# Electronic structure: Subshells

### Electronic structure: Subshells

#### Lessons

In this lesson, we will learn:
• To understand energy level diagrams of electron shells.
• How to construct energy level diagrams using electron subshells.
• How to write electron configurations using electron subshell notation.
• How to write electron configurations using noble gas or 'core' notation.
• How an element’s electron configuration relates to its chemical properties.

Notes:
• In Electronic Configuration 1 a basic electron structure was introduced with the 2-8-8 rule. This is useful only for the first 3 rows of the periodic table before the transition metals and it doesn’t look at electron subshells which we need for later elements.

• Evidence that electrons exist in quantum shells comes from the results of atomic emission spectroscopy. When they’re heated or electrified, samples of elements like hydrogen give off photons that always have a fixed wavelength – we call these its spectral lines and they act like a fingerprint for that element. These precise wavelengths and energies are evidence that the electrons shifted from a state of one specific amount of energy (let’s call it n=2) to another state with another specific amount of energy (let’s call it n=1). The important point is that the photons have fixed wavelength – the specific amount of, or quantized energy, is why they are quantum shells. Ionization energies (see Periodicity: Ionization energies) are also evidence of quantum shells and subshells because of the distinct gaps in energy required at regular intervals as you progress through the periodic table in order of atomic number.

• Each energy level (shell) has orbitals (sub-shells). An orbital is a region of space which can hold up to two electrons each, and there are a few types of orbitals:

• For the first energy level (n=1), one s-orbital exists which contains up to 2 electrons. S orbitals are spherical shaped; think “s for sphere”.

• In the second energy level (n=2), three p-orbitals also exist, which can contain up to 6 electrons. P orbitals are lobe shaped; each p orbital is two lobes running along an axis in the opposite direction. With three of these and one s orbital, the second energy level holds a total of 8 electrons.

• In the third energy level (n=3), five d-orbitals also exist which can contain up to 10 electrons. This gives the third energy level a total of 18 electrons.

• In the fourth energy level (n=4), seven f-orbitals also exist, but you won’t need to deal with f-block elements here.

• IMPORTANT: “Electron shells” don’t fill electrons in exactly this order, they fill with electrons according to the energy level diagram, explained below.

• The electron configuration in an atom of any element can be shown using this information in two ways:

• You can use an energy level diagram, just by filling in the labelled orbitals from lowest to highest. The energy level diagram is shown below. It reflects a few pieces of IMPORTANT information that is essential to writing electron configurations correctly:

• An s orbital (say, 3s) is lower energy than a same-level p orbital (3p), which is lower energy than a d-orbital (3d) which is lower than an f orbital.

• Electrons always fill lower energy subshells before higher energy subshells. For example, 2s will always fill before 2p. This is known as the Aufbau principle.

• Electrons must fill orbitals singly first and only pair up after this. They must pair up with ‘opposite spin’; one pointing up, one pointing down.

• The gaps between increasing energy levels get ever smaller, which is why 3d is higher energy and fills up after 4s, which is why it is the 2-8-8 rule and not 2-8-18!

• Up to including the f-orbitals, the shells fill with the following number of electrons: 2 (1s); 8 (2s, 2p); 8 (3s, 3p); 18 (4s, 3d, 4p); 18 (5s, 4d, 5p); 32 (6s, 4f, 5d, 6p).

• You can show electron configuration by writing in subshell notation (below).

• For atoms with more than one electron (called 'polyelectronic'), the energy levels and the sub shells need to be filled in order of the energy level diagram. Visually, this is filling the subshells on the ELD from bottom to top. This is because electrons always fill the lowest energy orbital available first.

• Subshell notation is written in the format: nx$^y$ where n is the energy level, x is the subshell and y is the number of electrons in that subshell.
• For example: 2p4 would mean there are four electrons in the 2nd level p-subshell.

• To write the configuration for ions, see whether the ion is negative (it has gained an electron) or positive (it has lost an electron) and add or remove electrons as needed. If a subshell was the last to fill up, it is the first to empty! The only exception to this is with the 3d subshell.

• There is another way to write electron configuration, known as noble gas notation or 'core' notation. This uses the chemical symbols of noble gases in square brackets, and only writes the subshells for the highest incomplete energy level. Noble gases are used because their electron configuration are always of full shells; it is writing a shorthand.
• For example: [Ar] has the configuration (2,8,8) so the configuration of K could be written: [Ar] 4s1.

• The highest (partially) filled subshell for an element is how we say what “block” an element is in.
• For example, lithium’s electron configuration of 1s2 2s1 shows its highest energy electron in an s-orbital, so it is an s-block element.
• Likewise, boron is called a p-block element because its configuration of 1s2 2s2 2p1 shows its highest energy electron in a p-orbital.
Because the highest energy electrons are what make bonds and take part in chemical reactions, an element’s electron configuration is what determines its chemical properties.
• Introduction
Building on electron structure
a)
Electron Shells: Recap and introduction to sub-shells.

b)
The evidence for electron shells or 'quantum' energy levels.

c)
Electron subshells.

d)
The energy level diagram.

• 1.
Apply knowledge of electron subshells to write electron configuration.
Write the electron configuration of the following species using subshell notation:
a)
B

b)
Ca

c)
F$^-$

d)
Mg$^{2+}$

• 2.
Apply knowledge of energy level diagrams to write subshell electron configuration.
Write the electron configuration of the following species using an energy level diagram:
a)
Al

b)
Ne

c)
O$^{2-}$

d)
As

• 3.
Apply knowledge of core notation to write subshell electron configuration.
Write the electron configuration of the following species using noble gas or 'core' notation:
a)
Be

b)
N$^{3-}$

c)
Br$^-$

d)
Se