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Bonding for AQA A-level Chemistry

Bonding

This page covers the following topics:

1. Ionic bonding
2. Covalent bonding
3. Metallic bonding
4. Polar covalent bonds
5. Dative covalent bonding

Ionic bonds form between cations (positive ions) and anions (negative ions). Many ionic compounds (salts) are formed by metals and non-metals. When an ionic bond is formed by a metal (usually from groups 1, 2) and a non-metal (usually from groups 6, 7), electrons are transferred from the metal atoms to the non-metal atoms to form metal cations and non-metal anions. The electrostatic attraction between these ions creates an ionic bond, which can be depicted using a dot-and-cross diagram.

Ionic bonding

Covalent bonds arise from the sharing of pairs of electrons between two non-metal atoms. Covalent bonds are stronger than ionic or metallic bonds. Most molecules have covalent bonds. Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine exist naturally as diatomic molecules Hโ‚‚, Nโ‚‚, Oโ‚‚, Fโ‚‚, Clโ‚‚, Brโ‚‚, Iโ‚‚. For these diatomic molecules, the number of shared electron pairs in a molecule is the same as the number of unpaired electrons in a single atom. A single bond is formed when 2, a double when 4 and a triple when 6 electrons are shared.

Covalent bonding

Metallic bonding arises from the electrostatic attraction between positive metal cations and negative delocalised electrons. Delocalised electrons aren't associated with any particular metal atom, they move freely around the whole giant metallic structure. Since these delocalised electrons are free to move across a material, metals are good conductors of electricity. Metallic bonding can be present between atoms of a single metal or a mixture of different metals called alloys.

Metallic bonding

Not every bond between two different atoms falls within the strict definitions of ionic or covalent bonds. A covalent bond formed between atoms of two elements that have different electronegativities is called a polar covalent bond. Such two atoms have an unequal attraction to a bonding electron pair which results in unsymmetrical electron distribution across the bond. ฮดโˆ’ partial charge is formed on the side of the bond that has a higher electron density, while ฮด+ is formed on the side of the bond that has a lower electron density.

In some molecules the partial charges across polar covalent bonds result in a permanent dipole, in which the molecule is considered polar. Molecules that are 3D symmetric are likely to be non-polar even if they have polar covalent bonds. Molecules that are not 3D symmetric and have polar bonds are likely to be polar.

Polar covalent bonds

Dative covalent bonding (sometimes called coordinate bonding) is the sharing of a pair of electrons between two atoms, with both of the electrons originating from the same atom. The dative covalent bond is usually depicted as an arrow, illustrating where the shared electrons originate from. A dative bond is usually formed in between an atom in a molecule that lacks electrons and has empty orbitals available in its outer shell, and an atom that has lone pairs of electrons in its outer shell.

Dative covalent bonding

1

Which atom in a Cโ€”O bond would have a ฮด+ partial charge?

An unequal attraction to a bonding electron pair represented by electronegativity results in unsymmetrical electron distribution across the bond. ฮด+ partial charge is formed on the side of the bond that has a lower electron density. Carbon is less electronegative than oxygen; thus, carbon has a lower electron density and a ฮด+ partial charge.

C

Which atom in a Cโ€”O bond would have a ฮด+ partial charge?

2

Draw a dot-and-cross diagram to represent an ionic bond in KBr.

When an ionic bond is formed by a metal (usually from groups 1, 2) and a non-metal (usually from groups 6, 7), electrons are transferred from the metal atoms to the non-metal atoms to form metal cations and metal anions. The electrostatic attraction between these ions creates an ionic bond, which can be depicted using a dot-and-cross diagram.

Potassium is in group 1 and it has 1 valence electron. Bromine is in group 7 and has 7 valence electrons. Thus, bromide ion has 7 dots and 1 cross (the electron from potassium).

image

Draw a dot-and-cross diagram to represent an ionic bond in KBr.

3

Which two groups of the periodic table are most cations in ionic compounds from?

Cations (positive ions) are frequently formed by metals from groups 1 and 2.

1, 2

Which two groups of the periodic table are most cations in ionic compounds from?

4

Which of the following compounds have metallic bonding within their structures?

โ‹… water
โ‹… barium
โ‹… sodium chloride
โ‹… methane
โ‹… nickel
โ‹… silicon

Metallic bonding arises from the electrostatic attraction between positive metal cations and negative delocalised electrons. The only two metals in the provided list are barium and nickel.

barium, nickel

Which of the following compounds have metallic bonding within their structures? 

โ‹… water 
โ‹… barium 
โ‹… sodium chloride 
โ‹… methane 
โ‹… nickel 
โ‹… silicon

5

Explain which of the options provided is a correct depiction of dative covalent bond between AlClโ‚ƒ and Clโป.

The dative covalent bond is usually depicted as an arrow, illustrating where the shared electrons originate from. A dative bond is usually formed in between an atom in a molecule that lacks electrons and has empty orbitals available in its outer shell, and an atom that has lone pairs of electrons in its outer shell. In this example aluminium has an empty electron orbital, while chlorine has a lone pair of electrons. Thus, the arrow would be pointing from Cl to Al.

B, the electrons in a dative bond are provided by chlorine.

Explain which of the options provided is a correct depiction of dative covalent bond between AlClโ‚ƒ and Clโป.

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