TRENDS IN ELECTRONEGATIVITY OF 3rd PERIOD

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Electronegativity is a measure of the tendency of an atom to attract a shared pair of electrons towards itself.

Elements
Na
Mg
Al
SI
P
S
Cl
Ar
Electronegativity
0.93
1.13
1.81
2.02
2.19
2.58
3.16
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Note that EN of Ar is not included. Electronegativity is related to the tendency of an atom to attract the shared pair to attract the shared pair of electrons. Since Argon does not form covalent bonds, you obviously cannot assign it an electronegativity value.
Moving from one elements to the next across the period, nuclear charge increases by 1 unit and one electron is added to the outer shell. As the positive charge on the nucleus rises, the atom has increasing electron attracting power and therefore an increasing electronegativity.

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