Particulate Nature of Matter – SS1 Chemistry Lesson Note (First Term)

Matter is anything that has mass and occupies space. It exists as solid, liquid or gas, and is made up of tiny particles called atoms, molecules and ions. The three states differ in how freely their particles move. Matter can undergo physical changes, in which no new substance forms, or chemical changes, in which new substances form.

Behavioural objectives

By the end of the lesson, students should be able to:

• describe the arrangement and movement of particles in solids, liquids and gases;
• compare the three states of matter;
• distinguish physical changes from chemical changes and give examples of each;
• define atom, molecule, atomicity, ion and radical;
• state the postulates of Dalton’s atomic theory and the modifications made to them.

What is the nature of matter?

Matter is anything that has mass and occupies space. It exists in three states, namely solid, liquid and gas.

The fundamental difference between the three states of matter depends on the degree of movement of the particles they are made of.

Solid state

The particles of a solid are tightly packed and held together by a strong electrostatic force.

The particles only vibrate to and fro about an equilibrium or fixed position. Solids have a definite shape and volume and are very difficult to compress.

Liquid state

The forces of attraction between molecules of liquids are weak compared to those of solids. Hence they have slight movements. This is why liquids can flow. They have definite volume but not definite shape.

Gaseous state

Because of the large distance between the molecules of a gas, the cohesive forces between them are negligible, so they move randomly. Gases have no definite shape or volume. They assume the shape of the containing vessel.

How do solids, liquids and gases compare?

What are the types of change in matter?

When a substance is heated, it may change. There are two types of change: physical and chemical.

Physical change

A physical change is a change which is easily reversible and in which no new substances are formed. Examples are:

1. Dissolution of common salt.
2. Changes in states of matter, such as melting of solids, freezing of liquids, evaporation of liquids, liquefaction of gases and sublimation of solids.
3. Magnetisation and demagnetisation of iron nails.
4. Separation of mixtures by evaporation, distillation, fractional distillation etc.

Chemical change

A chemical change is a change which is not easily reversible and in which new substances are formed. Examples are:

1. Rusting of iron and other metals.
2. Dissolution of metals and limestone in acids.
3. Fermentation and decay of substances.
4. Changes in electrochemical cells.
5. The addition of water to quicklime.
6. Burning of materials.

How do physical and chemical changes compare?

What are atoms, molecules and ions?

Matter is made up of discrete particles. The main ones are atoms, molecules and ions.

Atoms and molecules

An atom is the smallest part of an element which can take part in a chemical reaction.

A molecule is the smallest particle of a substance that can exist alone and still retain the chemical properties of that substance. Molecules are made up of atoms.

Atomicity

The atomicity of an element is the number of atoms in one molecule of the element.

Elements whose molecules contain one, two or three atoms are described as monatomic, diatomic and triatomic respectively.

Examples:

Ions and radicals

An ion is an atom or group of atoms which carries an electric charge. Groups of atoms that carry either a positive or a negative charge are called radicals.

An acid radical is a small group of atoms carrying a negative charge that keeps its identity. Examples include SO₄^2-, NO₃^– etc.

Ions are grouped as cations and anions.

• Cations are positively charged ions, e.g. Ca²⁺, Na⁺, NH₄⁺ etc.
• Anions are negatively charged ions, e.g. CO₃^2-, SO₄^2-, Cl^–, OH^– etc.

What is Dalton’s atomic theory?

John Dalton, a British physicist and chemist, proposed the atomic theory in 1808 as follows:

1. All elements are made up of small indivisible particles called atoms.
2. Atoms can neither be created nor destroyed in any chemical reaction.
3. Atoms of the same element are exactly alike in every respect and are different from atoms of all other elements.
4. Atoms of different elements can combine in simple whole-number ratios to form compounds.
5. All chemical changes result from the combination or separation of atoms.

How has Dalton’s atomic theory been modified?

Because of discoveries in the twentieth century, Dalton’s atomic theory cannot be accepted in full and needed modification.

• The first statement was proved wrong by the discovery of protons, electrons and neutrons as constituents of the atom. An atom is not an indivisible solid piece.
• The second statement still holds good for ordinary chemical reactions. During nuclear reactions, however, the nucleus can be broken into simpler atoms, giving out large amounts of heat (nuclear fission). This destroys the atoms involved.
• The discovery of isotopes makes the third statement unacceptable. Chlorine, for example, has two kinds of atoms with different numbers of neutrons in the nucleus, and hence different relative atomic masses, although they have the same proton number.
• The fourth statement is true only for inorganic compounds which contain a few atoms per molecule. Carbon forms very large organic molecules such as proteins, starch and fats, which contain thousands of atoms.

Evaluation

1. Give two differences between physical and chemical changes.
2. Give three processes which involve a physical change.
3. What is atomicity? Give one example of a diatomic element.
4. Differentiate between a cation and an anion, with one example of each.

Assignment

1. Give any two postulates of Dalton’s atomic theory.
2. Differentiate an atom from a molecule.
3. How will an atom become an ion?