Page 11 - Chemistry_Form_2

Chemistry
for Secondary Schools

core) of the atom. Within the nucleus, there are positively charged particles called
protons. This was the second sub-atomic particle to be discovered. Rutherford’s
discovery helped scientists discover nuclear energy used to produce electricity
and in radiotherapy.

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Rutherford’s findings are summarised as follows:
1. Protons, the positively charged particles of an atom, are located in the
nucleus.
2. Most of the mass of the atom is located in the nucleus.

3. The nucleus has a relatively smaller volume compared to the whole atom.
4. Electrons have very small masses compared to the protons.
5. Most of the space in an atom is empty.

6. Electrons are the negatively charged particles in an atom. They move
around the nucleus in orbits.

Rutherford thus developed the planetary model of the atom shown in Figure 1.3.

Electron in orbit
around the nucleus

Orbit

Nucleus (with positive charge)

Empty space

Figure 1.3: Rutherford’s planetary model of the atom

In 1932, a scientist named James Chadwick discovered the neutrons, which also
forms part of the nucleus. Figure 1.4 shows the locations of neutrons and other
sub-atomic particles in an atom. Neutrons have the same mass as protons but
no charge. They are located in the nucleus of an atom. They were the third sub-
atomic particles to be discovered. The Chadwick discoveries have made nuclear
power possible, helping to produce electricity and develop medical treatments
such as cancer therapy.
The properties of neutrons are summarised as follows:

1. They have no charge (are neutral).
2. They have nearly the same mass as the corresponding protons.
3. They have a mass nearly 1840 times the mass of an electron.

4 Student’s Book Form Two

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CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 4
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 4 01/08/2025 11:20:50