Atomic Structure

The Structure of the Atom

The Structure of the Atom

Protons, neutrons and electrons

Atoms are composed of three fundamental particles: protons, neutrons and electrons.

  • Protons are found in the nucleus and collectively known as nucleons.
  • Electrons orbit the nucleus.
  • In between electrons and the nucleus is empty space.

The basic properties of these particles is illustrated in the table below.

Particle Charge Mass
Proton +1 unit Approximately 1 unit
Neutron No charge Approximately 1 unit
Electron -1 unit Approximately 1/1840 units

1 unit of charge is equivalent to 1.602 x 10-19 coulombs

1 unit of mass is equivalent to 1.661 x 10-27 kg. To simplify, however, atomic mass units are used instead.

The mass of protons and neutrons depends on the nucleus and so varies a little. Therefore, one nucleus is taken as standard in order to define an atomic mass unit: carbon-12. This nucleus was chosen because its mass per nucleon is about average. Everything is then measured against this quantity.

Atomic numbers, mass number and isotopes

Atoms are named after the number of protons they contains in their nucleus.

The atomic number if the number of protons found in the nucleus (Z).

The number of protons and neutrons in the nucleus is known as its mass number (A).

The nucleus of an atom can be represented by both its mass number and atomic number.

Atoms which have the same atomic number but different mass numbers are known as isotopes.

A neutral atom consists of the same number of protons and neutrons. However, most elements do not exist is this state. Instead, they exist as ions which contain different numbers of protons and neutrons. This means that the overall charge of an ion is either negative or positive.

Relative atomic mass

The relative atomic mass (RAM) of an isotope is the ratio of the mass of one atom of that isotope to 1/12th of the mass of one carbon-12 atom.

Remember that; Different isotopes have different proton and neutron masses. The RAM is never a whole number.

Certain compounds and elements are molecules. The RAM of a molecule is the ratio of the average mass of the molecule to 1/12th of the mass of a carbon-12 atom. In other words, the RAM of a molecule is the RAM if its atoms summed together.