In the modern periodic table, the period indicates the value of: (a) atomic number (b) atomic mass (c) princip

Exercises · 3.33

Q. In the modern periodic table, the period indicates the value of:

(a) atomic number

(b) atomic mass

(d) azimuthal quantum number.

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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 period number in the modern periodic table corresponds to the principal quantum number ( $n$ ) of the outermost shell being filled. The correct answer is (c).

The modern periodic table is arranged by increasing atomic number, but the period (the horizontal row) tells you something deeper about the electron configuration. Each period begins when a new electron shell starts to fill. That shell is defined by its principal quantum number, $n$ .

Think of it like floors in a building. The ground floor is period 1 ( $n=1$ ), the first floor is period 2 ( $n=2$ ), and so on. The period number is the value of $n$ for the valence shell of the elements in that row.

Let’s check each option:

Atomic number (a) – This increases across a period, but it’s not what the period indicates. The period number is the same for elements with very different atomic numbers (e.g., Li with Z=3 and Ne with Z=10 are both in period 2). So this is wrong.
Atomic mass (b) – Atomic mass generally increases across a period, but it’s not a fixed value for the period. Elements in the same period have wildly different atomic masses. This is also wrong.
Principal quantum number (c) – This is correct. The principal quantum number $n$ defines the main energy level (shell). Period 1 has $n=1$ , period 2 has $n=2$ , period 3 has $n=3$ , and so on. For example, sodium (Na) is in period 3 because its outermost electron is in the $n=3$ shell. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

TG EAPCET 2021Set eng-2021-08-06-FN1 markMCQ
$Q. Which of the following is not a periodic property? (A) Atomic size (B) Electron affinity (C) Radioactivity (D) Ionisation potential$
›Reveal solutionSolution
The question asks which property is not periodic across the elements. Radioactivity is a nuclear phenomenon, not a periodic property of the elements' electronic structure. The answer is (C).

The idea behind periodic properties is simple: when elements are arranged in order of increasing atomic number, their electronic configurations repeat in a pattern. Since chemical and many physical properties depend on how electrons are arranged in shells and subshells, those properties also repeat — that's what "periodic" means. Atomic size, electron affinity, and ionisation potential all depend on the ease with which an atom gains, loses, or holds onto electrons, and these trends repeat across periods and groups.

Radioactivity, however, is a property of the nucleus — it depends on the instability of the proton-neutron ratio inside the nucleus, not on the electron arrangement. There is no regular, repeating pattern of radioactivity as you move across a period or down a group. Some heavy elements are radioactive, some light ones are not, and the distribution is not periodic.

Let's check each option:

Atomic size — This is the distance from the nucleus to the outermost electron. Across a period, atomic size decreases (more protons pull electrons in); down a group, it increases (more shells). This trend repeats for every period. So it is a periodic property.

Electron affinity — This is the energy change when an atom gains an electron. Across a period, it generally becomes more negative (easier to add an electron); down a group, it becomes less negative. The pattern repeats. So it is a periodic property. …
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Q.In the modern periodic table, the period indicates the value of:

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