Question

In second period of the long form of the periodic table an element \(X\) has second lowest first ionization enthalpy and element \(Y\) has second highest first ionization enthalpy values. What are \(X\) and \(Y\)?

• A. B, F
• B. Be, Ne
• C. Be, O
• D. C, O

Hint:

Ionization enthalpy generally increases across a period. Noble gases have the highest ionization enthalpy values due to their stable electronic configuration.

Answer 1

The Correct Option is A

Step 1: List the second period elements.
The second period contains \[ Li, Be, B, C, N, O, F, Ne \] We must find element $X$ with the second lowest first ionization enthalpy and element $Y$ with the second highest first ionization enthalpy.
Step 2: Recall the general trend.
First ionization enthalpy broadly increases from left to right across the period because atomic size decreases and nuclear pull increases, with small dips at boron and oxygen due to electronic configuration effects.
Step 3: Find the lowest and second lowest.
Lithium has the lowest first ionization enthalpy in the period. The next lowest is boron, because removing boron's single $2p$ electron, which is shielded by the filled $2s$ pair, is relatively easy. So $X = B$.
Step 4: Find the highest and second highest.
Neon, a noble gas with a complete octet, has the highest first ionization enthalpy. The next highest is fluorine, which has a very high effective nuclear charge and a nearly complete shell. So $Y = F$.
Step 5: Combine the results.
Element $X$ is boron and element $Y$ is fluorine, giving the pair $B, F$.
Step 6: Match with options.
This corresponds to the option listing $B$ and $F$, matching the key.
\[ \boxed{B, F} \]