Question

The correct order of energy of $2s$-orbitals in $H, Li, Na$ and $K$, is

• A. $K < Na < Li < H$
• B. $Na < Li < K < H$
• C. $Na < K < H < Li$
• D. $H < Na < Li < K$

Answer 1

Answer: (A)

As the atomic number increases, the energy of orbital decreases. This is because the atomic radii decreases (nuclear charge increases) with increase in atomic number. The atomic number of $H, Li, Na$ and $K$ respectively, are $1, 3, 11$ and $19$. Thus, the correct order of energy of $2s$-orbitals is
$K < Na < Li < H$.