The group number, number of valence electrons and valency of an element with atomic number 15, respe

Q: Why do we add 10 to the valence electrons to find the group number?

In the modern periodic table, the $p$-block starts after the $s$-block (Groups 1 and 2) and the $d$-block (Groups 3 to 12). So, to find the correct group number for a $p$-block element, we must account for those 10 $d$-block groups by adding 10 to the number of valence electrons.

Q: Why is the valency 3 and not 5?

Valency is the number of electrons an atom needs to gain, lose, or share to achieve a stable octet. With 5 valence electrons, it is energetically much easier to gain 3 electrons ($8 - 5 = 3$) than to lose all 5. However, Phosphorus can exhibit a covalency of 5 in certain compounds by expanding its octet.

Q: How do I know if an element is in the s, p, d, or f block?

Look at the electronic configuration. The block is determined by the subshell into which the *last* electron enters. For $Z=15$, the configuration ends in $3p^3$, meaning the last electron enters a $p$-orbital, making it a $p$-block element.

Q: Is valency the same as oxidation state?

Not exactly. Valency is strictly the combining capacity and is always a positive whole number (e.g., 3). Oxidation state is the hypothetical charge an atom would have if all bonds were 100% ionic, and it can be positive, negative, or zero (e.g., -3, +3, +5 for Phosphorus).

Q: Could the configuration be written as $[Ne] 3s^2 3p^3$?

Absolutely! Using the noble gas core is a great shortcut. Neon ($Ne$) has an atomic number of 10, so $[Ne]$ represents the $1s^2 2s^2 2p^6$ part. This makes it even easier to spot the $3s^2 3p^3$ valence shell.

Question

The group number, number of valence electrons and valency of an element with atomic number 15, respectively, are

Options:

(A) 16, 5 and 2

(B) 15, 5 and 3

(C) 16, 6 and 3

(D) 15, 6 and 2

Sigma Percentile · Accepted Answer

Imagine you are an atomic detective, and you've just been handed a mysterious element with an atomic number of $Z = 15$. Your mission? To uncover its group number, its valence electrons, and its valency. Let's break down this atomic puzzle step by step.

### Decoding the Electronic Configuration

The atomic number tells us exactly how many protons and electrons this neutral atom has. With 15 electrons to place, we follow the Aufbau principle, filling the lowest energy orbitals first.

The electronic configuration unfolds as: $1s^2 2s^2 2p^6 3s^2 3p^3$.

Notice the principal quantum numbers. The innermost shell ($n=1$) holds 2 electrons, the second shell ($n=2$) holds 8, and the outermost shell ($n=3$) holds the remaining 5 electrons. This outermost shell is our **valence shell**, the frontier where all the chemical magic happens.

### Counting the Valence Electrons

Valence electrons are simply the electrons residing in the outermost shell. For our element, the outermost shell is the third shell ($n=3$).

If we look closely at the configuration $3s^2 3p^3$, we can see there are 2 electrons in the $s$-orbital and 3 electrons in the $p$-orbital. Adding them up, we get a total of **$5$ valence electrons**.

### Pinpointing the Group Number

The periodic table is beautifully structured. Because the last electron of our element enters a $p$-orbital, it belongs to the **$p$-block** of the periodic table.

For $p$-block elements, the group number is calculated by adding 10 (representing the intervening $d$-block elements) to the number of valence electrons.

Therefore, the Group Number is $10 + 5 = \mathbf{15}$. Our mystery element is Phosphorus, sitting proudly in Group 15!

### Determining the Valency

Valency is an element's combining capacity—how many bonds it wants to form to achieve a stable, noble-gas-like octet.

With 5 valence electrons, Phosphorus has a choice: lose all 5 (which requires immense energy) or simply gain 3 more to reach the magic number of 8. Energetically, gaining 3 is much easier.

Thus, its valency is calculated as $8 - 5 = \mathbf{3}$.

### The Final Verdict

Putting all our clues together, we have:
- **Group Number:** 15
- **Valence Electrons:** 5
- **Valency:** 3

This perfectly aligns with option (b). But remember, chemistry always has a twist! While its normal valency is 3, Phosphorus can expand its octet using empty $3d$ orbitals to show a valency of 5 (as seen in $PCl_5$). Always keep your eyes open for these fascinating exceptions!

Animated Solution for Chemistry - Periodicity in Properties: The group number, number of valence electrons and valency of an element with atomic number 15, respectively, are

Select Answer:

Visualized Solution

$Z = 15$

$Electronic Configuration$

$Valence Electrons$

$Group Number$

$Valency$

$Final Answer$

$The Way Forward$

The Sigma Insight: Periodic Table and Periodic Properties

Decoding the Electronic Configuration

Counting the Valence Electrons

Pinpointing the Group Number

Determining the Valency

The Final Verdict

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