L.O. To understand isotopes and atomic mass

Most naturally-occurring carbon exists as carbon-12, about 1% is carbon-13 and a much smaller amount is carbon-14.

6 protons

7 neutrons

6 electrons

6 protons

8 neutrons

6 electrons

6 protons

6 neutrons

6 electrons

Particle Mass Charge

proton 1 +1

neutron 1 0

electron almost 0 -1

The atoms of an element contain equal numbers of protons and electrons and so have no overall charge.

The isotopes of an element are virtually identical in their chemical reactions.

The uncharged neutrons make no difference to chemical properties but do affect physical properties such as melting point and density.

Natural samples of elements are often a mixture of isotopes.

This is because they have the same number of protons and the same number of electrons.

Hydrogen-1 makes up the vast majority of the naturally-occurring element but two other isotopes exist.

hydrogen deuterium tritium

1 proton

0 neutrons

1 electron

1 proton

1 neutron

1 electron

1 proton

2 neutrons

1 electron

About 75% of naturally-occurring chlorine is chlorine-35 and 25% is chlorine-37.

17 protons

18 neutrons

17 electrons

17 protons

20 neutrons

17 electrons

What are the particle

numbers in each isotope?

8

8

8

8

8

10

oxygen-16 oxygen-18

protons

neutrons

electrons

Almost all of naturally-occurring oxygen is oxygen-16 but about 0.2% is oxygen-18.

Atomic Mass of elements The atoms of each element have a different mass. The relative atomic mass is a way of saying how heavy

atoms are in comparison to others. Carbon is given a relative atomic mass (RAM) of 12Carbon is given a relative atomic mass (RAM) of 12.. The RAM of other atoms compares them with carbon. Eg. Hydrogen has a mass of only one twelfth that of carbon

and so has a RAM of 1. Below are the RAMs of some other elements.

Element Symbol Times as heavy as carbon R.A.M

Helium He one third

Beryllium Be three quarters

Molybdenum Mo Eight

Krypton Kr Seven

Oxygen O One and one third

Silver Ag Nine

Calcium Ca Three and one third

4

9

96

84

16

108

40

Complete the Isotopes and atoms sheet

Many elements are a mixture of isotopes. The RAM given in the periodic table takes account of this.

For example, chlorine exists as two isotopes:chlorine-35 (75%) and chlorine-37 (25%).

To calculate the RAM of a mixture of isotopes, multiply the percentage of each isotope by its

atomic mass and add them together.

= (0.75 x 35) + (0.25 x 37)

= 26.25 + 9.25

= 35.5

RAM of chlorine = (75% x 35) + (25% x 37)

Bromine contains 50.5% bromine-79 and 49.5% bromine-81.

= (0.505 x 79) + (0.495 x 81)

= 39.895 + 40.095

= 79.99

= 80 (the RAM is usually rounded to the nearest whole number)

RAM of bromine = (50.5% x 79) + (49.5% x 81)

What is the RAM of naturally-occurring bromine?

1. Calculate the average atomic mass of iron if its abundance in nature is 15% iron-55 and 85% iron-56.

2. What is the average atomic mass of silicon if 92.21 % of its atoms have a mass of 27.977 amu, 4.07 % have a mass of 28.976 amu, and 3.09 % have a mass of 29.974 amu?

3.  Calculate the average atomic mass for neon if its abundance in nature is 90.5% neon-20 (19.922 amu), 0.3% neon-21 (20.994 amu), and 9.2% neon-22 (21.991 amu).

4.  Calculate the average atomic mass of silver if 13 out of 25 atoms are silver-107 and 12 out of 25 atoms are silver-109.

5.  Calculate the average atomic mass of chromium.

6.  

Isotope Mass (amu) Relative Abundance Cromium – 50 49.946 0.043500 Chromium – 52 51.941 0.83800 Chromium – 53 52.941 0.095000 Chromium – 54 53.939 0.023500