order each of the following sets of five atoms according to decreasing size?

Question

1- H
2- C
3- Al
4- B
5- Mg

Answer 1

Look at their atomic mass on the periodic table, the larger the mass, the larger the atom.

Al - 26.98
Mg - 24.31
C - 12.01
B - 10.81
H - 1.01

Answer 2

The trick to answering this is to realise that atomic radius goes down going from left to right in the periodic table.
So Mg is the biggest, then Al.
Similarly, we then have B bigger than C.
And H is the smallest.

Answer 3

Al
Mg
C
B
H