Isotopes

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Neutrons

The Mass Number (In DP this is called the Relative Atomic Mass) is the Sum of the Neutrons and the Protons in the Nucleus of an atom. It is the largest number in the elements box.

We can use this number to calculate the number of Neutrons in the nucleus:

Mass Number = Proton Number + Neutron Number
Example (Helium)
4 = . 2 . + Neutron Number This can rearrange
Neutron Number = 4 - 2
The Neutron Number of Helium = 2
This agrees with our diagram above as the nucleus has 4 particles in it

Isotopes - Changing Numbers of Neutrons

Although atoms cannot have different numbers of protons the Nucleus of an atom of an element can have different numbers of neutrons

An element that has the same Proton Number but Different Neutron Number is called an ISOTOPE

On the left here you can see 3 ISOTOPES of the element Hydrogen

They are the same element but have different numbers of neutrons

Properties of Isotopes

Sometimes extra neutrons can make the nucleus unstable, so its radioactive.
Different isotopes of the same element can have different physical properties, e.g. Density.
Different isotopes of the same element ALWAYS the same chemical properties because their reactions depend on their electron structure.

Calculating Relative Atomic Mass

Relative Atomic Mass is defined as 'Ar, the average mass of naturally occurring atoms of an element on a scale where the 12C atom has a mass of exactly 12 units '

Chlorine has 2 isotopes

Chlorine 35 and Chlorine 37

75% of one mole of atoms are Cl-35

25% of one mole of atoms are Cl-37

General Formula

Ar = (Ar of isotope 1 x Abundance of isotope 1) + (Ar of isotope 2 x Abundance of isotope 2)

100

Chlorine

Ar = (35 x 75) + (37 x 25) = 35.5

100

Isotopes Activity

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