1b.3 Isotopes and relative atomic mass

Question 1

What is an isotope?

Answer 1

Isotope = different forms of the same element - same number of protons but different numbers of neutrons

This means they have the same atomic number but different mass numbers

Question 2

Give an example of an element which has isotopes

Answer 2

Carbon (atomic number 6)

Most (99%) have mass 12 (6 neutrons)
Some (1%) have mass 13 (7 neutrons)

Note one in every trillion carbon atoms has mass number 14 (8 neutrons)

Question 3

What is the relative atomic mass (and the symbol)?

Answer 3

Relative atomic mass = Ar (subscript “r”)

= the avergae mass of one atom of an element, compared to 1/12 of the mass of one atom of carbon-12

Question 4

Why may a relative atomic mass not be a whole number?

Answer 4

If an element has more than one isotope, the average of the mass numbers of different isotopes may not be a whole number

Question 5

GIve an example of an element with a non-integer relative atomic mass.

Answer 5

Chlorine (atomic number 17)

Two isotopes: chlorine-35 (18 neutrons) and chlorine-37 (20 neutrons)

Chlorine-35 makes up around 75% of the atoms, and chlornie-37 about 25%, so the relative atomic mass is 35.5

Question 6

State the formula to calculate relative atomic mass

Answer 6

Question 7

Calculate the relative atomic mass of copper: Cu-63 (69.2%), Cu-65 (30.8%)

Answer 7