Electronegativity is the power of an atom to attract the electron density in a covalent bond towards itself.
Electronegativity depends on:
- the nuclear charge
- the atomic radii (the distance between the nucleus and the outer shell electrons)
- the shielding of the nuclear charge by electrons in inner shell
Trends in electronegativity
Across a period:
- electronegativity increases
- due to an increase in nuclear charge (more protons)
Down a group:
- electronegativity decreases
- due to an increase in atomic radii
- due to a greater shielding by electrons in inner shells (because there are more shells)
Polar Covalent Bonds
Polar covalent bonds are bonds between atoms in which the electronegativity varies.
The atom with the larger electronegative value attracts the electrons towards it more than the atom with the smaller value, and therefore the electrons move nearer to that atom. Due to this movement, that atom has is partially negative and the other atom is partially positive.
Covalent AND ionic bonds
A bond can be both covalent and ionic - well, partly anyway...
If there is a large difference between the two atoms electronegative value, the electrons will move much closer to the atom with the larger electromagnetic value than to the atom with the smaller value. In fact, the electrons will be so much more attracted to the atom with the larger value that it's almost as though the atom with the smaller value has given the other atom its electron (as they do in ionic bonds).
Non-polar covalent bonds
A non-polar covalent bond occurs when the electronegativity of the two atoms is the same.
'Polar' is when the electrons are more attracted to one atom than to the other, so 'non-polar' is when the electrons are attracted equally to both atoms.
This is what happens when two atoms of the same element form a covalent bond e.g. H2, but it also happens with atoms that are of different elements but have the same electronegativity.
Examples of polar and non-polar covalent bonds
From this, you can see that elements such as beryllium and aluminium have the same electronegative value, even though they are not the same element, so they would form a non-polar covalent bond. However, magnesium and fluorine have very different electronegative values. As fluorine has the higher value, the electrons will be more attracted to the fluorine atom. The fluorine atom will have a partially negative charge, and the magnesium atom will have a partially positive charge.