The Structure of an Atom

Atoms are the fundamental units of matter. They are made up of a positively charged nucleus (consisting of protons and neutrons), negatively charged electrons, and a lot of empty space.

The nucleus has a diameter 100,000 times smaller than that of the atom, but still more than 99.9% of the mass of the atom is found in the nucleus.

Electrons are small subatomic particles. Due to their negative charge, they are attracted to the positively charged nucleus, which leads them to orbit around it in successive layers called electron shells.

Two diagrams representing simplified models of an atom.On the left, a Bohr model of an atom is shown, with a central nucleus composed of protons and neutrons, and three electrons in defined circular orbits or shells around the nucleus. Each component is labelled accordingly.On the right, there is a more abstract representation of an atom, depicting a nucleus with protons and neutrons at the centre and a cloud of electrons represented by blue orbitals around the nucleus.

Subatomic Particles

Atoms are made up of three types of smaller particles:

  • Protons
  • Neutrons
  • Electrons

The way they interact defines an atom’s properties. Because they are smaller than an atom, they are called subatomic particles.

Since subatomic particles are such small structures, their mass cannot be measured in actual masses like grams of kilograms. Instead, a relative mass is established for each of them. This describes their mass relative to one another. Protons and neutrons have the same relative mass, but electrons have a much smaller relative mass. This is why almost all of the mass of an atom is collected in the nucleus.

Protons are positively charged, neutrons have no charge (they are neutral), and electrons are negatively charged. These charges are also depicted relative to one another: a proton has a relative charge of +1, a neutron a relative charge of 0, and an electron has a relative charge of -1.

The basic properties of the three main subatomic particles are represented in the table below:

Subatomic particle Relative mass Relative charge

The Nucleus of an Atom

The nucleus is the central part of an atom. Because it is made up of positively charged protons and neutrons with no charge, it overall has a positive charge. It holds more than 99.9% of the total mass of the atom.

The number of protons in atoms of the same element is always the same – in fact, an element is defined by the number of protons it has. This means that there will never be two different elements containing the same number of protons in their atoms, or two atoms of the same element with different number of protons. So, if we know the number of protons in an atom, we can identify the element.

Although neutrons have no charge, they do have mass. Both protons and neutrons have a relative mass of 1.

Typically, an atom will have equal numbers of protons, neutrons and electrons. If an atom has a different number of protons and neutrons, it is called an isotope. If it has a different number of protons and electrons, it is called an ion and will have a charge.

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