Metals and non-metals in the periodic table

Elements can be grouped into metals and non-metals according to how they behave in chemical reactions.

Metal atoms can lose electrons, forming positive ions. Losing negatively charged electrons leaves the atom with more positive charge than negative charge. Non-metal atoms do not normally form positive ions in this way. Instead, they commonly gain electrons to form negative ions, or share electrons with other atoms in covalent bonds.

Position in the periodic table

Most elements in the periodic table are metals. They are found mainly on the left-hand side and towards the bottom of the table. Non-metals are found mainly towards the right-hand side and the top.

An element’s position therefore gives a useful guide to whether it is a metal or a non-metal, but it is not an absolute rule. For example, hydrogen is a non-metal even though it is placed on the left-hand side of the table.

Electron arrangement and the reactivity of metals and non-metals

Atoms react according to the arrangement of their electrons, especially the electrons in their outer shell. These are the electrons that can be lost, gained or shared when atoms react.

An element’s atomic number is the number of protons in each of its atoms. A neutral atom has equal numbers of protons and electrons, so its atomic number also tells us how many electrons it has. Those electrons occupy shells around the nucleus. As atomic number changes from one element to the next, the electron arrangement changes too.

Outer-shell electrons and periodic-table position

An element’s position in the periodic table is linked to its electron arrangement, and therefore to how it tends to react.

• Metals, found mostly on the left, tend to have only a few outer-shell electrons. They tend to lose these electrons when they react.
• Non-metals, found mostly on the right, often have nearly full outer shells. They tend to gain electrons or share electrons with other atoms when they react.

Losing, gaining or sharing electrons can give atoms a more stable electron arrangement, often with a full outer shell. These different tendencies help explain why metals and non-metals show different reaction patterns.

For example, a neutral sodium atom has 11 electrons arranged as 2, 8, 1. It can lose its one outer-shell electron. A neutral chlorine atom has 17 electrons arranged as 2, 8, 7, so it can gain one electron to complete its outer shell. When sodium reacts with chlorine, an electron can transfer from a sodium atom to a chlorine atom.

Properties of metals and non-metals

Elements can be recognised as metals or non-metals by looking at patterns in their physical properties and how their atoms behave in reactions. These patterns are useful guides, although individual properties have exceptions.

Physical properties

Metals are typically good conductors of heat and electricity. They are often strong and shiny, and many are malleable: they can be hammered or pressed into shape without breaking. Metals usually have relatively high melting and boiling points.

Non-metals are generally poor conductors of heat and electricity. When solid, they are usually dull and brittle, meaning they break rather than bend when force is applied. Many non-metals have lower melting and boiling points than metals.

These are general patterns, not rules without exceptions. For example, graphite, a form of the non-metal carbon, conducts electricity.

Behaviour of electrons

Metal atoms tend to lose electrons in reactions. Because electrons have a negative charge, an atom that loses electrons becomes a positive ion. For example, a sodium atom, $$\ce{Na}$$, can lose one electron to form $$\ce{Na+}$$.

Non-metal atoms do not normally form simple positive ions. They commonly gain electrons to form negative ions: for example, a chlorine atom, $$\ce{Cl}$$, can gain one electron to form $$\ce{Cl-}$$. Non-metal atoms can also share electrons with other atoms in covalent substances; sharing electrons does not mean that the atoms form negative ions.