Electron

An electron is a subatomic particle in the shells with a relative charge of −1 and a relative mass of about 1/1840, which is so small it is treated as negligible. Malay: elektron · Chinese: 电子.

Electrons move around the nucleus in shells, or energy levels. Because their mass is negligible, they add almost nothing to the mass of the atom; their importance is their negative charge. In a neutral atom the number of electrons equals the number of protons, so the charges cancel. It is the electrons, especially the outermost ones, that decide how an atom reacts chemically.

Example. A sodium atom has 11 electrons arranged as 2.8.1. The single electron in its outermost shell is its valence electron, and losing it turns the atom into a sodium ion, Na⁺. Because chemical reactions involve gaining, losing or sharing electrons, the electron arrangement of an atom controls its chemistry.

Confusion to avoid. Do not confuse the electron with the proton or the neutron. The electron is outside the nucleus, has charge −1, and has negligible mass; the proton and neutron are inside the nucleus and each have a relative mass of 1. A frequent mistake is to count electrons when working out the nucleon number, electrons are not nucleons, so they are never counted in it.

In the exam you may be asked for the relative charge and mass of the electron, to write an electron arrangement such as 2.8.1, or to identify the number of valence electrons. Remember that the electrons fill the shells from the one nearest the nucleus outwards, and that only the outermost electrons take part in reactions. Because their mass is negligible, electrons are never included when you calculate the nucleon number.

The electron is what links this chapter to the rest of chemistry: electron arrangement, the formation of ions and, later, chemical bonding all depend on it. Our teachers spend real time on electron arrangement here because so much of SPM Chemistry rests on getting it right.