Structure of Matter

Anything that has mass and occupies space is called matter.

Every substance is made of extremely small particles. At the school level, atoms and molecules are two of the most important particles used to explain the structure and behaviour of matter.

A molecule is the smallest particle of many substances that can exist independently and retains the characteristic chemical properties of that substance. Molecules are made of atoms. Atoms themselves contain smaller subatomic particles, but in ordinary chemical reactions atoms are treated as the basic units that are rearranged.

Pure substances can be classified into two main types

Elements

Compounds

👉 Elements

A pure substance that contains only one type of atom and cannot be broken down into simpler substances by ordinary chemical methods is called an element.

For example, a hydrogen molecule (H₂) contains only hydrogen (H) atoms.

👉 Compounds

A pure substance that can be chemically broken down into two or more different elements is called a compound. In a compound, atoms of different elements are chemically combined in a fixed proportion.

For example, a carbon dioxide molecule (CO₂) contains carbon (C) and oxygen (O) atoms. The physical and chemical properties of carbon dioxide are very different from those of elemental carbon and oxygen.

🔹 The properties of a pure substance do not depend on its source.

Oxygen (O₂) obtained by heating potassium chlorate (KClO₃), oxygen obtained from air, and oxygen obtained by electrolysis of water all have the same fundamental chemical identity and properties when they are pure.

🔹 Different substances have different molecular masses and chemical properties.

Oxygen (O₂) and hydrogen (H₂) are molecules of two different substances and therefore have different molecular masses and chemical properties.

👉 When two or more substances are mixed physically in any proportion without undergoing a chemical reaction, the resulting material is called a mixture.

Mixtures will be discussed separately in another lesson.

👉 Atom

An atom is the smallest unit of an element that takes part in chemical reactions while retaining the identity of that element.

The names of atoms are represented briefly by chemical symbols. Different elements have different symbols.

For example: hydrogen (H), oxygen (O), carbon (C), calcium (Ca), sodium (Na), etc.

👉 Molecule

A molecule is a particle made of one or more atoms chemically bonded together that can exist independently and has the characteristic chemical properties of the substance.

Molecules can be discussed in two broad groups at this level

Elemental Molecules

Compound Molecules

⭐ A molecule formed only from atoms of the same element is called an elemental molecule.

Examples include oxygen (O₂) and ozone (O₃), which are made only of oxygen atoms, and hydrogen (H₂), which is made only of hydrogen atoms.

⭐ A molecule formed when atoms of different elements combine chemically in a fixed whole-number ratio is called a compound molecule.

Carbon dioxide (CO₂) is a compound molecule made of carbon (C) and oxygen (O) atoms. In one molecule of carbon dioxide, the ratio of carbon atoms to oxygen atoms is 1:2.