom in a way that will make it easy to remember. Thomson proposed that an atom is similar to a Christmas pudding. Imagine a spherical pudding with dried fruits or currants embedded in it. Similarly, in an atom, the electrons are embedded in a sphere of positive charge, like currants in a pudding. Another analogy is a watermelon - the positive charge in the atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere, like the seeds in a watermelon.

Thomson proposed two main points: first, an atom consists of a positively charged sphere with electrons embedded in it. Second, the negative and positive charges are equal in magnitude, so the atom as a whole is electrically neutral. This model explained why atoms are electrically neutral - the positive and negative charges balance each other.

However, students, Thomson's model had limitations. The results of experiments carried out by other scientists could not be explained by this model. One of those experiments was performed by Ernest Rutherford, and it completely changed our understanding of the atom.

Now let's talk about Rutherford's model of an atom. Ernest Rutherford was very interested in knowing how electrons are arranged within an atom. So he designed a very famous experiment known as the alpha-particle scattering experiment. In this experiment, fast-moving alpha particles were made to fall on a thin gold foil.

Why did Rutherford choose gold? He selected a gold foil because he wanted it to be as thin as possible - about 1000 atoms thick. Alpha particles are doubly-charged helium ions with a mass of 4 units. Since they are heavy and fast-moving, they have a considerable amount of energy.

Rutherford expected that alpha particles would be deflected by the sub-atomic particles in the gold atoms. But because alpha particles are much heavier than protons, he did not expect to see large deflections.

Let me explain this with a simple analogy. Imagine a child standing in front of a solid wall with his eyes closed, throwing stones at the wall from a distance. The child will hear a sound when each stone strikes the wall. If he throws ten stones, he will hear ten sounds. But now imagine the child throwing stones at a barbed-wire fence instead. Most of the stones would pass through the gaps in the fence, and no sound would be heard. This is because there are lots of gaps in the fence.

Rutherford reasoned in a similar way. He observed that most of the alpha particles passed through the gold foil without getting deflected. This showed that most of the space inside the atom is empty. Very few particles were deflected from their path, indicating that the positive charge occupies very little space. And a very small fraction of alpha particles were deflected by 180 degrees, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

From this data, Rutherford calculated that the radius of the nucleus is about 10⁵ times less than the radius of the atom. That means if the atom were the size of a cricket stadium, the nucleus would be just a small marble in the center! Isn't that amazing, students?

Based on this experiment, Rutherford proposed the nuclear model of an atom with the following features: first, there is a positively charged center in an atom called the nucleus, and nearly all the mass of an atom resides in the nucleus. Second, the electrons revolve around the nucleus in circular paths. Third, the size of the nucleus is very small compared to the size of the atom.

But students, Rutherford's model had a major drawback. According to the laws of physics, any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. So the revolving electron would lose energy and finally fall into the nucleus. If this were true, atoms should be highly unstable, and matter as we know it would not exist. But we know that atoms are quite stable! This was a serious objection to Rutherford's model.

To overcome the objections raised against Rutherford's model, a scientist named Neils Bohr came up with a new model. Let me tell you a bit about Bohr before we discuss his model. Neils Bohr was born in Copenhagen in 1885. He was appointed professor of physics at Copenhagen University in 1916 and received the Nobel Prize for his work on the structure of the atom in 1922.

Bohr proposed the following postulates about the model of an atom: first, only certain special orbits known as discrete orbits of electrons are allowed inside the atom. Second, while revolving in discrete orbits, the electrons do not radiate energy. These orbits or shells are called energy levels.

Students, think of this like this - imagine a solar system where planets can only revolve around the sun at certain specific distances, not at any distance they want. Similarly, electrons in an atom can only revolve in certain allowed orbits or shells. These shells are represented by the letters K, L, M, N, and so on, or by numbers n=1, 2, 3, 4, and so on.

Now, let's discuss the discovery of neutrons. In 1932, J. Chadwick discovered another sub-atomic particle which had no charge and a mass nearly equal to that of a proton. This particle was named the neutron. Neutrons are present in the nucleus of all atoms, except hydrogen. A neutron is represented as 'n'. The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.

So students, the three sub-atomic particles of an atom are: electrons, protons, and neutrons. Electrons are negatively charged, protons are positively charged, and neutrons have no charge. The mass of an electron is about 1/2000 times the mass of a hydrogen atom, while the mass of a proton and a neutron is taken as one unit each.

Now let's answer the questions from this section. The first question asks - on the basis of Thomson's model of an atom, explain how the atom is neutral as a whole. According to Thomson's model, the atom has a positively charged sphere with electrons embedded in it. The negative and positive charges are equal in magnitude, so they cancel each other out, making the atom electrically neutral.

The second question asks - on the basis of Rutherford's model of an atom, which sub-atomic particle is present in the nucleus of an atom? According to Rutherford's model, protons are present in the nucleus. Later, neutrons were also discovered to be present in the nucleus.

The third question asks - draw a sketch of Bohr's model of an atom with three shells. Students, in your notebook, you should draw the nucleus in the center with protons and neutrons. Then draw three circular orbits around it - the innermost orbit is K, the next is L, and the outermost is M. Place electrons on these orbits according to the element you are representing.

The fourth question asks - what do you think would be the observation if the alpha-particle scattering experiment is carried out using a foil of a metal other than gold? Students, the observations would be similar regardless of the metal used, as long as the foil is very thin. Most alpha particles would pass through undeflected, a few would be deflected, and very few would bounce back. However, the extent of deflection might differ depending on the atomic number of the metal - higher atomic number would mean more positive charge in the nucleus, causing slightly more deflection.

Now let's move on to the next topic - how electrons are distributed in different orbits or shells. The distribution of electrons into different orbits was suggested by Bohr and Bury. They proposed certain rules for writing the number of electrons in different energy levels or shells.

The first rule is: the maximum number of electrons present in a shell is given by the formula 2n², where n is the orbit number or energy level index. So for the first orbit or K-shell, n=1, so maximum electrons = 2 × 1² = 2. For the second orbit or L-shell, n=2, so maximum electrons = 2 × 2² = 8. For the third orbit or M-shell, n=3, so maximum electrons = 2 × 3² = 18. For the fourth orbit or N-shell, n=4, so maximum electrons = 2 × 4² = 32, and so on.

The second rule is: the maximum number of electrons that can be accommodated in the outermost orbit is 8.

The third rule is: electrons are not accommodated in a given shell unless the inner shells are filled. That is, the shells are filled in a step-wise manner.

Now let's look at Table 4.1 from your textbook, which shows the composition of atoms of the first eighteen elements. Let me explain some examples. Hydrogen has atomic number 1, so it has 1 proton and 1 electron. This electron is in the K-shell. Helium has atomic number 2, so it has 2 protons and 2 electrons, both in the K-shell. Lithium has atomic number 3, so it has 3 protons and 3 electrons - 2 in the K-shell and 1 in the L-shell. And so on.

Now let's answer the questions from this section. The first question asks - write the distribution of electrons in carbon and sodium atoms. Carbon has atomic number 6, so its electron distribution is K=2, L=4. Sodium has atomic number 11, so its electron distribution is K=2, L=8, M=1.

The second question asks - if K and L shells of an atom are full, then what would be the total number of electrons in the atom? If the K-shell is full, it has 2 electrons. If the L-shell is full, it has 8 electrons. So the total would be 2 + 8 = 10 electrons.

Now let's discuss valency. We have learned how electrons are arranged in different shells. The electrons present in the outermost shell of an atom are known as valence electrons. From the Bohr-Bury scheme, we know that the outermost shell of an atom can accommodate a maximum of 8 electrons. It was observed that atoms of elements completely filled with 8 electrons in the outermost shell show little chemical activity. In other words, their combining capacity or valency is zero. These are the inert or noble gases - helium has 2 electrons in its outermost shell (which is full for the first shell), and all other noble gases have 8 electrons in their outermost shell.

The combining capacity of atoms of elements, that is, their tendency to react and form molecules with atoms of the same or different elements, was explained as an attempt to attain a fully-filled outermost shell. An outermost shell with 8 electrons is said to have an octet. Atoms react so as to achieve an octet in the outermost shell. This is done by sharing, gaining, or losing electrons. The number of electrons gained, lost, or shared to make the octet of electrons in the outermost shell gives us the combining capacity of the element, that is, the valency.

For example, hydrogen, lithium, and sodium atoms contain one electron each in their outermost shell. Each one of them can lose one electron to achieve a full outer shell. So they are said to have a valency of one. Similarly, magnesium has two electrons in its outermost shell, so it can lose two electrons - valency is two. Aluminium has three electrons in its outermost shell, so its valency is three.

Now, what about elements whose outermost shell is close to being full? For example, fluorine has 7 electrons in its outermost shell. It could lose 7 electrons or gain 1 electron to achieve an octet. It is easier for fluorine to gain one electron rather than lose seven. So its valency is determined by subtracting 7 from 8, which gives us 1. Similarly, oxygen has 6 electrons in its outermost shell. It can gain 2 electrons to complete its octet, so its valency is 2.

Now let's answer the question from this section. The question asks - how will you find the valency of chlorine, sulphur, and magnesium? Let's look at their electron distributions. Chlorine has 17 electrons, so its distribution is K=2, L=8, M=7. It has 7 electrons in the outermost shell, so it needs to gain 1 electron to complete its octet. Therefore, its valency is 1. Sulphur has 16 electrons, so its distribution is K=2, L=8, M=6. It has 6 electrons in the outermost shell, so it needs to gain 2 electrons to complete its octet. Therefore, its valency is 2. Magnesium has 12 electrons, so its distribution is K=2, L=8, M=2. It has 2 electrons in the outermost shell, so it can lose 2 electrons to complete its octet. Therefore, its valency is 2.

Now let's discuss atomic number and mass number. We know that protons are present in the nucleus of an atom. The number of protons in an atom determines its atomic number. It is denoted by 'Z'. All atoms of an element have the same atomic number. In fact, elements are defined by the number of protons they possess. For hydrogen, Z=1 because a hydrogen atom has only one proton in its nucleus. Similarly, for carbon, Z=6. So atomic number is defined as the total number of protons present in the nucleus of an atom.

Now, what about mass number? After studying the properties of sub-atomic particles, we can conclude that the mass of an atom is practically due to protons and neutrons alone, which are present in the nucleus. These are also called nucleons. So the mass of an atom resides in its nucleus. For example, the mass of carbon is 12 atomic mass units because it has 6 protons and 6 neutrons - 6 + 6 = 12. Similarly, the mass of aluminium is 27 atomic mass units because it has 13 protons and 14 neutrons.

Mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. It is denoted by 'A'. In the notation for an atom, the atomic number, mass number, and symbol of the element are written together. For example, nitrogen is written as ¹⁴₇N, where 14 is the mass number, N is the symbol, and 7 is the atomic number.

Now let's answer the questions from this section. The first question asks - if the number of electrons in an atom is 8 and the number of protons is also 8, then what is the atomic number of the atom, and what is the charge on the atom? The atomic number is 8 because it is equal to the number of protons. Since the number of electrons equals the number of protons, the atom is electrically neutral, so the charge on the atom is zero.

The second question asks - with the help of Table 4.1, find out the mass number of oxygen and sulphur atoms. Oxygen has 8 protons and 8 neutrons, so its mass number is 8 + 8 = 16. Sulphur has 16 protons and 16 neutrons, so its mass number is 16 + 16 = 32.

Now let's discuss isotopes. In nature, a number of atoms of some elements have been identified which have the same atomic number but different mass numbers. For example, hydrogen has three atomic species - protium (¹₁H), deuterium (²₁H or D), and tritium (³₁H or T). The atomic number of each is 1, but the mass number is 1, 2, and 3 respectively. Other examples are carbon - ¹²₆C and ¹⁴₆C, and chlorine - ³⁵₁₇Cl and ³⁷₁₇Cl.

Isotopes are defined as atoms of the same element having the same atomic number but different mass numbers. So there are three isotopes of hydrogen - protium, deuterium, and tritium.

Many elements consist of a mixture of isotopes. Each isotope of an element is a pure substance. The chemical properties of isotopes are similar, but their physical properties are different.

Now, let's understand how to calculate the average atomic mass when an element has isotopes. Chlorine occurs in nature in two isotopic forms with masses 35 and 37 in the ratio of 3:1. That means 75% of chlorine atoms have mass 35, and 25% have mass 37. The average atomic mass would be (35 × 75/100) + (37 × 25/100) = (105/4) + (37/4) = 142/4 = 35.5 atomic mass units.

This does not mean that any one atom of chlorine has a fractional mass of 35.5. It means that if you take a sample of chlorine, it will contain both isotopes, and the average mass is 35.5.

Now let's discuss some applications of isotopes. An isotope of uranium is used as fuel in nuclear reactors. An isotope of cobalt is used in the treatment of cancer. An isotope of iodine is used in the treatment of goitre.

Now let's discuss isobars. Consider two elements - calcium with atomic number 20, and argon with atomic number 18. The number of protons in these atoms is different, but the mass number of both is 40. Atoms of different elements with different atomic numbers but the same mass number are known as isobars.

Now let's answer the questions from this section. The first question asks - for the symbol H, D, and T, tabulate three sub-atomic particles found in each of them. For protium (H), deuterium (D), and tritium (T), each has 1 proton. Protium has 0 neutrons, deuterium has 1 neutron, and tritium has 2 neutrons. Each has 1 electron. So the three sub-atomic particles are protons, neutrons, and electrons.

The second question asks - write the electronic configuration of any one pair of isotopes and isobars. For isotopes, take carbon-12 and carbon-14. Both have the same electron configuration - 2 electrons in K-shell and 4 in L-shell. For isobars, take calcium-40 and argon-40. Calcium has electron configuration K=2, L=8, M=8, N=2. Argon has electron configuration K=2, L=8, M=8.

Now let's move on to the exercises at the end of the chapter. I'll solve each one for you.

Exercise 1: Compare the properties of electrons, protons, and neutrons. Electrons have negative charge (-1), negligible mass (1/2000 of hydrogen atom), and are present outside the nucleus. Protons have positive charge (+1), mass of 1 unit, and are present in the nucleus. Neutrons have no charge (neutral), mass of 1 unit, and are present in the nucleus.

Exercise 2: What are the limitations of J.J. Thomson's model of the atom? Thomson's model could not explain the results of experiments like Rutherford's alpha-particle scattering experiment. It could not explain how the positive charge was concentrated in a small nucleus.

Exercise 3: What are the limitations of Rutherford's model of the atom? According to Rutherford's model, electrons revolve around the nucleus in circular orbits. But according to physics, any charged particle in circular motion would undergo acceleration and radiate energy. So the electrons would lose energy and fall into the nucleus, making atoms unstable. But we know atoms are stable, so this was a major limitation.

Exercise 4: Describe Bohr's model of the atom. Bohr proposed that electrons revolve around the nucleus in certain allowed orbits called energy levels or shells. These orbits are designated as K, L, M, N or n=1, 2, 3, 4. While revolving in these discrete orbits, electrons do not radiate energy. Only when electrons jump from a higher energy level to a lower energy level, they emit energy, and when they jump from lower to higher, they absorb energy.

Exercise 5: Compare all the proposed models of an atom given in this chapter. Thomson's model was like a Christmas pudding - positive sphere with electrons embedded in it. It explained electrical neutrality but couldn't explain other experimental results. Rutherford's model had a small positively charged nucleus with electrons revolving around it. It explained alpha-particle scattering but couldn't explain stability of atoms. Bohr's model was similar to Rutherford's but with discrete energy levels or shells where electrons revolve without radiating energy. This explained the stability of atoms.

Exercise 6: Summarise the rules for writing distribution of electrons in various shells for the first eighteen elements. Rule 1: Maximum electrons in a shell = 2n², where n is the shell number. So K(n=1) has 2, L(n=2) has 8, M(n=3) has 18. Rule 2: Maximum electrons in outermost shell is 8. Rule 3: Shells are filled step by step - inner shells are filled before outer shells.

Exercise 7: Define valency by taking examples of silicon and oxygen. Valency is the combining capacity of an atom. Silicon has atomic number 14, electron distribution K=2, L=8, M=4. It has 4 electrons in its outermost shell, so it can lose 4 electrons or gain 4 electrons to complete its octet. So its valency is 4. Oxygen has atomic number 8, electron distribution K=2, L=6. It has 6 electrons in its outermost shell, so it needs to gain 2 electrons to complete its octet. So its valency is 2.

Exercise 8: Explain with examples atomic number, mass number, isotopes, and isobars. Give any two uses of isotopes. Atomic number (Z) is the number of protons in the nucleus. For example, carbon has Z=6. Mass number (A) is the sum of protons and neutrons in the nucleus. For example, carbon-12 has A=12. Isotopes are atoms of the same element with same atomic number but different mass number. For example, ¹²₆C and ¹⁴₆C are isotopes. Isobars are atoms of different elements with same mass number but different atomic number. For example, ⁴⁰₂₀Ca and ⁴⁰₁₈Ar are isobars. Two uses of isotopes: uranium-235 is used as fuel in nuclear reactors, cobalt-60 is used in cancer treatment.

Exercise 9: Na⁺ has completely filled K and L shells. Explain. Na⁺ is a sodium ion that has lost one electron. Sodium has 11 electrons in its neutral state - 2 in K, 8 in L, and 1 in M. When it loses one electron to become Na⁺, it has 10 electrons - 2 in K and 8 in L. Both shells are completely filled according to the 2n² rule.

Exercise 10: If bromine atom is available in the form of two isotopes ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass of bromine atom. Average atomic mass = (79 × 49.7/100) + (81 × 50.3/100) = (79 × 0.497) + (81 × 0.503) = 39.263 + 40.743 = 80.006, approximately 80 u.

Exercise 11: The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes ¹⁶₈X and ¹⁸₈X in the sample? Let the percentage of ¹⁶X be a and of ¹⁸X be (100-a). Then average mass = (16 × a/100) + (18 × (100-a)/100) = 16.2. Multiply by 100: 16a + 18(100-a) = 1620. 16a + 1800 - 18a = 1620. -2a = 1620 - 1800 = -180. a = 90. So ¹⁶X is 90% and ¹⁸X is 10%.

Exercise 12: If Z = 3, what would be the valency of the element? Also, name the element. Z=3 means the element has 3 protons, so it is lithium. Lithium has electron distribution K=2, L=1. It has 1 electron in its outermost shell, so it can lose 1 electron to achieve a full K-shell. Therefore, its valency is 1.

Exercise 13: Composition of the nuclei of two atomic species X and Y are given as under. X has 6 protons and 6 neutrons. Y has 6 protons and 8 neutrons. Give the mass numbers of X and Y. What is the relation between the two species? Mass number of X = protons + neutrons = 6 + 6 = 12. Mass number of Y = 6 + 8 = 14. Both have the same atomic number (6 protons), so they are isotopes of carbon.

Exercise 14: For the following statements, write T for True and F for False. (a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. This is False. Thomson proposed that electrons are embedded in a positive sphere. (b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. This is False. A neutron is a separate sub-atomic particle. (c) The mass of an electron is about 1/2000 times that of proton. This is True. (d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. This is True.

Now let's look at questions 15, 16, and 17. Put tick against correct choice and cross against wrong choice.

Question 15: Rutherford's alpha-particle scattering experiment was responsible for the discovery of (a) Atomic Nucleus. This is correct - tick. (b) Electron - cross. (c) Proton - cross. (d) Neutron - cross.

Question 16: Isotopes of an element have (a) the same physical properties - cross. (b) different chemical properties - cross. (c) different number of neutrons - tick. (d) different atomic numbers - cross.

Question 17: Number of valence electrons in Cl⁻ ion are: Cl⁻ is a chloride ion that has gained one electron. Chlorine has 17 electrons, so Cl⁻ has 18 electrons. The electron configuration is K=2, L=8, M=8. So it has 8 valence electrons. The answer is (b) 8.

Question 18: Which one of the following is a correct electronic configuration of sodium? Sodium has atomic number 11, so it has 11 electrons. The correct distribution is K=2, L=8, M=1, which is written as 2,8,1. The answer is (d) 2,8,1.

Now let's complete the table in question 19. I'll go row by row.

Row 1: Atomic Number = 9, Mass Number = ?, Number of Neutrons = 10, Number of Protons = ?, Number of Electrons = ?, Name of Atomic Species = ?. Since atomic number is 9, it has 9 protons and 9 electrons. Mass number = protons + neutrons = 9 + 10 = 19. The element with atomic number 9 is Fluorine (F). So the species is ¹⁹₉F.

Row 2: Atomic Number = 16, Mass Number = 32, Number of Neutrons = ?, Number of Protons = ?, Number of Electrons = ?, Name of Atomic Species = Sulphur. Since atomic number is 16, it has 16 protons and 16 electrons. Number of neutrons = mass number - atomic number = 32 - 16 = 16. The species is ³²₁₆S.

Row 3: Atomic Number = ?, Mass Number = 24, Number of Neutrons = ?, Number of Protons = 12, Number of Electrons = ?, Name of Atomic Species = ?. Since it has 12 protons, atomic number is 12. The element with atomic number 12 is Magnesium (Mg). Number of neutrons = mass number - atomic number = 24 - 12 = 12. It has 12 electrons. The species is ²⁴₁₂Mg.

Row 4: Atomic Number = ?, Mass Number = 2, Number of Neutrons = ?, Number of Protons = 1, Number of Electrons = ?, Name of Atomic Species = ?. Since it has 1 proton, atomic number is 1. The element is Hydrogen (H). Number of neutrons = mass number - atomic number = 2 - 1 = 1. It has 1 electron. The species is ²₁H or Deuterium (D).

Row 5: Atomic Number = ?, Mass Number = 1, Number of Neutrons = 0, Number of Protons = 1, Number of Electrons = 0, Name of Atomic Species = ?. Since it has 1 proton but 0 electrons, this is a hydrogen ion (H⁺). The species is ¹₁H or simply H⁺.

Now students, let me give you a brief summary of everything we have learned in this chapter.

In this chapter, we learned about the structure of the atom. We started with the discovery of charged particles in matter. J.J. Thomson discovered the electron, and E. Goldstein discovered canal rays, which led to the discovery of the proton. J. Chadwick later discovered the neutron.

We studied three major atomic models. Thomson's model proposed that electrons are embedded in a positive sphere, like currants in a Christmas pudding. This explained electrical neutrality but couldn't explain other experimental results. Rutherford's model proposed a small positively charged nucleus with electrons revolving around it, based on the alpha-particle scattering experiment. However, it couldn't explain the stability of atoms. Bohr's model proposed that electrons revolve in discrete energy levels or shells without radiating energy, which explained the stability of atoms.

We learned about the distribution of electrons in shells using the Bohr-Bury rules - maximum electrons in a shell is 2n², the outermost shell can have maximum 8 electrons, and shells are filled step by step.

We learned about valency - the combining capacity of an atom. Atoms try to achieve an octet (8 electrons) in their outermost shell by gaining, losing, or sharing electrons.

We learned about atomic number (Z) - the number of protons in the nucleus, and mass number (A) - the sum of protons and neutrons in the nucleus.

We learned about isotopes - atoms of the same element with the same atomic number but different mass numbers, and isobars - atoms of different elements with the same mass number but different atomic numbers.

We also learned about the applications of isotopes in various fields like medicine and nuclear energy.

Students, this is a very important chapter that forms the foundation for understanding chemistry. The concepts we learned here will help you understand chemical bonding, periodic table, and many other topics in the future. Make sure you practice all the questions and understand the concepts clearly.

That's all for today, students. Thank you for your attention and patience. Keep studying and keep exploring the fascinating world of science. See you in the next lesson!