A covalent bond is usually formed when 2 non-metals atoms join and share a pair of electrons EQUALLY between them. You should be familiar with covalent bonds from your gcse chemistry course. The diagram below shows a covalent bond formed between 2 atoms of hydrogen. You should recall that the two electrons in the covalent bond between the hydrogen atoms are attracted to each of the positively charged hydrogen nuclei. The two electrons will feel the same positive attractive force from each nucleus since both nuclei contain 1 positively charged proton. This means that that the two electrons are shared equally. You will no doubt have learned from your gcse course that a covalent bond involves the equal sharing of a pair of electrons.
However what happens if the molecule contains different atoms? Will the electrons in the covalent bond still be shared equally this time? Consider as an example a molecule of hydrogen fluoride.
The fluorine atom has a much larger nuclear charge (+9) compared the the (+1) charge
in the hydrogen nucleus, so the fluorine atom will be better at attracting the electrons in the
covalent bond than hydrogen atom ; we say that fluorine
more electronegative than the hydrogen atom. Electronegativity
is the ability of an atom in a covalent bond to attract
electron density towards it.
|Metal||Electron arrangement||Number of shielding electrons (all electrons except those in the last shell)||Effective nuclear charge|
Small atoms with large effective nuclear charges
have large electronegativity values. The most
in the periodic table is fluorine while the least electronegative
element is francium. The image above illustrates
the trends in electronegativity across the periodic table.
As we descend a group the atomic radius of the atom
increases while the effective nuclear charge remains constant;
this means that electronegativity
decreases down a
group in the periodic table. As we cross a period in the periodic table the atomic radius
decreases while the
effective nuclear charge increases; this means that the electronegativity increases across a period. The alkali
metals in group 1 of the periodic table have low electronegativity values whereas the halogens in group 7 have
higher electronegativity values.
Electron affinity is the ability of an isolated atom to attract an electron and ionisation energy is a measure of how easy or hard it is to remove an electron from an atom. So one of the ways to measuring electronegativity is to take an average of the electron affinity and ionisation energy. A scale can be produced from this and a number assigned to each element to represent its electronegativity (note the scale or number has no units). On this scale the most electronegative element fluorine is assigned a value of 4 and all other elements electronegativity values are measured relative to this. The table below lists some values of electronegativity for the elements you are likely to meet. You can see that N,O and F are the most electronegative elements in the periodic table. It is also clear to see the trends in the electronegativity values across a period and down a group in the periodic table.
It is worth mentioning that if you look online or in textbooks you may see slightly different values of electronegativity for some elements; this is simply due to the way in which it is measures. However the same patterns or trends should be obvious no matter which method is chosen.
There are no values listed for the noble gases; this is simply because they rarely form covalent compounds and have no affinity for electrons.