Question

What is the trend in Atomic Radii? Which elements have a bigger Radius?

Answer 1

Down a group, the atomic radius increases. Across a period, the atomic radius decreases.
Explanation;
$1.$Down a group
THE ATOMIC RADIUS INCREASES
$\bullet$ the number of energy levels increases by $1$ as we go son a group. This makes the atom larger.
$\bullet$ An increasing number of electrons shells increases the nuclear shielding. Thus, outer e- are protected from the positive charge of the nucleus by filled inner e-shells.
$\bullet$ The nuclear charge (ie. the number of protons) does increase down a group and the nucleus become more positive. How ever, the incred nuclear shielding protects the outer e- from the increases pull.
For example, $Rb$ has a bigger atomic radius than $Na$
$2$. Across a period.
THE ATOMIC RADIUS DECREASES
$\bullet$ From $L$ to $R$, no new energy levels are added to the atom (ie. electrons are not farther away).
$\bullet$ This means that the nuclear shielding stays the same (ie. these number if filled inner e- shells stays the same from $L$ to $R$).
$\bullet$ However, the nuclear charge increases. More protons are added to the nucleus, making it more positive. As such, it is able to pull the outer e- inwards. The atom becomes smaller
For example, $C$ has a smaller atomic radius than $Li$.