Question

re - order each list in the table below, if necessary, so that the atoms or ions

atoms or ions	atoms or ions in order of decreasing size
br, br⁻, cl	, ,
ar, na, al	, ,

Explanation:

Step1: Consider atomic and ionic size trends

Atomic size decreases across a period from left - to - right due to increasing nuclear charge. Cations are smaller than their parent atoms as they lose electrons. Anions are larger than their parent atoms as they gain electrons.

Step2: Analyze the first set ($Na$, $Mg^+$, $Mg$)

$Na$ is in group 1 and period 3, $Mg$ is in group 2 and period 3. $Mg^+$ is a cation of $Mg$. $Na$ has a larger atomic radius than $Mg$ because atomic radius decreases across a period. $Mg^+$ is smaller than $Mg$ because it has lost an electron. So the order is $Na$, $Mg$, $Mg^+$.

Step3: Analyze the second set ($Br$, $Br^-$, $Cl$)

$Br$ and $Cl$ are in the same group (group 17), with $Br$ below $Cl$. Atomic radius increases down a group. $Br^-$ is an anion of $Br$, so it is larger than $Br$. The order is $Br^-$, $Br$, $Cl$.

Step4: Analyze the third set ($Ar$, $Na$, $Al$)

$Na$, $Al$, and $Ar$ are in the same period (period 3). Atomic radius decreases across a period. So the order is $Na$, $Al$, $Ar$.

Answer:

$Na$, $Mg$, $Mg^+$
$Br^-$, $Br$, $Cl$
$Na$, $Al$, $Ar$