Electron Shell Examples: 46 Problems with Answers

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Electron Shell.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A discrete energy level surrounding the atomic nucleus where electrons reside, with each shell ( $n = 1, 2, 3, ...$ ) holding a maximum of $2n^2$ electrons.

Electrons live in 'floors' around the nucleus. Lower floors fill first.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Electron Shell starts by naming the element, charge, and relevant protons, neutrons, or electrons.

Common stuck point: Students often know a formula related to electron shell but skip the recognition step: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?

Worked Examples

Example 1

easy

How many electrons can each of the first three electron shells hold? Show the maximum capacity for shells

n = 1

n = 2

, and

n = 3

Answer

n = 1 : 2, n = 2 : 8, n = 3 : 18

First step

The maximum number of electrons in a shell is given by

2n^2

where

n

is the shell number.

Full solution

2
Shell 1: $2(1)^2 = 2$ electrons. Shell 2: $2(2)^2 = 8$ electrons.
3
Shell 3: $2(3)^2 = 18$ electrons.

Electron shells are energy levels surrounding the nucleus. Inner shells are lower in energy and fill first. The formula

2n^2

determines the capacity of each shell and explains why the periodic table has rows of different lengths.

Example 2

medium

Draw the electron shell diagram (Bohr model) for a sodium atom (

Z = 11

). How many electrons are in each shell?

Na: 2, 8, 1 — one valence electron in the outermost shell

Example 3

medium

Draw the electron shell diagram for sodium (Na,

Z=11

). How many electrons are in each shell?

Na electron shell arrangement: 2, 8, 1

Example 4

medium

Why does the third shell appear to hold only

8

electrons in the configuration of argon (

Z=18

) even though

2n^2 = 18

Example 5

medium

Compare the outer-shell configurations of lithium (

Z=3

), sodium (

Z=11

), and potassium (

Z=19

). Why are their chemistries similar?

Example 6

hard

An ion has configuration

2, 8, 8

and a

+1

charge. Identify the element it came from.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium

Determine the electron shell arrangement for chlorine (

Z = 17

). How many electrons are in the outermost shell, and what does this predict about chlorine's chemical behavior?

Cl: 2, 8, 7 — 7 valence electrons, one short of a full octet

Example 2

hard

Compare the electron shell arrangements of neon (

Z = 10

) and argon (

Z = 18

). Explain why both are chemically inert despite having different numbers of total electrons.

Example 3

easy

What is an electron shell?

Example 4

easy

What is the maximum number of electrons the first shell (

n=1

) can hold?

Example 5

easy

Using

2n^2

, how many electrons fit in the second shell (

n=2

Example 6

easy

Which shell fills first as electrons are added to an atom?

Example 7

easy

How many electrons does a neutral neon atom (10 electrons) place in its shells?

Ne: 2, 8 — both shells completely filled

Example 8

easy

How many electrons are in the outer shell of a neutral oxygen atom (8 electrons)?

Example 9

easy

Electrons in shells follow fixed planetary orbits — true or false?

Example 10

easy

Why do electrons occupy the lowest available shell first?

Example 11

medium

A neutral sodium atom has 11 electrons. Write its electron-shell configuration.

Na: 2, 8, 1

Example 12

medium

Using

2n^2

, how many electrons can the third shell (

n=3

) hold at maximum?

Example 13

medium

A neutral chlorine atom has 17 electrons. Give its shell configuration and the outer-shell count.

Cl: 2, 8, 7 — outer shell has 7 electrons

Example 14

medium

Why do sodium (2, 8, 1) and lithium (2, 1) have similar chemistry?

Example 15

medium

Distinguish a shell from a subshell using the

n=2

shell as an example.

Example 16

medium

A neutral atom has the configuration 2, 8, 8. How many electrons does it have and what element is it?

Example 17

medium

How many electrons does a neutral magnesium atom (12 electrons) have in its outermost shell?

Mg: 2, 8, 2 — 2 electrons in the outermost shell

Example 18

medium

A neutral fluorine atom has 9 electrons. Write its electron-shell configuration and outer-shell count.

F: 2, 7 — 7 electrons in the outer shell

Example 19

medium

How many electrons can the fourth shell (

n=4

) hold using

2n^2

Example 20

challenge

Potassium has 19 electrons. Its configuration is 2, 8, 8, 1 rather than 2, 8, 9. Explain why the fourth shell starts before the third is full.

Example 21

challenge

A neutral atom has 2 full shells and 4 electrons in its third shell. Find its atomic number and element.

Example 22

challenge

Why do noble gases like neon (2, 8) and argon (2, 8, 8) rarely react?

Example 23

easy

Using

2n^2

, find the maximum capacity of shell

n=5

Example 24

easy

How many electrons fit in the first three shells combined (

n = 1, 2, 3

Example 25

easy

What is the electron-shell configuration of a neutral aluminum atom (

Z = 13

Al: 2, 8, 3

Example 26

easy

Write the shell configuration of a neutral silicon atom (

Z = 14

Si: 2, 8, 4

Example 27

easy

How many electrons does a neutral beryllium atom (4 electrons) have in its outermost shell?

Example 28

medium

A neutral sulfur atom has

Z = 16

. Give its shell configuration and outermost electron count.

S: 2, 8, 6 — 6 electrons in the outer shell

Example 29

medium

A neutral calcium atom has

Z = 20

. Give its electron-shell configuration.

Ca: 2, 8, 8, 2 — 4s fills before 3d

Example 30

medium

How many valence electrons does carbon (

Z=6

) have?

Example 31

medium

Which element has the configuration

2, 8, 5

Example 32

medium

A neutral atom has configuration

2, 8, 18, 1

. Identify it.

Example 33

medium

How many electrons does a neutral nitrogen atom (

Z=7

) have in its outer shell?

Example 34

medium

How many electrons does a

\text{Mg}^{2+}

ion have, and what is its shell configuration?

Mg²⁺: 10 electrons, configuration 2, 8

Example 35

medium

How many electrons does

\text{O}^{2-}

have, and what is its shell configuration?

O²⁻: 10 electrons, configuration 2, 8 (isoelectronic with Ne)

Example 36

hard

Write the shell configuration of a neutral bromine atom (

Z=35

Example 37

hard

What is the shell configuration of a neutral iron atom (

Z=26

Example 38

hard

Why does adding a third shell to sodium (config

2, 8, 1

) lower its ionization energy compared to lithium (

2, 1

Example 39

challenge

A particle has

17

protons and

18

electrons. Give its shell configuration, identity, and net charge.

Example 40

challenge

An emission line forms when an electron drops from

n=3

n=2

in hydrogen. Is the photon emitted in this transition higher or lower energy than one from

n=4

n=2

← Back to Electron Shell Practice Mode

Related Concepts

Electron Atom Valence Electron Electron Configuration

Background Knowledge

These ideas may be useful before you work through the harder examples.

electronatom