The modern periodic table follows increasing atomic number. Main-group elements in the same group have related outer-electron arrangements. For the examples here, period number identifies the highest occupied shell, while block identifies the subshell being filled. Across a main-group period, radius generally decreases and electronegativity increases as nuclear attraction increases within the same shell.
Down a main group, added occupied shells and shielding generally increase radius. Metals commonly form positive ions by losing electrons; non-metals often gain or share electrons. Use comparable radius definitions: ionic, covalent and metallic values describe different situations. Read supplied data before making a trend comparison, because qualitative patterns do not determine exact numerical differences.
Example 1
Locate an atom with configuration [Ne]3s23p3.
Its highest shell is 3, so period 3; five outer electrons give group 15; the last occupied subshell is p, so it is p-block.
Example 2
Why is a sodium atom generally larger than a chlorine atom?
Both use shell 3, but chlorine has a larger effective nuclear attraction across that period, drawing its electron cloud closer.
Watch out: It generally decreases across a main-group period and increases down a group.