Page 10 - Chemistry_Form_2

Chemistry
for Secondary Schools

These modifications include the following:
1. Atoms can be either created or destroyed by means of nuclear reactions.
The atom can change form through special processes such as nuclear
fusion (combining the atomic nuclei) or nuclear fission (splitting the atomic
nucleus). For example, an atom of uranium-235 can be split into two separate
FOR ONLINE READING ONLY
atoms.
2. Some elements have atoms of more than one kind which differ slightly in
mass. Such atoms are called isotopes. For example, carbon has three isotopes
known as carbon-12, carbon-13, and carbon-14.

3. An atom is made up of smaller sub-atomic particles called protons, neutrons,
and electrons.
4. Atoms of different elements may chemically combine in many different
ratios to form compounds.
The modern atomic theory builds on Dalton’s original ideas by recognising sub-
atomic particles, isotopes, and nuclear reactions while retaining the ideas of
chemical combinations and reactions. Dalton’s discovery thus helped scientists
understand chemical reactions and how substances combine.

Sub-atomic particles

In 1897, J. J. Thomson carried out experiments and described an atom as a sphere
of positive charge, with negative particles called electrons spread throughout the
sphere. This model of the atom was referred to as plum pudding model, as shown
in Figure 1.2.

A positively
Electrons charged sphere

Figure 1.2: Thomson’s plum pudding model of the atom

Thomson, therefore, managed to discover the electron among the three sub-
atomic particles. His discovery led to inventions of electronic devices such
as televisions, radios and computers. However, another scientist called Ernest
Rutherford reasoned that if Thomson’s model was correct, then the mass of the
atom was evenly spread throughout the atom. He carried out experiments and
discovered that most of the mass of an atom is concentrated in the nucleus (central

Student’s Book Form Two 3

01/08/2025 11:20:50
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 3 01/08/2025 11:20:50
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 3