An element X has electron configuration 1s2 2s2 2p6 3s2 3p5. Which of the following statements is correct about the element?

1. Analyze the Question

We are given the electronic configuration of an element X: $1s^2 2s^2 2p^6 3s^2 3p^5$. We need to determine its chemical properties and position in the periodic table based on this configuration.

2. Explain the Concept

• Determining the Group: The group of an element is determined by the total number of electrons in its outermost shell (valence shell). For this element, the highest principal quantum number is 3. The electrons in the 3rd shell are $3s^2$ and $3p^5$. • Total valence electrons = $2 + 5 = 7$. • Elements with 7 valence electrons belong to Group 7 (or Group VII).
• Identifying the Element: Elements in Group 7 are known as Halogens. They are highly reactive non-metals that need one more electron to achieve a stable octet configuration (a full shell).
• Orbital Filling: A "completely filled p-orbital" would mean the p-subshell contains 6 electrons ($p^6$). Here, we have $p^5$, meaning it is one electron short of being full.

3. Show the Solution

• Step 1: Identify the valence shell. The configuration ends in $3s^2 3p^5$. The valence shell is the 3rd shell.
• Step 2: Count the valence electrons. $2 (from\ s) + 5 (from\ p) = 7$ electrons.
• Step 3: Evaluate the options: • A: Incorrect. It has 5 electrons in the p-orbital, not 6. It is one electron short of being filled. • B: Incorrect. It has 7 electrons in its outermost shell, not 5. • C: Incorrect. Elements in Group II have 2 valence electrons (ending in $ns^2$). • D: Correct. Elements with 7 valence electrons are classified as halogens.

4. Identify the Answer

Based on the analysis, the element is a halogen because it has 7 valence electrons and requires one electron to complete its octet.

5. Exam Tip

The "Octet Rule" Shortcut: For elements in the p-block (Groups 13–18), you can quickly find the group number by adding 10 to the number of valence electrons. Here, $7 + 10 = 17$. Group 17 is the Halogen group. Remember: Halogens ($F, Cl, Br, I$) always end in $ns^2 np^5$.