5 - Electron Arrangments in Atoms

What is the arrangement of electrons in shells of the modern model?

Electrons are found in specific energy levels known as shells surrounding the nucleus. Additionally, Shells that are further from the nucleus hold electrons with higher energy compared to those closer.

How are each shells defined?

Each shell is defined by a principal quantum number (n = 1, 2, 3...), indicating its relative distance from the nucleus

What are the shells divided into and what is the arrangement?

Shells are further divided into sub-shells, which have distinct energy levels and are labelled as s, p, d, and f.

Where are electrons located within sub - shells?

Electrons are located in orbitals, which are regions with a high probability of finding an electron.

What is an orbital?

Defined as a region around the nucleus that can accommodate up to two electrons with opposite spins

The capacity of holding electrons in the s subshell is what (according to how many orbitals) ?

1 orbital, max of 2 electrons

The capacity of holding electrons in the p subshell is what (according to how many orbitals) ?

3 orbitals, max of 6 electrons

The capacity of holding electrons in the d subshell is what (according to how many orbitals) ?

5 orbitals, max of 10 electrons

The capacity of holding electrons in the f subshell is what (according to how many orbitals) ?

7 orbitals, max of 14 electrons

What is the distribution of sub-shells across the first four shells, including the total number of electrons in each shell?

Shell 1: 1s sub-shell, total of 2 electrons

Shell 2: 2s, 2p sub-shells, total of 8 electrons

Shell 3: 3s, 3p, 3d sub-shells, total of 18 electrons

Shell 4: 4s, 4p, 4d, 4f sub-shells, total of 32 electrons

What is an atomic orbital?

A region around the nucleus that can hold up to 2 electrons with opposite spins

What is an electronic configuration and what 2 ways can it be depicted in?

Arrangement of electrons in an atom is called its electron configuration. This can be depicted in multiple ways:

• 1.) Sub-shell notation

• 2.) Electrons-in-boxes notation

What does the sub-shell notation involve and use example to explain (neon) ?

It uses superscripts to indicate the number of electrons within each sub-shell.

For example, the electron configuration of neon (10 electrons) is 1s² 2s² 2p⁶

What does the electrons-in-boxes notation involve and use example to explain (neon) ?

• Orbitals are depicted as boxes, with electrons shown as arrows.

• Oppositely directed arrows represent electrons with opposite spins.

• Electron pairing within orbitals occurs only with opposite spins.

e.g: Neon (10 electrons)

How does one determine the electron configurations i.e: how are they arranged?

• Electron configurations are arranged to minimise the overall energy of the atom or ion.

• This lowest energy arrangement corresponds to the most stable electronic structure

What is Step 1 of deducing an atom’s electron configuration?

Electrons fill the lowest energy orbitals first ( Aufbau Principle)

What is Step 2 of deducing an atom’s electron configuration?

Electrons first occupy orbitals of equal energy singly before pairing up. So, they fill all the ‘boxes’ individually and then fill up the boxes again with the leftover electrons, to form pairs. ( Hund's rule)

What is Step 3 of deducing an atom’s electron configuration?

Ensure that when two electrons occupy the same orbital, they must have opposite spins (up and down) to minimise electron-electron repulsion.

What is Step 4 of deducing an atom’s electron configuration?

Add to or remove electrons from the highest occupied sub-shell, for the s and p blocks. e.g.:

The electronic configuration of Mg²⁺ is 1s²2s²2p⁶

The electronic configuration of Cl⁻ is 1s²2s²2p⁶3s²3p⁶

How are noble gases useful when it comes to writing down electronic configurations?

Noble gas notation can simplify representations, using square brackets for the electron configuration of the preceding (previous) noble gas.

• For example, calcium's configuration (1s²2s² 2p⁶3s²3p⁶ 4s²) is abbreviated as [Ar] 4s², where [Ar] represents 1s²2s² 2p⁶ 3s² 3p⁶

What are the 2 transition metals with an unusual electronic configuration?

Chromium (Cr) and copper (Cu) exhibit unusual electron configurations

What is the electronic configuration of Cr - Chromium?

1s2 2s2 2p6 3s2 3p6 3d5 4s1 (instead of 3d4 4s2)

What is the electronic configuration of Cu - Copper?

1s2 2s2 2p6 3s2 3p6 3d10 4s1 (rather than 3d9 4s2)

Why do these exceptions occur for these 2 transition metals with an unusual electronic configuration?

These exceptions occur because configurations with a half-filled (d5) or fully filled (d10) d sub-shell are energetically more favourable.

Where does the loss of electrons occur in transition metals for them to become ions?

For transition metals forming ions, the loss of electrons happens from the 4s orbital first, then the 3d orbital

Why are electrons lost from the 4s orbital first then the 3d orbital?

Because the 4s orbital is of a lower energy than 3d

What is the exception for 3d subshell when it comes to filling up electrons?

This sub-shell is more stable when its either half full or completely full.

Orbitals of exactly the same energy are said to be what?

Degenerate