Question #70261

1 Answer
Nov 1, 2015

Answer:

Because that's what the electronegativity trends lead to.

Explanation:

As you know the periodic trends in electronegativity can be described as

  • electronegativity increases as you move from left to right across a period
  • electronegativity decreases as you move down a group

These trends make a special relationship, called the diagonal relationship, be possible for some elements from periods 2 and 3.

http://www.chemguide.co.uk/atoms/bonding/electroneg.html

Basically, these element pairs

  • #"Li " - " Mg"#
  • #"Be " - " Al"#
  • #"B " - " Si"#

have very similar properties, including electronegativity values. That happens because the two electronegativity trends compete against each other.

In your case, beyllium is located in period 2 and aluminium in period 3.

This means that if you go by the way electronegativity changes down a group, beryllium would be expected to have a higher electronegativity value.

However, beryllium is also located in group 2 and aluminium in group 3.

This means that if you go by the way electronegativity changes across period, aluminium would be expected to have a higher electronegativity value.

So which trend wins?

In these cases, neither one, and so the two atoms have very similar electronegativity values.