Page 17 - Chemistry_Form_2

Chemistry
for Secondary Schools

Determination of atomic number and mass number

Task 1.5

Access an online simulation on how to build an atom. Explore the simulation
by adding protons, neutrons, and electrons to form different elements. Observe
FOR ONLINE READING ONLY
changes in atomic number, atomic mass, and stability of the atoms.

The sub-atomic particles of an atom, namely protons, neutrons, and electrons,
relate to the atomic number and mass number of the atom.

Atomic number
The atomic number is the number of protons in an atom. It is also known as
the proton number. For example, the atomic number of hydrogen is 1 since it

has only one proton. A sodium atom has 11 protons in the nucleus; therefore,
its atomic number is 11. Since the number of protons is equal to the number of
electrons in the neutral atom, its atomic number is not only the number of protons
but it is also the number of electrons.

Therefore, for the neutral atoms:
Atomic number = Number of protons = Number of electrons

Mass number

Protons and neutrons are found in the nucleus of an atom and are called nucleons.
The sum of the protons and neutrons in one atom of an element is called the mass
number or nucleon number or atomic mass. This number is actually taken as the

mass of the atom since the mass of the electron is negligible.
Thus,

Number of protons + Number of neutrons = Mass number
For example:

(i) Hydrogen has 1 proton and 0 neutrons. Therefore, its atomic number is 1,
and mass number is 1 + 0 = 1.

(ii) Boron has 5 protons and 6 neutrons. Its atomic number is 5 and mass
number is 5 + 6 = 11.

(iii) Nitrogen has 7 protons and 7 neutrons. Its atomic number is 7 and mass
number is 7 + 7 = 14.

10 Student’s Book Form Two

01/08/2025 11:20:50
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 10
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 10 01/08/2025 11:20:50