Atoms: protons, neutrons and electrons

Last time

Before we start: last time, in Metallic bonding and comparing structures, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Three particles

Every substance you have ever touched is built from atoms, and every atom is built from just three kinds of particle. Protons, neutrons and electrons. Once you know how many of each an atom has, you know exactly which element it is and what it can do.

The structure of an atom

An atom has a tiny nucleus in the centre. The nucleus contains the protons and the neutrons. The electrons move around the nucleus in shells, which are also called energy levels. Almost all the mass of the atom is in the nucleus, but the electrons take up almost all of the space. An atom has no overall charge, because the number of protons is equal to the number of electrons.

An atom drawn

Here is a lithium atom. It has three electrons: two in the first shell and one in the second. So it must also have three protons, to make it neutral.

The three particles

Here are the facts you must know. The proton has a relative charge of plus one and a relative mass of one. The neutron has no charge, and a relative mass of one. The electron has a relative charge of minus one, and almost no mass: one eighteen-hundred-and-fortieth.

Details that earn marks

Write the charge of the proton as plus one, with the plus sign. A plain one loses the mark. The electron has a very small mass, so we say its mass is negligible. Protons and neutrons are together called nucleons, because they are both in the nucleus.

Proton number

The proton number, also called the atomic number, is the number of protons in the nucleus. It decides the element. Eleven protons is always sodium, and six protons is always carbon. The Periodic Table is arranged in order of proton number.

Nucleon number

The nucleon number, also called the mass number, is the total number of protons and neutrons in the nucleus. So to find the number of neutrons, subtract the proton number from the nucleon number. The electrons are not counted in the nucleon number, because their mass is so small.

Reading the symbol

In the symbol for sodium, the top number, twenty-three, is the nucleon number. The bottom number, eleven, is the proton number. So sodium has eleven protons, and twenty-three take away eleven is twelve neutrons. And as a neutral atom, it has eleven electrons.

Ions

An ion is an atom that has gained or lost electrons, so it has an overall charge. When an atom loses electrons, it becomes a positive ion. Sodium loses one electron to become Na⁺. When an atom gains electrons, it becomes a negative ion. Chlorine gains one electron to become Cl⁻. Only the number of electrons changes. The number of protons never changes.

A sodium ion

Here is the sodium ion. It has lost the one outer electron, so it has ten electrons, arranged two, eight. But it still has eleven protons, so the charge is eleven minus ten, which is plus one.