Why does electronegativity increase across a period?
Consider sodium at the beginning of period 3 and chlorine at the end (ignoring the noble gas, argon). Think of sodium chloride as if it were covalently bonded.
Both sodium and chlorine have their bonding electrons in the 3-level. The electron pair is screened from both nuclei by the 1s, 2s and 2p electrons, but the chlorine nucleus has 6 more protons in it. It is no wonder the electron pair gets dragged so far towards the chlorine that ions are formed.
Electronegativity increases across a period because the number of charges on the nucleus increases. That attracts the bonding pair of electrons more strongly.
Why does electronegativity fall as you go down a group?
Think of hydrogen fluoride and hydrogen chloride.
The bonding pair is shielded from the fluorine's nucleus only by the 1s2 electrons. In the chlorine case it is shielded by all the 1s22s22p6 electrons.
In each case there is a net pull from the centre of the fluorine or chlorine of +7. But fluorine has the bonding pair in the 2-level rather than the 3-level as it is in chlorine. If it is closer to the nucleus, the attraction is greater.
As you go down a group, electronegativity decreases because the bonding pair of electrons is increasingly distant from the attraction of the nucleus.
Diagonal relationships in the Periodic Table
What is a diagonal relationship?
At the beginning of periods 2 and 3 of the Periodic Table, there are several cases where an element at the top of one group has some similarities with an element in the next group.
Three examples are shown in the diagram below. Notice that the similarities occur in elements which are diagonal to each other - not side-by-side.
For example, boron is a non-metal with some properties rather like silicon. Unlike the rest of Group 2, beryllium has some properties resembling aluminium. And lithium has some properties which differ from the other elements in Group 1, and in some ways resembles magnesium.
There is said to be a diagonal relationship between these elements.
There are several reasons for this, but each depends on the way atomic properties like electronegativity vary around the Periodic Table.
So we will have a quick look at this with regard to electronegativity - which is probably the simplest to explain.
Explaining the diagonal relationship with regard to electronegativity
Electronegativity increases across the Periodic Table. So, for example, the electronegativities of beryllium and boron are:
So, comparing Be and Al, you find the values are (by chance) exactly the same.
The increase from Group 2 to Group 3 is offset by the fall as you go down Group 3 from boron to aluminium.
Something similar happens from lithium (1.0) to magnesium (1.2), and from boron (2.0) to silicon (1.8).
In these cases, the electronegativities aren't exactly the same, but are very close.
Similar electronegativities between the members of these diagonal pairs means that they are likely to form similar types of bonds, and that will affect their chemistry. You may well come across examples of this later on in your course.
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