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

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

1 Answer
anor277
Sep 4, 2016

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#.

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