The Structure of the Atom

Protons, Neutrons and Electrons

Atoms are the smallest units of an element and serve as the basic building blocks of matter.

  • The radius of an atom is 1 \times 10^{-10} \:\text{m}

An atom has a nucleus in the centre, which contains protons and neutrons. The nucleus has an overall positive charge because protons have a positive charge and neutrons have no electric charge.

Negatively charged electrons orbit the nucleus and they are arranged in a series of energy levels that are at specific distances from the nucleus.

  • The radius of the nucleus is over 10,000 times smaller than the whole atom

Atoms have no net charge. Electrons are negatively charged and the nucleus is positively charged, so these opposite charges balance out. The table below shows the properties of these particles:

Subatomic particleRelative chargeRelative massLocation
Electron-11/2,000 (close to 0)Electron shells around the nucleus

Mass Numbers and Atomic Numbers

Protons and neutrons make up nearly all of an atom’s mass. The radiuses of the electron orbits are much larger than the radius of the nucleus. This means that most of the atom’s volume is empty space.

An element is defined by the number of protons in its nucleus, which means that atoms of the same element have the same number of protons. This is also the atomic number of the atom.

The mass number and the atomic number of an element can be shown like this:

  • The mass number is the number of protons + the number of neutrons. 6 protons + 6 neutrons = 12, so carbon has a mass number of 12.

  • The atomic number refers to the number of protons in the atom. Carbon has 6 protons, so it has an atomic number of 6.

In a stable atom, the number of protons is equal to the number of electrons. This means the overall electrical charge of an atom is 0. However, if an atom becomes ionised, the atom will lose or gain electrons.

Electron Energy Levels

Electrons in an atom are arranged in different energy levels, which are at specific distances from the nucleus. These energy levels are often referred to as shells, with each successive shell being farther from the nucleus.

Electrons tend to stay at one energy level. However, with just the right amount of energy, an electron can jump to a higher energy level in its excited state. This energy comes from electromagnetic radiation. However, this is typically followed by the electron dropping to a lower energy level and emitting electromagnetic radiation.

  • An electron can move to a higher energy level by absorbing electromagnetic radiation
  • An electron can move to a lower energy level by emitting electromagnetic radiation

Electrons in higher energy levels have greater potential energy due to the greater distance between the electron and the nucleus.