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Exercises · 3.39

Q.Considering the elements B, C, N, F, and Si, the correct order of their non-metallic character is:

(a) B > C > Si > N > F
(b) Si > C > B > N > F
(c) F > N > C > B > Si
(d) F > N > C > Si > B
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Non-metallic character increases across a period (left to right) and decreases down a group. Among B, C, N, F, and Si, the correct order from most non-metallic to least is F > N > C > B > Si, which matches option (c).

Why This Approach Works

Non-metallic character is the tendency of an element to gain electrons and form negative ions (anions). It's the opposite of metallic character. The key periodic trends are:

  • Across a period (left to right): Non-metallic character increases. Why? As you move right, nuclear charge increases while electrons are added to the same shell. This pulls the electron cloud tighter, making it easier to attract additional electrons.
  • Down a group: Non-metallic character decreases. Why? New electron shells are added, increasing atomic size. The outer electrons are farther from the nucleus and more shielded, so the atom holds onto its own electrons less tightly and attracts new ones less strongly.

So to compare any set of elements, first locate them on the periodic table, then apply these two rules.

Step-by-Step Reasoning

1. Locate the elements on the periodic table

ElementPeriodGroup
B (Boron)213
C (Carbon)214
N (Nitrogen)215
F (Fluorine)217
Si (Silicon)314

All except Si are in Period 2. Si is directly below C in Group 14.

2. Order Period 2 elements first

For B, C, N, and F — all in Period 2 — non-metallic character increases from left to right. So:

B (least non-metallic in Period 2) < C < N < F (most non-metallic in Period 2)

This gives: F > N > C > B

3. Place Si relative to C

Si is directly below C in Group 14. Since non-metallic character decreases down a group, Si is less non-metallic than C. So:

C > Si

4. Place Si relative to B

B is in Group 13, Period 2. Si is in Group 14, Period 3. Two factors are at play here:

  • Across a period: B (Group 13) is less non-metallic than C (Group 14), and Si is in Group 14.
  • Down a group: Si is less non-metallic than C. …

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