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Bonding for SQA Higher Chemistry

Bonding

This page covers the following topics:

1. Ionic bonding
2. Covalent bonding
3. Polar covalent bonds

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

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

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

A bond between a silicon atom and a nitrogen atom is shown in the diagram. Should silicon be X or by Y? Explain your answer.

An unequal attraction to a bonding electron pair represented by electronegativity results in unsymmetrical electron distribution across the bond. Silicon is less electronegative than nitrogen; thus, silicon has a lower electron density and is represented by Y.

Y, silicon is less electronegative than nitrogen and has a lower electron density in a bond.

A bond between a silicon atom and a nitrogen atom is shown in the diagram. Should silicon be X or by Y? Explain your answer.

5

Explain what holds ions in an ionic compound together.

When an ionic bond is formed between cations and anions, the electrostatic attraction between these ions creates an ionic bond that holds the ions together.

electrostatic attraction between ions

Explain what holds ions in an ionic compound together.

End of page

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