Everything Science Grade 10

CHAPTER 4. THE ATOM 4.5

Calculate the average relative atomic mass for chlorine.

SOLUTION

Step 1:Calculate the mass contribution of chlorine- 35 to the average relative

atomic mass

75%of the chlorine atoms has a mass of of 35 u

Contribution of Cl-35 = ( 10075 ×35) = 26, 25 u

Step 2: Calculate the contribution of chlorine- 37 to the average relative

atomic mass

25%of the chlorine atoms has a mass of of 37 u

Contribution of Cl-37 = ( 10025 ×37) = 9, 25 u

Step 3:Add the two values to arrive at the average relative atomic mass of

chlorine

Relative atomic mass of chlorine= 26, 25 u+ 9, 25 u= 35, 5 u

If you look on the periodic table (see front of book), the average relative atomic mass for
chlorine is 35 , 5 u. See simulation: ( Simulation: VPcyv at http://www.everythingscience.co.za))

Exercise 4 - 3

1. Atom A has 5 protons and 5 neutrons, and atom B has 6 protons and 5

neutrons. These atoms are:

a. allotropes

b. isotopes

c. isomers

d. atoms of different elements

2. For the sulphur isotopes,^3216 S and^3416 S, give the number of:

a. protons

b. nucleons

c. electrons

d. neutrons

3. Which of the following are isotopes of^3517 Cl?

a.^1735 Cl

b.^3517 Cl

c.^3717 Cl

Chemistry: Matter and Materials 77