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Discovering chemistry
Discovering chemistry

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3.1 Electronegativity and the periodic table

In the last section you considered two elements, which clearly had markedly different electronegativities; chlorine, near the end of period 3, has a greater electronegativity than sodium, at the beginning.

This contrast applies generally: the electronegativities of atoms increases across a period of the periodic table; in addition electronegativities also increase up a group from the bottom to the top. This is illustrated in Figure 4. To keep things simple, only a portion of the periodic table is shown here.

This image demonstrates variation in electronegativity over the periodic table.
Figure 4 Variation in electronegativity over the periodic table
  • Account for the variation in electronegativity shown in Figure 4. (Hint: think back to what you know about the structure of atoms).

  • Across a period of the periodic table, the atomic number, or positive charge on the nucleus, increases. This increases the attraction of the outer electrons to the nucleus, so the electronegativity of the elements also increases.

The principle quantum number of the outer electrons decreases from the bottom to the top of the group. This means that they get closer to the positively charged nucleus. The result is, again, that the outer electrons are attracted more strongly, and the electronegativity increases.

  • Suggest why the noble gases have been omitted from Figure 4

  • These are completely unreactive, in fact at normal temperatures helium, neon and argon form no compounds; hence electronegativities are not assigned to them.

So taking everything into account, top of the league table for electronegativity is fluorine, followed by oxygen and chlorine. The three most electronegative elements are shown on a green background in Figure 4.

But how does electronegativity impact on bonding?

This will be considered in the next section.