C2aL2 Mass Number and Isotopes

Key Words

Atoms - the smallest particle into which an element can be broken down

Element - a substance that consists of one kind of atom only.

Mass Number - total number of protons and neutrons.

Neutrons - neutrally charged particle in the nucleus.

Protons - positively charged particles in the nucleus.

Relative mass - mass compared to the mass of a proton.

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Grade E

Elements are substances that contain only one kind of atom.

 

The element is determined by the number of protons.  This atom is lithium that has three protons.  If we added a fourth proton the element would change to beryllium.

If we change the number of neutrons, we do not change the element; instead we have an isotope.  If we added an extra neutron to the lithium atom, it will still be lithium, but its mass number would increase from 7 to 8.  This is because the number of neutrons would have increased from 4 to 5.  The number of protons would still be3.

Hydrogen has three isotopes.

  • Normal hydrogen, that has 1 proton, 0 neutrons;

  • Hydrogen-2 (deuterium), 1 proton and 1 neutron;

  • Hydrogen-3 (tritium), 1 proton and 2 neutrons.

Elements are shown in the periodic table.

Grade C

The relative atomic mass is based on carbon-12, which has a mass number of exactly 12.

Mass number

12              

C

6               

Atomic number

It has the symbol Ar

The relative atomic mass gives an average value of the mass of the element, as most elements have different isotopes.  For example, chlorine is made up of:

  • 25 % mass 37;

  • 75 % mass 35.

If we had 100 atoms, we could work out their total atomic mass:

25 × 37 = 925

75 × 35 = 2625

925 + 2625 = 3550.

Now divide by 100 to get the average mass per atom:

3550 ÷ 100 = 35.5

Be careful NOT to say that the 0.5 bit is due to there being half a neutron.

 

Isotopes have the same number of protons, so they will have the same number of electrons.  Therefore their chemical reactions are the same.  The physical properties may be different.

Grade A

Some isotopes are unstable, which means that they will decay radioactively.  Uranium decays to lead.  The proportion of uranium nuclei to lead nuclei gives scientists the age of the Earth and other objects.  The Earth is 4500 million years old.