(e) The elements with atomic numbers 35, 53 and 85 are all ____. (i) noble gases (ii) halogens (iii) heavy met

NCERT Exemplar · Q16

Q. (e) The elements with atomic numbers 35, 53 and 85 are all ________.

(i) noble gases

(ii) halogens

(iii) heavy metals

(iv) light metals

Yanam BieapMCQ· 1mImportance★★★★★est

61% · 66/109 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

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) …

Start your 14-day free trial to unlock the full solution →

Elements with atomic numbers 35, 53, and 85 all have 7 valence electrons, placing them in Group 17 of the periodic table, which are known as halogens. The correct option is (B).

The identity and chemical properties of an element are fundamentally determined by its atomic number, which represents the number of protons in its nucleus. In a neutral atom, the number of electrons equals the number of protons. The arrangement of these electrons, particularly the valence electrons (those in the outermost shell), dictates how an atom will interact with other atoms and thus determines its chemical behavior and its position in the periodic table. Elements in the same group (vertical column) of the periodic table share similar valence electron configurations and, consequently, similar chemical properties.

To identify the type of elements with atomic numbers 35, 53, and 85, we need to determine their electron configurations and identify their respective groups in the periodic table.

Element with atomic number $Z = 35$ :

This element is Bromine (Br). Its electron configuration can be written using the noble gas core notation. The noble gas preceding Bromine is Argon (Ar) with $Z = 18$ .

The electron configuration for Bromine is $[Ar] 3d^{10} 4s^2 4p^5$ .

The outermost shell is the 4th shell, containing $4s^2 4p^5$ electrons. This means it has $2 + 5 = 7$ valence electrons.

Elements with 7 valence electrons in their $s$ and $p$ orbitals belong to Group 17 (or VIIA) of the periodic table.
Element with atomic number $Z = 53$ :

This element is Iodine (I). The noble gas preceding Iodine is Krypton (Kr) with $Z = 36$ .

The electron configuration for Iodine is $[Kr] 4d^{10} 5s^2 5p^5$ .

The outermost shell is the 5th shell, containing $5s^2 5p^5$ electrons. This means it has $2 + 5 = 7$ valence electrons.

Like Bromine, Iodine also belongs to Group 17.
Element with atomic number $Z = 85$ : …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ15 — (d) The electronic configuration of the element which is just above th…→Next questionQ17 — Electronic configurations of four elements A, B, C and D are given bel…

Q.(e) The elements with atomic numbers 35, 53 and 85 are all ________.

🎓Unlock everything free for 14 days

Unlock everything free for 14 days