Quantum theory and electric configuration

What is meant by a quantum shell?

A main energy level where electrons are likely to be found, represented by the principal quantum number n.

What is the principal quantum number (n)?

A number indicating the main energy level or shell occupied by an electron.

What happens to the energy of an electron as n increases?

It increases and the electron is farther from the nucleus.

What are subshells?

Divisions within a main shell (s, p, d, f) that contain orbitals of slightly different energies.

What is an orbital?

A region of space around the nucleus where the probability of finding an electron is maximum.

How many electrons can one orbital hold?

Two electrons with opposite spins.

What is the shape of an s orbital?

Spherical.

What is the shape of a p orbital?

Dumbbell-shaped.

How many p orbitals are there in each p subshell?

Three (px, py, pz).

What is the maximum number of electrons in an s subshell?

2.

What is the maximum number of electrons in a p subshell?

6.

What is the maximum number of electrons in a d subshell?

10.

What is the maximum number of electrons in an f subshell?

14.

How many orbitals are there in each subshell?

s = 1, p = 3, d = 5, f = 7.

How many electrons can the first four main energy levels hold?

2, 8, 18, 32 respectively.

What is meant by the term ‘electron configuration’?

The arrangement of electrons in shells, subshells, and orbitals around the nucleus.

What is the ground state of an atom?

The lowest energy arrangement of electrons in an atom.

What does the Aufbau principle state?

Electrons fill the lowest energy orbitals first before moving to higher ones.

What is Hund’s rule?

Electrons occupy orbitals singly before pairing, to minimise repulsion.

What is the Pauli exclusion principle?

No two electrons in the same atom can have the same set of four quantum numbers.

What does ‘opposite spin’ mean?

One electron spins clockwise and the other anticlockwise in the same orbital, reducing repulsion.

What is the order of filling of orbitals up to krypton (Z=36)?

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.

Why is 4s filled before 3d?

Because the 4s orbital has slightly lower energy before filling begins.

Why do 3d electrons lose energy before 4s in transition metals?

After filling, 4s becomes higher in energy, so electrons are lost from 4s first.

What is the shorthand electron configuration?

It uses the symbol of the previous noble gas in brackets followed by remaining subshells.

Give the shorthand configuration of sodium (Z=11).

[Ne] 3s¹.

Give the shorthand configuration of calcium (Z=20).

[Ar] 4s².

Give the shorthand configuration of iron (Z=26).

[Ar] 3d⁶ 4s².

Why do chromium (Cr) and copper (Cu) have irregular configurations?

They gain extra stability from half-filled (Cr) or fully filled (Cu) 3d orbitals.

What is the electron configuration of chromium?

[Ar] 3d⁵ 4s¹.

What is the electron configuration of copper?

[Ar] 3d¹⁰ 4s¹.

What is the configuration of Cr³⁺?

[Ar] 3d³ (4s electrons and some 3d electrons are removed first).

What is the configuration of Fe²⁺?

[Ar] 3d⁶.

What is the configuration of Fe³⁺?

[Ar] 3d⁵.

What is the configuration of Zn²⁺?

[Ar] 3d¹⁰.

What are the four quantum numbers?

Principal (n), angular momentum (l), magnetic (mₗ), and spin (mₛ).

What does the angular momentum quantum number (l) describe?

The shape of the orbital (s=0, p=1, d=2, f=3).

What does the magnetic quantum number (mₗ) describe?

The orientation of the orbital in space.

What does the spin quantum number (mₛ) describe?

The direction of electron spin (+½ or –½).

How many orbitals are in the 3p subshell?

3 orbitals.

How many orbitals are in the 3d subshell?

5 orbitals.

How many electrons can the third shell hold?

18.

How many orbitals are in the n = 3 shell?

9 orbitals (1 from 3s, 3 from 3p, 5 from 3d).

What does it mean for orbitals to be degenerate?

They have the same energy (e.g., the three p orbitals in a subshell).

What is meant by electron promotion?

An electron moves to a higher energy orbital when energy is absorbed.

How is energy absorbed or emitted by electrons?

When electrons move between energy levels; absorbed when going up, emitted when falling.

What does the emission spectrum of hydrogen show?

Discrete lines corresponding to transitions between energy levels.

What does a line in an emission spectrum represent?

A specific energy change as an electron falls between levels.

Why is the hydrogen emission spectrum evidence for energy levels?

Because only certain frequencies (energies) are emitted, showing quantised levels.

What happens to the spacing between energy levels as n increases?

Levels get closer together.

What are orbitals within the same subshell like in energy?

They are degenerate (equal energy).

Why does the 4s orbital fill before 3d but empty first?

4s has slightly lower energy initially, but becomes higher once 3d is occupied.

What is the maximum number of electrons in a p orbital?

2.

What is the maximum number of electrons in the p subshell?

6.

What is the total number of orbitals in the second energy level?

4 orbitals (1 s + 3 p).

Explain how the arrangement of electrons is related to an element’s position in the periodic table.

Period number = number of shells; Group number = number of outer electrons for main group elements.

What is meant by a transition element?

A d-block element that forms at least one stable ion with a partially filled d subshell.

Why are transition metals coloured?

Because d–d electron transitions absorb certain wavelengths of visible light.

Why are Zn and Sc not coloured transition metals?

Zn²⁺ has a full 3d¹⁰ subshell; Sc³⁺ has an empty 3d subshell, so no d–d transitions occur.