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Isotopes

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Isotopes

Definition

- All atoms of the same element have the same atomic number, but may have different masses.
- Having the same atomic number, means having the same number of protons (and electrons).
- The difference in mass is due to the difference in the number of neutrons a specific atom may have.

Isotopes

Atomic mass

Examples

Isotopes

- Carbon-12
- Carbon-13
- Carbon-14

Atomic number

12C

13C

14C

35Cl

37Cl

- Chlorine-35
- Chlorine-37

17

17

6

6

6

Neutron Formula

- Atomic mass – Atomic number = # neutrons

Isotopes

Examples

Atomic mass

12 – 6 = 6 no

12C

14C

6

6

Atomic number

14 – 6 = 8 no

Explanation

- So the atomic mass refers to the average mass of all the atoms of an element.

Atomic mass calculations

Example

100 total atoms of carbon:

99 atoms of carbon-12

1 atom of carbon-13

99 x 12 = 1188 u

1 x 13 = 13 u

Total 1201 u

Average mass = 1201 u / 100 = 12.01 u

Natural Abundance

Atomic mass calculations

98.89 %

1.11 %

Example a

Steps

mass of carbon-12 => 12 x 0.9889 = 11.8668 u

mass of carbon-13 => 13 x 0.0111 = 0.1443 u

Average mass of

naturally occurring Carbon 12.0111 u

12C

13C

6

6

Example b

Calculate the atomic mass given the information:

IsotopeAbundance (%)Mass (u)

Cl-3575 35

Cl-3725 37

Atomic mass calculations

Steps

mass of Cl-35 => 35 x 0.75 = 26.25 u

mass of Cl-37 => 37 x 0.25 = 9.25 u

Average mass of Cl 35.50 u

Example c

If 50 % of Bromine atoms have a mass number

of 79 and 50 % have a mass number of 81.

Calculate the average mass (atomic mass ) of Bromine atoms.

Atomic mass calculations

Steps

79 x 0.50 = 39.50 u

81 x 0.50 = 40.50 u

Average mass of Bromine 80.00 u

Example d

If 80 % of Boron atoms have a mass number

of 11. The rest have a mass number of 10.

What is the atomic mass of Boron?

Atomic mass calculations

Steps

11 x 0.80 = 8.8 u

10 x 0.20 = 2.0 u

Average mass of B 10.8 u

Example e

Calculate the atomic mass of Fe.

73 % of the atoms have a mass of 55.

15 % of the atoms have a mass of 56.

12 % of the atoms have a mass of 54.

Atomic mass calculations

Steps

55 x 0.73 = 40.15 u

56 x 0.15 = 8.40 u

54 x 0.12 = 6.48 u

Average mass of Fe 55.03 u