C3.2 Explain shell and subshell electron configurations | Ontario Grade 12 Chemistry (SCH4U)

Ontario Grade 12 Chemistry

Structure and Properties of Matter

How electron arrangements are represented and read

Atoms of different elements behave differently because their electrons are arranged differently. An electron configuration is a written description of that arrangement. In this lesson, you will move from the shell model to subshell notation and practise checking a configuration against an element’s electron count. The model is a useful way to organize electrons; it is not a picture of electrons travelling on fixed tracks.

What you will learn

Describe shells and subshells in an atom’s electron arrangement.
Use subshell capacities and the filling order to write an electron configuration.
Check that a configuration accounts for the atom’s total number of electrons.

1. From shells to subshells

A neutral atom has equal numbers of protons and electrons. The atomic number gives the number of protons, so it also gives the number of electrons in a neutral atom. For example, a neutral atom with atomic number 8 has 8 electrons. This count is the starting point for writing its electron configuration.

In the introductory shell model, electrons are grouped into energy levels called shells. Shells are labelled with whole numbers: shell 1 is closest to the nucleus in the model, followed by shells 2, 3, and so on. A shell is not a hard circular path. It is a way to group electrons by energy.

Each shell is divided into smaller groups called subshells. The letters used for subshells are

s

,

p

,

d

, and

f

. For the elements commonly studied in introductory configurations, shell 1 has an

s

subshell; shell 2 has

s

and

p

; shell 3 has

s

,

p

, and

d

; and shell 4 has

s

,

p

,

d

, and

f

. A subshell is a group within a shell, not an additional shell.

Subshells hold different maximum numbers of electrons. An

s

subshell holds 2 electrons, a

p

subshell holds 6, a

d

subshell holds 10, and an

f

subshell holds 14. These capacities help you check whether a written arrangement is possible. For example,

2p^7

would be invalid because a

p

subshell cannot hold 7 electrons.

The superscript in subshell notation tells you how many electrons are in that subshell. In

2p^4

, the 2 identifies the shell,

p

identifies the subshell, and the superscript 4 means that subshell contains 4 electrons. The superscript is a count, not a charge.

s^2,\ p^6,\ d^{10},\ f^{14}

The atomic number gives the electron count for a neutral atom.
A shell contains subshells; the notation names the shell, then the subshell, then its electron count.
Maximum subshell capacities are $s^2$ , $p^6$ , $d^{10}$ , and $f^{14}$ .

2. How electrons fill subshells

To write a configuration, fill the subshells in an established energy order. For the first 20 elements, the order needed here is

1s

,

2s

,

2p

,

3s

,

3p

,

4s

, then

3d

. Notice that

4s

is filled before

3d

in this sequence. Use the order rather than assuming that every subshell in one shell fills before the next shell begins.

The filling rule says to place electrons into the available subshells in that order, without exceeding each subshell’s capacity. Begin with the atom’s total electron count. Fill the first subshell, subtract those electrons from the total, and continue until no electrons remain.

A full configuration lists every occupied subshell, from the beginning of the filling order onward. For instance, a configuration may start with

1s^2

and continue with

2s^2

. Do not skip a subshell that should already have been filled. In a configuration for an atom with fewer electrons than a subshell’s capacity, the final occupied subshell may have a smaller superscript.

This arrangement is a model of electron distribution, not a claim that electrons are small objects moving around the nucleus in neat rings. The notation is useful because it states which shells and subshells contain the electrons and how many are in each.

1s\rightarrow2s\rightarrow2p\rightarrow3s\rightarrow3p\rightarrow4s\rightarrow3d

Use the filling order, not shell number alone, to decide what comes next.
For the first 20 elements, $4s$ is filled before $3d$ .
A configuration’s superscripts must add to the total number of electrons.

3. Reading and checking a configuration

A reliable check uses three questions. First, do the superscripts add to the atom’s electron count? Second, is the filling order followed? Third, is the capacity of each subshell respected? A configuration that fails any one of these checks needs correction.

For a neutral atom, compare the sum of the superscripts with the atomic number. If you are given an ion rather than a neutral atom, the electron count differs from the atomic number according to the ion’s charge. This lesson focuses on explaining shell and subshell configurations; always identify whether the stated particle is an atom or an ion before counting electrons.

Shell totals can also be found by adding the electrons in subshells that share the same leading number. For example, electrons in

2s

and

2p

belong to shell 2. This lets you connect the detailed subshell notation to the broader shell description.

Do not confuse a subshell label with its electron count. The letter

p

does not mean 3 electrons; it identifies a type of subshell. The superscript gives the number actually present, up to the subshell’s maximum.

N_e=\sum_i n_i

Check the total electron count, filling order, and subshell capacities.
Subshells with the same leading number belong to the same shell.
The letter identifies the subshell; the superscript records its electron count.

Worked example

Write the configuration for chlorine

A neutral chlorine atom has atomic number 17. Write its shell and subshell electron configuration, then check the total.

Find the electron count
Chlorine has atomic number 17. Because the atom is neutral, it has 17 electrons.
$N_e=17$
Fill subshells in order
Place up to 2 electrons in $1s$ , then continue through $2s$ , $2p$ , $3s$ , and $3p$ . Stop when all 17 electrons have been assigned.
$1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^5$
Check the result
The superscripts add to 17. Each subshell is within its capacity, and the subshells appear in the required filling order. Shell 1 contains 2 electrons, shell 2 contains 8, and shell 3 contains 7.
$2+2+6+2+5=17$

Answer: Chlorine’s subshell configuration is

1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^5

. Its shell totals are 2, 8, and 7.

Check: The shell totals also add to 17:

2+8+7=17

.

Common mistakes and how to avoid them

Treating the shell number as the number of electrons in that shell.

Correction: The leading number labels the shell. The superscripts across its subshells give the shell’s electron total.

Filling

3d

before

4s

for the first 20 elements.

Correction: Follow the listed filling order:

4s

comes before

3d

.

Writing more electrons in a subshell than it can hold.

Correction: Check against the capacities: 2 for

s

, 6 for

p

, 10 for

d

, and 14 for

f

.

Lesson summary

Electron configurations describe how electrons are distributed among shells and subshells.
Subshell notation uses a shell number, a letter, and a superscript electron count.
Use the filling order and subshell capacities, then confirm that the superscripts equal the electron total.

Check your understanding

Question 1

How many electrons are in the subshell

3p^4

?

3
4
6
7

Show answer and explanation

4

The superscript is the electron count. The subshell is in shell 3, and it contains 4 electrons.

Question 2

Which configuration correctly represents a neutral atom with 10 electrons?

$1s^2\ 2s^2\ 2p^6$
$1s^2\ 2s^2\ 2p^7$
$1s^2\ 2s^6\ 2p^2$
$1s^2\ 2s^2\ 3p^6$

Show answer and explanation

1s^2\ 2s^2\ 2p^6

The superscripts add to 10, the subshell capacities are respected, and the filling order is followed.

Key terms

Electron configuration: A symbolic description of how an atom’s electrons are arranged among shells and subshells.
Shell: An energy level used to group an atom’s electrons, labelled with a whole number.
Subshell: A subdivision of a shell, identified by a letter such as $s$ , $p$ , $d$ , or $f$ .
Superscript: A small raised number in the notation that states how many electrons occupy a subshell.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation C3.2. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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C3.2 · Explain shell and subshell electron configurations

Ontario Grade 12 Chemistry

How electron arrangements are represented and read

What you will learn

1. From shells to subshells

2. How electrons fill subshells

3. Reading and checking a configuration

Write the configuration for chlorine

Common mistakes and how to avoid them

Lesson summary

Check your understanding

Question 1

Question 2

Key terms

Continue through SCH4U

About this lesson and its review