Chemical Ideas 2.1.

A simple model of the atom.

What’s inside?In the

nucleus?

ProtonsAnd around the nucleus?

ElectronsNeutrons

How are they

different?Mass

Charge

Location

Mass on Ar scale

Charge

p 1 +1

n 1 0

e- 0.00055 -1

Nuclear symbols

Na23

11

Mass number =?

Atomic number =?

p + n

p ( = e-)p?

e-?

n?

•11

•11

•12

What are isotopes?

Cl35

17Cl

37

17

p?

e-?

n?

p?

e-?

n?

17

17

18 20

17

17

•Atoms of the same element with:

•The same atomic number but different mass numbers

•Same number of protons but different numbers of neutrons

Why is 35Cl O.K.?

Cl35

17Cl

37

17

Relative isotopic mass

Cl-35 Cl-37

Chlorine always has an atomic number of 17!

So what is relative atomic mass?

Cl-35

Cl-37

75%

25%

Relative atomic mass is the average of the relative isotopic

masses

Cl-35

Cl-37

75%

25%

How do we get a relative atomic mass of 35.5?

What 2 things did we need to know?

Relative isotopic masses.

The relative abundances.

How do we find them out?

Use a mass spectromete

r

Mass spectrometry

Ionisation andfragmentation

Sample in here

Magnetic fieldElectric field

Mg

e-

e-

Mg+

Ionisation andfragmentation

Sample in here

Magnetic fieldElectric field

Cl-35

Cl-37

A mass spectrum …

35 3736

20

40

60

80

100

%

mass

A mass spectrum …

24 2625

20

40

60

80

100

%

mass

Try W and Y!