# Unit F321 : Module 1 - Atoms and reactions

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Atomic Structure

Proton

Neutron

Electron

Mass

1

1 Charge

+1

0

-1

Protons and neutrons are in the nucleus of the atom, surrounded by atomic orbitals containing clouds of electrical charge (electrons).

In a neutral atom the number of electrically positive protons balances the number of electrically negative electrons.

The nucleus is the densest part of the atom, with the electrons taking up most space, but weighing very little.

The atomic (proton) number shows the number of protons in the nucleus of an atom.

The mass (nucleon) number shows the number of particles (protons and neutrons) in the nucleus.

Ions are positively or negatively charged atoms or groups of atoms (molecules). In order gain this charge the number of electrons must change – the number of protons and neutrons in the nucleus cannot.

Isotopes are atoms of the same element with different numbers of neutrons and so different masses. The number of protons and electrons don’t change, so the atomic (proton) number doesn’t change. Different isotopes react in the same way because neutrons make no difference to electrical reactivity.

Relative Masses

In order to find the mass of atoms chemists invented the unified atomic mass unit. The mass of an atom of carbon-12 is defined as 12, so the mass of one twelfth of a carbon-12 atom is 1. All atoms are measured against carbon-12.

Relative atomic mass, Ar, is the weighted mean mass of an atom of an element compared with one twelfth of the mass of one atom of carbon-12.

Relative isotopic mass is the weighted mean mass of an atom of an isotope compared with one twelfth of the mass of one atom of carbon-12.

To calculate the relative atomic mass of an element if given the abundances of its isotopes:

Isotope

50Cr

52Cr

53Cr

54Cr

Abundance

4.31%

83.76%

9.55%

2.38% =52.1 (3sf)

The mass of compounds can also be given.

The relative formula mass is the weighted mean mass of a formula unit compared with one twelfth of the mass of one atom of carbon-12. This is used for compounds with giant structures.

The relative molecular mass, Mr, is the weighted mean mass