(d) The electronic configuration of the element which is just above the element with atomic number 43 in the s

NCERT Exemplar · Q15

Q. (d) The electronic configuration of the element which is just above the element with atomic number 43 in the same group is ________.

(i) 1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^2

(ii) 1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^3 4p^6

(iii) 1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2

(iv) 1s^2 2s^2 2p^6 3s^2 3p^6 3d^7 4s^2

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Concept understanding — Aufbau Principle

The Aufbau Principle: Building Atoms from the Ground Up

Imagine you're building a house. You don't start with the roof — you lay the foundation first, then the walls, then the roof. Electrons in an atom behave the same way: they fill the lowest-energy "rooms" (orbitals) before moving to higher-energy ones. That's the core idea behind the Aufbau Principle.

The word Aufbau comes from German, meaning "building up" or "construction." It describes how electrons are added one by one to an atom as you move across the periodic table.

The Intuition: Why Lowest Energy First

An atom is most stable when its electrons are in the lowest possible energy states. Think of it like a ball rolling down a hill — it naturally settles at the bottom, not halfway up. If you try to put an electron into a high-energy orbital while a lower-energy orbital is empty, the atom would be unstable and the electron would "fall" down to the empty spot, releasing energy.

So nature follows the path of least energy: fill from the bottom up.

The Precise Statement

Aufbau Principle: In the ground state of an atom, electrons fill atomic orbitals in order of increasing orbital energy. Lower-energy orbitals are completely filled before electrons occupy higher-energy orbitals.

This is not a guess — it's a rule that emerges from quantum mechanics and is confirmed by experimental data (like atomic spectra and ionization energies).

The Order of Filling: The (n + l) Rule

But here's where it gets interesting: orbital energies don't simply increase with principal quantum number $n$ . For example, the 4s orbital ( $n=4, l=0$ ) actually has lower energy than the 3d orbital ( $n=3, l=2$ ). So electrons fill 4s before 3d.

To predict the correct order, use the (n + l) rule:

Tip

For an orbital with quantum numbers $n$ and $l$ , its energy increases with $(n + l)$ . If two orbitals have the same $(n + l)$ value, the one with lower $n$ fills first.

Let's check:

4s: $n + l = 4 + 0 = 4$
3d: $n + l = 3 + 2 = 5$

Since $4 < 5$ , 4s fills first. That's why potassium (atomic number 19) has the electron configuration $[\text{Ar}]\,4s^1$ , not $[\text{Ar}]\,3d^1$ .

The Complete Filling Order (Memory Aid)

Here's the sequence you'll use for most elements up to atomic number 118:

1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \rightarrow 3d \rightarrow 4p \rightarrow 5s \rightarrow 4d \rightarrow 5p \rightarrow 6s \rightarrow 4f \rightarrow 5d \rightarrow 6p \rightarrow 7s \rightarrow 5f \rightarrow 6d \rightarrow 7p

A common trick is to write the orbitals in rows by $n$ and draw diagonal arrows:

1s
2s  2p
3s  3p  3d
4s  4p  4d  4f
5s  5p  5d  5f
6s  6p  6d
7s  7p

Follow the diagonals from top-right to bottom-left — that gives you the filling order.

Important Exceptions (Don't Panic)

The Aufbau principle works beautifully for most elements, but there are a few well-known exceptions where a half-filled or fully-filled subshell provides extra stability:

Chromium ( $Z=24$ ): Expected $[\text{Ar}]\,4s^2 3d^4$ , but actual is $[\text{Ar}]\,4s^1 3d^5$ (half-filled d-subshell is more stable) …

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The element with atomic number 43 is technetium (Tc), in group 7 of the periodic table. The element just above it in the same group is manganese (Mn, Z = 25). Its electronic configuration is $1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^2$ , which corresponds to option (A).

The key here is to understand how the periodic table is arranged and how to locate an element from its atomic number. Atomic number 43 is not a common one you memorise — but you can work it out by knowing the order of filling of orbitals and the structure of the periodic table.

The periodic table is built by filling orbitals in order of increasing energy (the Aufbau principle). Each period corresponds to the filling of a particular principal quantum shell. The s-block elements fill the left two columns, the p-block the right six, the d-block the middle ten columns (groups 3–12), and the f-block sits below.

Atomic number 43 falls in the 5th period, because by the end of the 4th period (krypton, Z = 36) we have filled up to $4p^6$ . The 5th period then starts with rubidium (Z = 37) and strontium (Z = 38), followed by the 4d transition series from yttrium (Z = 39) to cadmium (Z = 48). So Z = 43 is in the middle of that 4d series.

Counting from yttrium (Z = 39, the first 4d element):

39: Y ( $4d^1 5s^2$ ), 40: Zr ( $4d^2 5s^2$ ), 41: Nb ( $4d^4 5s^1$ — an exception, but still group 5), 42: Mo ( $4d^5 5s^1$ ), 43: Tc ( $4d^5 5s^2$ ). So element 43 is technetium, in group 7.

Now, the element just above it in the same group is in the previous period — period 4, group 7. That element is manganese (Mn, Z = 25). Its electronic configuration is straightforward: it is the 3d transition series, period 4. Manganese has 25 electrons. Filling order: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^5$ . But conventionally we write the 3d before 4s when listing in order of principal quantum number: $1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^2$ .

Let's check the options:

Option (A): $1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^2$ — this is exactly manganese. Total electrons = $2+2+6+2+6+5+2 = 25$ . Correct. …

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Q.(d) The electronic configuration of the element which is just above the element with atomic number 43 in the same group is ________.

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