Atomic Structure

protons, neutrons, electrons, ions and isotopes

Basic Atomic Structure

The basic features of an atom are: Protons and Neutrons form the centre of an atom called the nucleus. Electrons orbit this in electron shells. Compared to the total volume of an atom, the nucleus is extremely small. However the nucleus is extremely dense and makes up most of the atom's mass. An atom mainly consists of empty space.


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Masses and Charges

Here are the relative masses and charges of protons, neutrons and electrons:


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Key Definition: Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.

Isotopes of the same element have:

  • different masses
  • the same number of protons and electrons
  • different numbers of neutrons in the nucleus

Isotopes of the same element all react in the same way, this is because:

  • Chemical reactions only involve electrons
  • Neutrons make no difference to the chemical reactivity of an element
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Carbon Isotopes

Carbon is one of the most common isotopes. Carbon exists as Carbon-12, Carbon-13 and carbon-14. All 3 forms of carbon have 6 protons, which means the number of neutrons causes the change in relative atomic mass


Youcan calculate the change in the number of neutrons by subtracting the proton number from the mass number.E.g. Carbon-13:

Mass number - Proton number = Number of neutrons

13     -        6              =   7

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Ions are charged particles created when an atom either loses or gains electrons.

Key Definition: an ion is a positively or negatively charged atom or covalently bonded group of atoms.

The ion will be positive if there are more protons than electrons. E.g. Na+ has 1 more proton than it has electron

The ion will be negative if there are more electrons than protons. E.g. Cl- has one more electron than it has proton

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