Hello, and welcome to today's chemistry lesson! Today, we are going to explore the fascinating world of elements, compounds, symbols, and formulae. By the end of this lesson, you will understand what makes up everything around us, how scientists classify matter, and how we use special shorthand notations to represent substances.
Let us begin with something very simple. Everything you can see, touch, or feel is made of matter. Matter exists in three states — solid, liquid, and gas. But today, we will look at matter differently — based on what it is made of chemically.
Think about salt and sugar. Both are white solids, yet they taste completely different and behave differently. Why? Because they are made of different kinds of atoms. This brings us to how chemists classify matter.
Matter can be divided into pure substances and impure substances. A pure substance has a definite composition and the same properties throughout. It is made of only one kind of particle — either atoms or molecules. Pure substances cannot be broken into simpler substances by any physical means. They have fixed melting and boiling points, and characteristic colours, odours, and tastes.
For example, pure water always boils at 100 degrees Celsius and freezes at 0 degrees Celsius. If you add salt, these temperatures change — the substance is no longer pure.
Pure substances are of two types — elements and compounds. Let us explore each one.
An element is defined as a pure substance made up of only one kind of atoms that cannot be converted into anything simpler than itself by any physical or chemical process.
Scientists have discovered 118 elements so far. Of these, 92 occur naturally on Earth — in rocks, soil, air, and water. The remaining 26 have been created artificially in laboratories. Elements are the basic building blocks from which all other substances are made.
Some common examples include oxygen, hydrogen, nitrogen, carbon, copper, silver, gold, iron, and zinc. Elements can exist alone or combined with other elements. Each element has unique properties that help us classify it.
Elements are grouped into four main categories.
First, metals. About 93 of the 118 elements are metals. Metals are usually shiny and hard — though sodium is surprisingly soft. Most are solids, except mercury, which is liquid. Metals have high melting and boiling points. They are ductile — meaning they can be stretched into thin wires. They are malleable — they can be beaten into thin sheets, though zinc is brittle and breaks instead. Metals conduct heat and electricity well, except tungsten, which is a poor conductor. They produce a ringing sound when struck, so they are called sonorous.
Second, non-metals. There are only 18 non-metals. They generally have dull surfaces, though iodine and carbon in the form of diamond and graphite do shine. Non-metals are either soft solids like sulphur and phosphorus, or gases like oxygen and nitrogen. Bromine is the only liquid non-metal. They have low melting and boiling points, except diamond and graphite. They are neither ductile nor malleable — they are brittle. They are poor conductors of heat and electricity, though graphite is an exception. They do not produce sound when struck.
Third, metalloids. These seven elements are neither fully metals nor non-metals. They show properties of both. Examples include boron, silicon, arsenic, antimony, germanium, tellurium, and polonium.
Fourth, noble or inert gases. These six gases — helium, neon, argon, krypton, xenon, and radon — do not react chemically with other elements. They exist in air only in traces.
Now, let us learn how scientists write elements quickly using symbols.
A symbol is a short form of an element's name. It represents not just the element, but also one atom of that element. The Swedish chemist Berzelius first used English letters for this purpose.
Usually, the symbol is the first one or two letters of the element's name. The first letter is always capital. For example, oxygen is O, and hydrogen is H.
When several elements start with the same letter, we use two letters. The first is capital, the second is small. Carbon is C, calcium is Ca, and cobalt is Co.
Sometimes, the symbol comes from the Latin name. Sodium is Na from Natrium. Copper is Cu from Cuprum. Iron is Fe from Ferrum. Potassium is K from Kalium.
Here are the first twenty elements arranged in the Periodic Table — a systematic arrangement scientists use for convenient study. The horizontal rows are called periods, and the vertical columns are called groups. You will learn more about this in higher classes.
Let us now turn to compounds.
A compound is defined as a pure substance formed by the chemical combination of two or more elements in a definite proportion by mass.
The smallest unit of a compound is a molecule, which contains atoms of two or more elements in a fixed ratio. Every molecule of a compound is identical, but different from molecules of other compounds. Each compound has its own specific properties.
Consider water — once thought to be an element, but actually a compound. Water forms when hydrogen and oxygen combine chemically in a fixed mass ratio of one to eight. Whether water comes from a river, a well, or rain, this ratio never changes. Its formula is always H₂O — two hydrogen atoms and one oxygen atom.
The properties of water are completely different from its constituent elements. Hydrogen and oxygen are gases that burn or support burning. Water is a liquid that puts out fires.
Water cannot be broken into hydrogen and oxygen by simple physical methods like boiling or freezing. It requires a chemical process called electrolysis, where electric current separates the elements.
Another example is common salt, or sodium chloride. Sodium is a reactive metal stored in kerosene. Chlorine is a poisonous greenish-yellow gas. Yet when they combine chemically, they form harmless white salt that we eat every day.
Key characteristics of compounds include: they contain two or more elements in fixed proportion; they are pure and homogeneous; their properties differ from their constituent elements; they can only be separated by chemical methods; and energy is absorbed or released during their formation.
Now we come to atoms and molecules — the tiny particles that make up everything.
An atom is defined as the smallest indivisible unit of an element which exhibits all the properties of that element and may or may not have independent existence.
A molecule is defined as the smallest unit of an element or a compound which exhibits all the properties of that element or compound and has independent existence. Molecules can be divided into atoms.
When two or more atoms of the same element combine, they form a molecule of that element. Two hydrogen atoms form H₂. Three oxygen atoms form ozone, O₃.
When atoms of different elements combine, they form a molecule of a compound. Water is H₂O. Carbon dioxide is CO₂ — one carbon atom and two oxygen atoms.
Finally, let us understand molecular formulae.
A formula is a short way of representing the molecule of an element or compound. It saves time, space, and effort, and is understood worldwide.
The number of atoms in a molecule of an element is called its atomicity. We show this as a small subscript number after the symbol.
Monoatomic molecules contain one atom — examples include helium He, sodium Na, and calcium Ca. Metals, metalloids, and inert gases are typically monoatomic.
Diatomic molecules contain two atoms — hydrogen H₂, oxygen O₂, nitrogen N₂, and chlorine Cl₂.
Triatomic molecules have three atoms — ozone O₃. Polyatomic molecules have more than three — phosphorus P₄ has four atoms, and sulphur S₈ has eight.
For compounds, we need to know each element's valency — its combining power. To write a formula: write the symbols side by side, write the valency below each, then interchange these numbers. If the numbers are the same, they cancel out.
For example, sodium has valency one, chlorine has valency one — they form NaCl. Calcium has valency two, chlorine has valency one — they form CaCl₂. Magnesium and oxygen both have valency two — they form MgO, not Mg₂O₂.
A formula tells us what elements are present, how many atoms of each, and the ratio between them. H₂SO₄ means two hydrogen, one sulphur, and four oxygen atoms in sulphuric acid. 2CO₂ means two molecules of carbon dioxide, containing two carbon atoms and four oxygen atoms total.
Before we close, let us see why different elements and compounds are useful in daily life.
Gold, platinum, and silver are lustrous and do not tarnish, so they make beautiful jewellery. Copper and aluminium conduct electricity well and can be drawn into wires, so they are used in electrical wiring. Iron is strong and abundant, making it ideal for tools and machines. Diamond, the hardest natural substance, cuts glass and serves as a gem. Graphite marks paper and makes pencil leads. Argon and neon fill electric bulbs because they do not react with the hot filament. Coal, wood, and natural gas burn to release heat energy as fuels. Water dissolves many substances, earning it the name universal solvent. Plastics insulate and make countless everyday items.
Let us quickly recap what we have learned today.
First, matter is classified as pure substances and impure substances. Pure substances include elements and compounds.
Second, an element contains only one kind of atom and cannot be broken down further. Elements are classified as metals, non-metals, metalloids, and noble gases.
Third, a compound contains two or more elements chemically combined in fixed proportion, with properties different from its constituents.
Fourth, atoms are the smallest units of elements; molecules are the smallest units of compounds with independent existence.
Fifth, elements are represented by symbols, and compounds by molecular formulae using subscripts to show atomicity.
Sixth, the different properties of elements and compounds determine their practical uses in our daily lives.
That brings us to the end of today's lesson on elements, compounds, symbols, and formulae. You have taken an important step into understanding the language of chemistry. Keep curious, keep exploring, and remember — everything around you is made of these amazing building blocks of nature. See you in the next lesson!