Page 26 - Chemistry_Form_2

Chemistry
for Secondary Schools

Table 1.4: Atomic numbers and relative atomic masses of some elements
Element Atomic number Relative atomic mass

Hydrogen 1 1
Helium 2 4
FOR ONLINE READING ONLY
Lithium 3 6.9
Beryllium 4 9
Boron 5 10.8

Carbon 6 12

Nitrogen 7 14
Oxygen 8 16
Fluorine 9 19

Neon 10 20.2

Sodium 11 23
Magnesium 12 24.3
Aluminium 13 27

Silicon 14 28.1
Phosphorus 15 31

Sulfur 16 32
Chlorine 17 35.5

Argon 18 39.9
Potassium 19 39.1

Calcium 20 40.1

Example 1.5

(a) Chlorine has two isotopes:
!
!
35 Cl 75%" and 37 Cl 25%"
17 17
 75   25 
The relative atomic mass of chlorine is: 35 ×  + 37 × 


 100   100 

Student’s Book Form Two 19

CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 19
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 19 01/08/2025 11:20:52
01/08/2025 11:20:52

 90.5   0.3   9.2 

 20 ×  + 21 ×  + 22 × 

 100   100   100 