Isotopes

Question 1

Relative atomic mass:

Answer 1

Don’t confuse relative atomic mass (A[r]) with mass number.

A[r] is the mean mass of the atoms of an elements, relative the 1/12th the weight of a [12]C atom.

A[r] values take into account the relative abundance or % of each isotope. A[r] values may not be whole numbers.

Question 2

Define isotope.

Answer 2

Isotopes are atoms of an element with:

• the same number of protons (atomic number)
• different numbers of neutrons (mass numbers)

An isotope is the same element with the same atomic number but with different mass numbers.

Question 3

Define mass number.

Answer 3

The total number of protons and neutrons in the nucleus.

Question 4

Define atomic number.

Answer 4

The number of protons in the nucleus.

Atoms of a given element have the same amount of protons (same atomic number that is unique to that element).

Question 5

For sodium (Na):

atomic number = 11
number of protons = 11
number of electrons = 11
(equal number of protons and electrons)

mass number = 23
neutrons = ?

Answer 5

neutrons = mass number - atomic number = 23 - 1 1 = 12

Question 6

Isotopes:

Answer 6

You can recognise isotopes of the same element because:

• same atomic number
• different mass numbers

Isotopes of an element have the same chemical properties because they have the same number of electrons.

Question 7

How do you work out relative atomic mass?

Answer 7

(mass a x %) + (mass b x %) / 100 = relative atomic mass

Question 8

An element has 2 isotopes, element-76 and element-x. The relative abundance of element-76 is 63.7%.
The relative atomic mass of the element is 67.288.
What is the value of x, to 3 significant figures?

Answer 8

(76 x 63.7) + (x x (100-63.7) / 100 = 67.288
(4841.2) + (x x 37.3) / 100 = 67.288 (x100)
= 6728.8 (-4841.2)
= 1887.6 (/37.3)
= 52

Question 9

Gallium has 2 isotopes: [69,31]Ga and [71,31]Ga (69 + 71 at the top), also written as gallium-69 and gallium-71. The relative abundance of gallium-69 is 60%. Calculate, to 1 decimal place, the relative atomic mass of gallium.

Answer 9

/ 100 if using percentage not decimals
= relative atomic mass

Relative abundance of gallium-71 = 100% - 60%
= 40%

(69 x 60) + (71 x 40) / 100
= 4140 + 2840 / 100
= 69.8

Question 10

Bromine has two natural isotopes, [79, 35]Br (bromine-79) and [81,35]Br (bromine-81). Explain, in terms of the numbers of subatomic particles, why these are isotopes of the same element.

Answer 10

Each atom has the same number of protons and (usually) neutrons surrounded by electrons in shells.
Both of bromine’s isotopes have the same number of protons, but a different number of neutrons, so it is still the same element.

Question 11

Chlorine has 2 natural isotopes, 75.8% [35,17]Cl (chlorine-35) and 24.2% [37,17]Cl (chlorine-37).
Calculate the relative atomic mass, A[r], of chlorine.
Give your answer to 1 decimal place.

Answer 11

(35 x 75.8) + (37 x 24.3) / 100
= 2653 + 895.4 / 100
= 3548.4 / 100
A[r] = 35.484 = 35.5

Question 12

How do you work out relative atomic mass?

Answer 12

(mass a x %) + (mass b x %) / 100 = relative atomic mass