# Question #70261

Nov 1, 2015

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.

Basically, these element pairs

• $\text{Li " - " Mg}$
• $\text{Be " - " Al}$
• $\text{B " - " Si}$

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.