Science

Ionic bonding

Ions are electrically charged particles formed when atoms [atoms: An atom is the smallest part of an element that still has the properties of that element, comprising electrons surrounding a nucleus of protons and neutrons. ] lose or gain electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ]. They have the same electronic structures as noble gases.

Metal atoms form positive ions, while non-metal atoms form negative ions. The strong electrostatic [electrostatic: An electrostatic force is generated by differences in electric charge (ie positive and negative) between two particles. It can also refer to electricity at rest. ] forces of attraction between oppositely charged ions are called ionic bonds.

Ionic compounds [ionic compound: An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). The ions swap electrons to achieve a full outer shell. ] have high melting and boiling points.

How ions form

Ions are electrically charged particles formed when atoms lose or gain electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ]. This loss or gain leaves a complete highest energy level, so the electronic structure of an ion is the same as that of a noble gas - such as a helium, neon or argon.

Metal atoms and non-metal atoms go in opposite directions when they ionise:

  • Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions.
  • Non-metal atoms gain an electron, or electrons, from another atom to become negatively charged ions.

Positively charged sodium and aluminium ions

Negatively charged oxide and chloride ions

How many charges?

There is a quick way to work out what the charge on an ion should be:

  • the number of charges on an ion formed by a metal is equal to the group number of the metal
  • the number of charges on an ion formed by a non-metal is equal to the group number minus eight
  • hydrogen forms H+ ions

Group 1Group 2Group 3Group 4Group 5Group 6Group 7Group 0
Example elementNaMgAlCNOClHe
Charge1+2+3+Note 13-2-1-Note 2
Symbol of ionNa+Mg2+Al3+Note 1N3-O2-Cl-Note 2

Note 1: carbon and silicon in Group 4 usually form covalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ] by sharing electrons.

Note 2: the elements in Group 0 do not react with other elements to form ions.

Metal ions

You need to be able to show the electronic structure of some common metal ions, using diagrams like these:

Lithium, Li

Diagrams of a lithium atom (2,1) with two electrons in its inner shell and one electron in its highest energy level, and a lithium ion (2)+ with two electrons in its highest energy level

Lithium is in Group 1. It has one electron [electron: An electron is a very small negatively-charged particle found in an atom in the space surrounding the nucleus. ] in its highest energy level. When this electron is lost, a lithium ion Li+ is formed.

Sodium, Na

Diagrams of a sodium atom (2,8,1) and a sodium ion (2,8)+

Sodium is also in Group 1. It has one electron in its highest energy level. When this electron is lost, a sodium ion Na+ is formed.

Structure of a neon atom. A black dot represents the nucleus. The small circle around this has two red dots on it, representing the first energy level with two electrons. A larger outer circle has eight red dots on it, representing the second energy level with eight electrons

Neon atom

Note that a sodium ion has the same electronic structure as a neon atom (Ne).

But be careful - a sodium ion is not a neon atom. This is because the nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] of a sodium ion is the nucleus of a sodium atom and has 11 protons - but the nucleus of a neon atom has only 10.

Magnesium, Mg

Diagrams of a magnesium atom (2,8,8,2) and a magnesium ion (2,8)2+

Magnesium is in Group 2. It has two electrons in its highest energy level. When these electrons are lost, a magnesium ion Mg2+ is formed.

A magnesium ion has the same electronic structure as a neon atom (Ne).

Calcium, Ca

Diagrams of a calcium atom (2,8,8,2) and a calcium ion (2,8,8)2+

Calcium is also in Group 2. It has two electrons in its highest energy level. When these electrons are lost, a calcium ion Ca2+ is formed.

A calcium ion has the same electronic structure as an argon atom (Ar).

Non-metal ions

You need to be able to show the electronic structure of some common non-metal ions, using diagrams like these:

Fluorine, F

Diagrams of a fluorine atom (2,7) and a fluoride ion (2,8)-

Fluorine is in Group 7. It has seven electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] in its highest energy level. It gains an electron from another atom in reactions, forming a fluoride ion, F-.

Note that the atom is called fluorine, but the ion is called fluoride.

Structure of a neon atom. A black dot represents the nucleus. The small circle around this has two red dots on it, representing the first energy level with two electrons. A larger outer circle has eight red dots on it, representing the second energy level with eight electrons

Neon atom

Note that a fluoride ion has the same electronic structure as a neon atom (Ne).

Once again, a fluoride ion is not a neon atom, because the nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] of a fluoride ion is the nucleus of a fluorine atom, with 9 protons, and not of a neon atom, with 10.

Chlorine, Cl

Diagrams of a chlorine atom (2,8,7) and a chloride ion (2,8,8)-

Chlorine is in Group 7. It has seven electrons in its highest energy level. It gains an electron from another atom in reactions, forming a chloride ion, Cl-.

Oxygen, O

Diagrams of an oxygen atom (2,6) and an oxide ion (2,8) 2-

Oxygen is in Group 6. It has six electrons in its highest energy level. It gains two electrons from one or two other atoms in reactions, forming an oxide ion, O2-.

Ionic compounds and ionic bonding

When metals react with non-metals, electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] are transferred from the metal atoms to the non-metal atoms, forming ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons. ]. The resulting compound is called an ionic compound [ionic compound: An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron): The ions swap electrons to achieve a full outer shell. ].

Consider reactions between metals and non-metals, for example,

  • sodium + chlorine → sodium chloride
  • magnesium + oxygen → magnesium oxide
  • calcium + chlorine → calcium chloride

In each of these reactions, the metal atoms give electrons to the non-metal atoms. The metal atoms become positive ions and the non-metal atoms become negative ions.

There is a strong electrostatic [electrostatic: An electrostatic force is generated by differences in electric charge (ie positive and negative) between two particles. It can also refer to electricity at rest. ] force of attraction between these oppositely charged ions, called an ionic bond. The animation shows ionic bonds being formed in sodium chloride, magnesium oxide and calcium chloride.

There are many ionic bonds in an ionic compound such as sodium chloride, arranged in giant lattice [lattice: A lattice is a regular grid-like arrangement of atoms in a material. ] structures. Ionic compounds have high melting and boiling points.

Dot-and-cross diagrams

You need to be able to draw dot-and-cross diagrams to show the ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons. ] in some common ionic compounds [ionic compound: An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). The ions swap electrons to achieve a full outer shell. ].

Sodium chloride, NaCl

Diagram of bonding in sodium chloride. A sodium ion (2,8)+ gives an electron to a chloride ion (2,8,8)-. Both ions have full highest energy levels.

Sodium ions have the formula Na+, while chloride ions have the formula Cl-. You need to show one sodium ion and one chloride ion. In the exam, make sure the dots and crosses are clear, but do not worry about colouring them.

Magnesium oxide, MgO

Diagram of bonding in magnesium oxide. A magnesium ion (2,8)2+ gives two electrons to an oxide ion (2,8)2-. Both ions have full highest energy levels

Magnesium ions have the formula Mg2+, while oxide ions have the formula O2-. You need to show one magnesium ion and one oxide ion.

Calcium chloride, CaCl2

Diagram of bonding in calcium chloride. A calcium ion (2,8,8)2+ gives one electron to a chloride ion (2,8,8)- and another electron to another chloride ion (2,8,8)-. All three ions have full highest energy levels

Calcium ions have the formula Ca2+. Chloride ions have the formula Cl-.

You need to show two chloride ions, because two chloride ions are needed to balance the charge on a calcium ion.

Now try a Test Bite

 

Back to Revision Bite