Topic 1.6 — Electrons in energy levels

What are electrons arranged in?

Electrons are arranged in energy levels, also called quantum shells.

how are the electrons shells numbered?

The shells are numbered 1, 2, 3, 4, etc., starting with the shell closest to the nucleus.

What are the maximum number of electrons in the first four shells?

The maximum numbers of electrons in the first four shells are 2, 8, 18 and 32, respectively.

What is each main shell divided into?

Each main shell is divided into subshells labelled s, p, d and f, where available.

What does the first shell contain?

The first shell contains only an s subshell.

What does the second shell contain?

The second shell contains s and p subshells.

what does the third shell contain?

The third shell contains s, p and d subshells.

What does the fourth shell contain?

The fourth shell contains s, p, d and f subshells.

What are subshells divided into?

Subshells are divided into atomic orbitals.

Define an orbital

An orbital is a region within an atom in which there is a 95% probability of finding an electron.

How many electrons can each orbital hold?

Each orbital can hold a maximum of two electrons, which must have opposite spins.

What is in orbital defined by?

An orbital is defined by its energy, shape and direction in space.

What is the general shape of an s orbital?

s orbital

• Spherical shape.

• One orbital per s subshell.

• Maximum of 2 electrons.

What is the general shape of a p orbital?

p orbitals

• Dumbbell-shaped.

• Three orbitals, at right angles to one another.

• Maximum of 6 electrons.

What are the sub shells and corresponding number of orbitals and electrons?

Subshell	Number of orbitals	Maximum electrons
s	1	2
p	3	6
d	5	10
f	7	14

What is the aufbau principle?

Electrons fill the orbital with the lowest available energy first.

This means electrons occupy lower-energy orbitals before higher-energy orbitals.

The filling order you need is:

1s → 2s → 2p → 3s → 3p → 4s → 3d

What is the Pauli exclusion principle?

Each orbital can hold a maximum of two electrons, and if two electrons occupy the same orbital, they must have opposite spins.

In electrons-in-boxes diagrams, this is represented by one upward arrow and one downward arrow.

What is hunds rule?

Where two or more orbitals have the same energy, electrons occupy them singly before pairing up.

Define electronic configuration

An electronic configuration describes the number and arrangement of electrons in an atom or ion.

What are the two main ways to represent electrons?

A. 1s notation

B. Electrons-in-boxes notation

How does 1s notation work?

The number before the letter indicates the shell, the letter indicates the subshell, and the superscript indicates the number of electrons in that subshell.

How does boxes notation work?

• Each box represents one orbital.

• Each arrow represents one electron.

• Opposite arrows represent electrons with opposite spins.

• Electrons occupy lower-energy orbitals first.

• Orbitals of equal energy fill singly before pairing.

How does the noble gas shorthand work?

A shortened electronic configuration uses the symbol of the previous noble gas in square brackets to represent the inner electrons.

How does atomic emission spectra provide evidence for quantum shells?

• Electrons can absorb energy and move to higher energy levels.

• When they return to lower energy levels, they emit energy as electromagnetic radiation.

• Only certain energies are emitted, producing discrete lines in an emission spectrum.

• These discrete lines provide evidence that electrons occupy specific, quantised energy levels rather than having any energy value.

Electrons can only occupy certain quantised energy levels. When electrons move from higher to lower energy levels, they emit photons with specific energies. This produces discrete lines in an emission spectrum, providing evidence for quantum shells.

How do successive ionisation energies provide evidence for quantum shells?

• Successive ionisation energies generally increase because each successive electron is removed from an increasingly positive ion.

• A particularly large jump occurs when an electron is removed from an inner shell rather than the outer shell.

• This provides evidence that electrons occupy different shells with different energies.

• The position of the large jump can help identify the number of electrons in the outer shell and the element's group.

• A large jump occurs when removal starts from an inner shell rather than the outer shell. This is because the inner-shell electrons experience greater nuclear attraction and less shielding.

How do first ionisation energies provide evidence for sub shells?

• First ionisation energies generally increase across a period as nuclear charge increases.

• However, there are small decreases, such as from nitrogen to oxygen and from phosphorus to sulfur.

• These decreases provide evidence that electrons occupy different subshells and experience different repulsions.