Question

Which of the following atoms will have the largest and the smallest size?

`Mg`
`Mg^(+2)`
`Al`
`Al^(+3)`

Answer 1

Atomic size DECREASES across a Period (left to right as we face the Table) and INCREASES down a Group.

Explanation:

So, with respect to the atoms, aluminum (`Z=13`) is SMALLER then magnesium (`Z=12`).

With respect to the ions, both `Al^(3+)` and `Mg^(2+)` have a Noble Gas configuration (cf. `Ne`, `Z=10`). Because aluminum ion is tripositive, its ionic radius should be smaller than that of `Mg^(2+)`, because the excess nuclear charge should compress the valence radii.

So we've made a prediction. Our job is not finished as physical scientists; we need data on the ionic radii. You will find this on the web; suitable units would be `"pm"`, `"1 pm "=10^-12m`.