Atoms

We are learning how to:

• define what an atom is

• sketch a diagram of an atom.

Elements and atoms

An element is a substance that cannot be split into simpler substances. Elements are made up of atoms, all of the same type.

The idea of small units of matter called atoms was first proposed around 600 BC by a Greek called Democritus. However, for the next 2000 years, both the word and the idea disappeared. About 200 years ago, the scientist John Dalton developed, from his experiments, a theory of atomic matter. Dalton’s basic theory is still used today, although much more research was needed to discover the nature of the atom itself.

Atoms are too small to see, so we need models to visualise them.

Atoms are defined as the tiniest indivisible part of an element that can exist on its own. An atom of gold is the smallest piece of gold we can get.

Each atom consists of three types of particles: protons, neutrons and electrons. These three particles are referred to as subatomic particles. At the centre of every atom is a nucleus, containing the protons and neutrons. The electrons orbit the nucleus in ring-like structures called shells or orbits.

b) c)

Atomic number

The number that each element has in the Periodic Table (Fig 8.2) is based on the characteristics of the atoms that make up the element. This number is called the atomic number of the element. The atomic number is the number of protons in the nucleus of each atom of that element.

The atomic number (Z ) is the number of protons (p), and this is equal to the number of electrons (e) that each atom of the element has; that is:

Z = number of p = number of e

Activity 8.3

Here is what you should do:

1. Copy and complete the following table. Refer to the Periodic Table in Fig 8.2.

Fun fact

A single glass of water contains more atoms than the oceans of the Earth contain glasses of water.

2. Compare your responses with those of your group members.

Check your understanding

1. How many electrons will orbit atoms of: a) neon? b) lithium? c) magnesium? d) calcium? e) phosphorus?

2. On a large piece of paper, draw your own diagram of an atom of: a) helium b) silicon c) sodium d) argon e) boron.

Key terms atom the smallest indivisible part of an element that can exist on its own subatomic particle particles found within an atom atomic number the number of protons in the nucleus of an atom; this number determines the place of each element in the Periodic Table