Question: Write the symbol for the element which has the smallest atomic radius Na, Si, Mg, P.
Answer: In general, atomic radius decreases going across a period of the Periodic Table.
Sodium (Na), magnesium (Mg), silicon (Si) and phosphorus (P) all belong to the third period.
In order of increasing atomic number they are: Na (Z=11), Mg (Z=12), Si (Z=14), P(Z=15)
Therefore, in order of decreasing atomic radius they are: Na > Mg > Si > P
So, phosphorus, symbol P, has the smallest atomic radius.
Read more about trends in atomic radius at AUS-e-TUTE: http://www.ausetute.com.au/trendar.html