Page 18 - Chemistry_Form_2

Chemistry
for Secondary Schools

It is also possible to calculate the number of neutrons and number of electrons of
an atom if its mass number and atomic number are given.

Example 1.1
FOR ONLINE READING ONLY
Atom Q has a mass number of 49 and an atomic number of 24. Calculate the
number of neutrons and the number of electrons in atom Q.

Solution
Mass number = 49; atomic number = 24

(a) Neutron number = mass number – atomic number = 49 – 24 = 25

(b) Number of electrons = number of protons = atomic number = 24

Note: For the mass number with fractions, such as chlorine (35.5), calculating
the number of neutrons and electrons involves only a whole number. In
this case, for chlorine, 35 is used.

Nuclide notation

Atoms of different elements can be represented by chemical symbols that indicate
their respective atomic numbers and mass numbers. Using an arbitrary element
X, the mass number (A) is placed on its upper left end, while its atomic number
A
(Z) is placed on the lower left end. Thus, element X is represented as X .
Z
This is known as the nuclide notation. The following are examples of nuclide
representations of different atoms:
1
(i) Hydrogen H (ii) Boron B
11
1 5
(iii) Nitrogen 14 N (iv) Oxygen O
16
7 8
With this information, it is possible to deduce the number
of neutrons and electrons in the atom, and to write the
electronic configuration. For example, in the oxygen
atom, the mass number is 16 and the atomic number is 8.
Therefore, the number of electrons is 8 and the number
of neutrons is 16 – 8 = 8. The nucleus of the oxygen
atom can therefore be represented as shown in Figure 1.7. Figure 1.7: The nucleus
of the oxygen atom

Student’s Book Form Two 11

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CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 11