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
$K<Na<Li<H$.