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Trend in electronegativity of Period 3 elements

What is here?

Electronegativity increases going across Period 3.  You can:

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Table of physical data

Element

Proton number

Symbol

Electronegativity
(scale 0 to 4)

sodium

11

Na

0.9

magnesium

12

Mg

1.2

aluminium

13

Al

1.5

silicon

14

Si

1.8

phosphorus

15

P

2.1

sulphur

16

S

2.5

chlorine

17

Cl

3.0

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Graph of physical data

Graph of electronegativity values of Period 3 elements

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Explanation of this trend

Electronegativity is the relative ability of an atom to attract the bonding electron pair in a covalent bond (to withdraw electron density). It cannot be measured directly. In the Pauling electronegativity index, the most electronegative element (fluorine) is given an electronegativity value of 4. Note that there are no values for the noble gases (Group 0) as they do not usually form covalent bonds.

Find out more about electronegativity at Electronegativity Totally Explained.

Going across Period 3:

  • the nuclear charge increases ...
  • the atomic radius decreases ...
  • there are more electrons, but the increase in shielding is negligible because each extra electron enters the same principal energy level ...
  • so electrons will be more strongly attracted to the nucleus.

Remember that ionisation energies are all endothermic processes (i.e. energy is needed to remove electrons), so atoms with low electronegativities do not have a tendency to lose electrons. They just have a weaker tendency than other atoms to attract the bonding  electron pair  in a covalent bond.

In Period 3, sodium with 11 protons is the least electronegative element, and chlorine with 17 protons is the most electronegative element. You might expect argon (with 18 electrons) to be the most electronegative element in Period 3, but its outer energy levels are full. Therefore, it does not form covalent bonds with other atoms, so it is given an electronegativity value of zero.

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