Metals and non-metals sit on opposite sides of the table
Definition
Atomic number
The number of protons in the nucleus of an atom of an element.
- Metals are found on the left and in the centre of the periodic table.
- Non-metals are found on the right, towards the top.
- A stepped line near the right separates most metals from the non-metals.
- Whether an element is a metal or non-metal depends on its electronic structure, especially how many outer electrons it has.
Key Idea
- Most elements are metals.
- Position in the table predicts whether an element loses or gains electrons.
Metals lose electrons to form positive ions
Definition
Ion
An atom or group of atoms with an overall electrical charge because electrons have been lost or gained.
- Metal atoms have few electrons in their outer shell.
- They lose these outer electrons to reach a full, stable outer shell.
- Losing negative electrons leaves the atom with more protons than electrons, so it becomes a positive ion.
- Sodium loses one electron: Na→Na++e−\text{Na} \rightarrow \text{Na}^+ + \text{e}^-Na→Na++e−.
- Magnesium loses two electrons: Mg→Mg2++2e−\text{Mg} \rightarrow \text{Mg}^{2+} + 2\text{e}^-Mg→Mg2++2e−.
- Typical metal properties are being shiny, good conductors of heat and electricity, malleable and having high melting points.
Note
The group number of a metal often matches the positive charge of the ion it forms.
Non-metals gain or share electrons
Definition
Outer-shell electron
An electron in the highest occupied electron shell of an atom.
- Non-metal atoms have almost full outer shells, with many outer-shell electrons.
- They gain electrons to complete their outer shell, forming negative ions.
- Chlorine gains one electron: Cl+e−→Cl−\text{Cl} + \text{e}^- \rightarrow \text{Cl}^-Cl+e−→Cl−.
- Oxygen gains two electrons: O+2e−→O2−\text{O} + 2\text{e}^- \rightarrow \text{O}^{2-}O+2e−→O2−.
- Non-metals can also share electrons with other non-metals instead of transferring them.
- Typical non-metal properties are being dull, poor conductors, brittle if solid, and often having low melting points.
Self review
- Where are metals and non-metals found in the periodic table?
- Answer: metals on the left and centre, non-metals on the right towards the top.
- Why do metals form positive ions?
- Answer: they lose their few outer electrons to gain a full outer shell.
- Write an equation for magnesium forming its ion.
- Answer: Mg→Mg2++2e−\text{Mg} \rightarrow \text{Mg}^{2+} + 2\text{e}^-Mg→Mg2++2e−.
- Why do non-metals form negative ions?
- Answer: they gain electrons to complete their outer shell.
