One number that names the element
Atomic number Z.
What makes an atom hydrogen?
Atoms of one element are alike, and different from the atoms of other elements.
What do you think makes an atom hydrogen and not helium?
What this lesson covers
The idea
The atomic number Z is the number of protons in the nucleus of an atom, equal to its number of electrons; it uniquely identifies the element and determines its chemical behaviour.
What makes an atom hydrogen?
Atoms of one element are alike, and different from the atoms of other elements.
What do you think makes an atom hydrogen and not helium?
- The number of protons in its nucleus
- The colour of the atom
- How big the atom looks in a picture
Change the number of protons
Use + and − to change the number of protons in the nucleus.
The atomic number
The number of protons is called the atomic number, with the symbol Z. Change it and you get another element. In a neutral atom the number of electrons is the same.
The atomic number Z is the number of protons in the nucleus of an atom, and it equals its number of electrons. It uniquely identifies the element and determines its chemical behaviour.
Notes
The atomic number Z is the number of protons, equal to the number of electrons. It identifies the element and decides its chemical behaviour.
Check yourself
The atomic number of an element is the number of …
Sodium has atomic number 11. How many electrons are there in a neutral sodium atom?
Answer: 11
In a neutral atom the electrons equal the protons, which equal the atomic number: 11.
Two atoms have different atomic numbers. What does this tell us?
Helium has atomic number 2. How many protons does a helium atom have?
Answer: 2
The atomic number is the number of protons: 2.
- neutrons in the nucleus. Protons plus neutrons make the mass number, not the atomic number.
- shells around the nucleus. The atomic number counts protons, not shells.
- protons in the nucleus — correct. Yes!
- They are atoms of different elements — correct. Yes!
- They are atoms of the same element. The atomic number identifies the element, so different numbers mean different elements.
- They have the same number of protons. The atomic number is the number of protons, so different atomic numbers mean different numbers of protons.