Electronegativity and Bonding

A covalent bond has a shared pair of electrons. In $H_{2}$, the bonded atoms are identical, with each atom having an equal share of the electrons. $H-H$ is non-polar, due to this equal distribution of charge.

When the bonded atoms are different, one of the atoms is more likely to attract the electrons. The bonded atom with the greater attraction for the electrons is said to be more electronegative.

Electronegativity

Electronegativity is a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond.

In a $HCl$ molecule, the $Cl$ atom is more electronegative meaning it has a greater attraction to the bonded electron pair. This causes a permanent dipole.

Permanent Dipole

A permanent dipole is a small charge difference across a bond due to a difference in electronegativity of the bonded atoms. In $HCl$, the permanent dipole is shown by:

Polar Molecules

Non-symmetrical molecules such as $HCl$ are polar molecules due to the polar bonds. For symmetrical molecules however any dipoles may cancel out meaning it can be non-polar overall. $CCl_{4}$ is an example of this as each $C-Cl$ bond is polar however the molecule is symmetrical.

Electronegativity is measured using the Pauling Scale. The higher the electronegativity, the higher the value on the Pauling Scale. The electronegativity of an element can be predicted from the position on the periodic table:

Bonding Type

In a bond where the atoms have a slight difference in electroneagtivity, one of the atom attracts the atom slightly more than the other resulting in a polar covalent bond.

In a bond where the atoms have a large difference in electronegativity, one of the atoms would have captured the electrons forming an ionic bond.

Intermolecular Forces

Unlike ionic and covalent bonds, intermolecular bonds act between molecules. Intermolecular bonds are formed by small dipoles. There are three types of intermolecular forces:

Permanent dipole-dipole interactions

A permanent dipole-dipole is a weak attractive force between permanent dipoles in neighbouring polar molecules.

$$ H^{\delta +}-Cl^{\delta - } --------- H^{\delta +}-Cl^{\delta -} $$

Van der Waals Forces

Van der Waals forces are induced dipoles between neighbouring molecules. They exist between all simple covalent molecules.

These forces are caused by the movement of electrons. This movement generates an instantaneous dipole. This induced dipoles induces more dipoles in neighbouring molecules, which results in the formation of van der Waals' forces.

Van der Waals forces increase with the number of electrons. The stronger the van der Waals forces, the higher the boiling point of the molecules.

Hydrogen Bonding

Molecules containing either $O-H$ or $N-H$ bonds are polar with strong permanent dipoles. These permanent dipole-dipole interactions are called hydrogen bonds. For example, in water each oxygen has four bonds: two covalent bonds and two hydrogen bonds. The slightly negative $O^{\delta -}$ form bonds with the slightly positive $H^{\delta +}$.

hydrogen_bond

These hydrogen bonds give two unexpected properties of water:

Metallic Bonding

The atoms in a solid metal are held together by metallic bonding. In metallic bonding, positive ions form an electrostatic attraction with electrons in a sea of delocalised electrons. Some of the properties are shown below:

Giant Ionic Lattice

Ionic bonds are formed by the transfer of electrons between a non metal and a metal. In a giant ionic lattice each ion is surrounded by oppositely charged ions. These ions attract each other forming a giant ionic lattice. Some of the properties are shown below:

The strength of the ionic lattice depends on the charge between the atoms. For example the electrostatic attraction between $Mg^{2+}$ and $O^{2-}$ is greater than between $Na^{+}$ and $Cl^{-}$.

Covalent Structures

Elements with covalent structures can have either of two structures:

Simple molecular lattice

These structures are made up from small, simple molecules such as $O_{2}$ and $N_{2}$. Molecules are held together through weak van der Waals' forces. The properties of simple molecular lattices are shown below:

Giant Covalent Structures

This is where atoms are held together by strong covalent bonds in a lattice. Some of the properties are shown below:

Properties of Diamond

Diamond has a tetrahedral structure held together by strong covalent bonds throughout the structure which makes diamond hard. It is poor conductor of electricity however, as there are no delocalised electrons.

Properties of graphite

Graphite has a strong hexagonal layer structure but with weak van der Waals' forces between the layers. This makes it a good conductor of electricity as delocalised electrons are between the layers. Graphite is also soft as the layers are able to slide above each other.

© Andrew Deniszczyc, 2018