Relative Atomic Mass & Isotopes 5.1

Until recently it was impossible to weigh individual atoms

The relative scale

• English chemist John Dalton (1766-1844)

simply assigned an arbitrary mass to the carbon* atom

• It was assigned an exact mass of 12• The mass of the elements were

calculated “relative” to carbon

* It was attempted using O and H but the relative masses were either too small or non-integral numbers

Relative Atomic Mass

• For example, – H is 12 times lighter than C– Mg is 2 times heavier than C

Why does carbon weigh 12.01?

6

C12.01 No units because

this is NOT the mass of an atom

(relative mass only)

Carbon is made up of several isotopes, not just carbon-12

Isotopes

6

C12.01 Average

Atomic Mass

Carbon-12

Carbon-13

Carbon-14

Naturally occurring carbon contains 3 isotopes that are not present in equal amounts

Isotopes

Atoms from the same element with different amounts of neutrons in the nucleus

MassSpectrometer

can find the relative abundance of each isotope in an element

How many isotopes does this element have?

A typical mass spectrum for an element.

How would you find the average mass of a group of objects that have different masses and abundances?

80% 10% 10%

9g

12g22g

= 9 + 12 + 22 = 14.3 g 3

Doesn’t take into account that 80% of the spoons are 9g. Must calculate the WEIGHTED average.

= (80% x 9) + (10% x 12) + (10% x 22)

= 10.6 g

This applies to atoms and their isotopes

80% 10% 10%

9g

12g22g

Mg-24 Mg-26Mg-25

80% 10% 10%

VS

Find the atomic mass for Mg (remember: no units!)

Relative Atomic Mass (also called Atomic Weight)

Relative Molecular Mass

• It is the sum of the relative weights of the atoms in the molecular formula

Eg. H2OMass of O: 15.9994

Mass of H: 1.00794 1.00794

Molecular mass: 18.0153

Find the molecular weight:

Eg. H2SO4

Molecular weight vs

Formula weight

H2SO4 is a discrete covalent molecule

vs

NaCl and SiO2 which are infinitely large ionic and covalent lattices, respectfully

Because these represent empirical formulas, it’s proper term is “formula weight” for the ratio of atoms present eg. NaCl is 1:1 (23 + 35.5 = 58.5)