Electronegativity is the measure of how an atom will attract electrons or electron density towards them. Electronegativity is often found mostly in covalent bonds where the electron density in a bond where the electrons are shared are pulled towards one of the atoms more than the other. Electronegativity is determined by the atomic number due to the influence of the number of protons in the nucleus and also the number of valence electrons. In general the larger the nucleus the more protons there are, but the more electrons an atom has the more shells and the further the valence electron from the nucleus. This means in general the larger the atom the more shells it will have this produces a shielding effect where the positive charge of the nucleus is reduced by the layers of electrons in shells.
Electronegativity was measured and developed by Linus Pauling in 1932 and he created the Pauling scale giving each element a value that corresponds to its ability to attract electrons. If we look at the example of Potassium it has a value of 0.8, a rather low electronegativity. Potassium has 19 protons in its nucleus but it also has 4 shells of electrons meaning there is a large number of electrons between the nucleus and the valence electron. When compared to Bromine an element in the same period has 35 protons but it has the same number of shells, 4. This means bromine has a much larger nuclear charge but nearly the same amount of shielding. Bromine has an electronegativity value of 2.8 which is much greater than that of potassium.
In terms of patterns and trends in general electronegativity increases across a period as the number of protons increases meaning the nucleus has a stronger charge but the number of shells stays the same. This means that whilst the nuclear charge increases it is more able to attract electrons to it in a bonded pair. Whilst in a group the electronegativity decreases down a group as whilst the nuclear charge increases the shielding effect of the electron shells also increases.
There s an exception to the rule with Gallium and Germanium that have higher electronegativities than Aluminium and Silicon this is because the d block of transition metals have small atomic radii as the 3d shell is not as effective at shielding nuclear charge.
Electronegativity has a strong link to the type of bonding in a compound. This gives rise to the bonding continuum. If the two elements in a compound have an electronegativity difference of 2.0 or more it is considered to have an ionic bond as one of the elements in the bond has enough nuclear charge to fully attract electrons in the bond creating a positive and negative ion. A common example is sodium and chlorine which have an electronegativity difference of 2.1.
Some compounds might be described as polar covalent molecules, these are compounds that have an unequal sharing of electrons where in a covalent bond the pair of electrons is closer to one of the atoms than another. For example in the molecule of water one oxygen atom is bonded to two hydrogen atoms. This covalent bond between oxygen and hydrogen is described as polar. This is because the difference in electronegativities is less than 1.6 but above 0.5. Polar covalent molecules often due to their differences in electronegativities have different charges meaning the molecule is polar.
If a molecule has an electronegativity difference between the atoms of 0.0-0.5 it is described as non polar covalent as the electrons are equally shared between the atoms. For example in a molecule of chlorine, two chlorine atoms are bonded in a diatomic molecule and they have the same electronegativity and the electrons in the covalent bond are equally shared.
The electron is the smallest sub atomic particle that make up the atom. Has a negative charge and is located in shells that orbit the nucleus
The nucleus is the term given to the centre of the atom comprising of the proton and neutron
Electronegativity is the ability of an atom to attract electrons in its bond determined by the nuclear charge, number of protons and the number of shells.
The valence electron is the outermost electron of an atom
A proton is a positive particle that makes up the atom in the nucleus with a positive charge
Linus Pauling was an American scientist who measured the tendency of atoms to attract electrons in a bond. He invented the Pauling scale
The Pauling scale is a numerical measure of the tendency of an atom to attract electrons in a bonded pair
Shielding is the effect of inner shell electrons close to the nucleus reducing the nuclear charge on the valence electron.
Nuclear charge is the attraction exerted by the nucleus on electrons due to the positive charge of the protons and negative charge of the electron
A bonded pair is the term used to describe a shared pair of electrons in a covalent bond
A polar molecule is a molecule that has an electronegativity between 0.5 and 1.6 showing that the electrons are unevenly distributed
A non polar molecules is a molecule that has an electronegativity under 0.5 meaning the electrons are evenly shared between the two atoms
A diatomic molecule is the term given to two atoms of the same element that are bonded covalently