What is the basic structure of an atom according to the Cambridge IGCSE Chemistry syllabus?

In the Cambridge IGCSE Chemistry syllabus, an atom is described as consisting of a central nucleus containing protons and neutrons, surrounded by electrons in shells. The nucleus is positively charged due to protons, while electrons are negatively charged and orbit the nucleus in defined energy levels or shells.

How do you determine the number of protons, neutrons, and electrons in an atom?

The number of protons in an atom is equal to its atomic number, which is unique for each element and found on the Periodic Table. The number of electrons in a neutral atom is equal to the number of protons. Neutrons can be calculated by subtracting the atomic number from the atomic mass number (rounded to the nearest whole number), which is also provided on the Periodic Table.

What is the significance of the Periodic Table in understanding elements and compounds?

The Periodic Table is a crucial tool in Chemistry as it organizes elements based on increasing atomic number and similar chemical properties. It helps predict the behavior of elements, their compounds, and the trends in their properties, such as electronegativity, ionization energy, and atomic radius, which are essential for understanding chemical reactions and bonding.

How are elements classified in the Periodic Table according to the Cambridge IGCSE syllabus?

In the Cambridge IGCSE syllabus, elements in the Periodic Table are classified into metals, non-metals, and metalloids based on their physical and chemical properties. They are also grouped into periods (horizontal rows) and groups (vertical columns), with elements in the same group having similar properties due to the same number of electrons in their outer shell.

What is the role of electron configuration in determining the chemical properties of an element?

Electron configuration describes the distribution of electrons in an atom's shells and subshells. It plays a vital role in determining an element's chemical properties, such as reactivity and bonding behavior. Elements with similar electron configurations, particularly in their outermost shells, exhibit similar chemical properties, which is why they are grouped together in the Periodic Table.

Why is "Confusing $Z$ (proton number) with $A$ (nucleon number)." a common mistake in Atomic Structure and the Periodic Table?

Why it happens: Two numbers sit close together in the nuclide symbol and look similar. How to avoid it: Z is at the BOTTOM = protons. A is at the TOP = protons + neutrons. Neutrons = A - Z. Source: 0620/42 — recurring

Why is "Writing the number of neutrons as $A$ instead of $A - Z$." a common mistake in Atomic Structure and the Periodic Table?

Why it happens: The top number is called the 'mass number', so students read it as the neutron count. How to avoid it: Neutrons are always nucleon number MINUS proton number: neutrons = A - Z.

Why is "Putting more than $8$ in a shell (e.g. writing K as $2,8,9$)." a common mistake in Atomic Structure and the Periodic Table?

Why it happens: Forgetting the simple shell-capacity model for the first 20 elements. How to avoid it: For H to Ca: shell 1 holds 2, shell 2 holds 8, shell 3 holds 8, then the fourth shell starts. So K is 2,8,8,1.

Why is "Forgetting to change the electron count for an ion (or changing the proton count)." a common mistake in Atomic Structure and the Periodic Table?

Why it happens: Students treat ions exactly like neutral atoms, or adjust the wrong particle. How to avoid it: Charge changes ONLY electrons. Positive ion = fewer electrons (lost); negative ion = more electrons (gained). Protons and neutrons are unchanged.

Cambridge IGCSE Chemistry (0620)

Atomic Structure and the Periodic Table

0% Complete

3 tasks remaining

Short Notes - Atomic Structure and the Periodic Table

Detailed Study Notes

Detailed notes on Atoms, Elements and Compounds for Cambridge IGCSE Chemistry, covering key concepts, explanations, examples, and exam-focused revision points.

Atomic Structure and the Periodic Table — Cambridge IGCSE 0620 Chemistry Extended (2026)

Protons, neutrons and electrons; proton number, nucleon number; electronic configurations; how electronic structure links to position on the Periodic Table.

At a glance

Proton: $+1$ , mass $1\,\text{u}$ , in nucleus.
Neutron: $0$ , mass $1\,\text{u}$ , in nucleus.
Electron: $-1$ , mass $\sim 1/1836\,\text{u}$ , in shells.
Proton number $Z$ = number of protons (defines the element).
Nucleon number $A$ = protons + neutrons.
Neutral atom: number of electrons = $Z$ .
Electron shells fill 2, 8, 8, 2 (for elements 1-20).
Group number = outer-shell electrons. Period number = number of occupied shells.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

1.4 — State the relative charges and approximate masses of protons, neutrons and electrons.
1.4 — Use proton and nucleon numbers to deduce the number of electrons, protons and neutrons.
1.4 — Describe the build-up of electrons in shells (2, 8, 8, 2).
1.4 — Relate the electronic configuration of an element to its position on the Periodic Table.

Subatomic particles and notation

Three particles: proton (+1), neutron (0), electron (-1). Use $^{A}_{Z}\text{X}$ to identify a nuclide.

The three subatomic particles.

Particle	Charge	Relative mass	Location
Proton	$+1$	$1$	Nucleus
Neutron	$0$	$1$	Nucleus
Electron	$-1$	$\sim 1/1836$	Shells around nucleus

Atomic notation. $^{A}_{Z}\text{X}$

$X$ : element symbol.
$Z$ : proton number (atomic number) = number of protons. Defines the element.
$A$ : nucleon number (mass number) = protons + neutrons.

Number of each particle in a neutral atom.

Protons: $Z$ .
Neutrons: $A - Z$ .
Electrons: $Z$ (matches protons in a neutral atom).

Worked. $^{27}_{13}\text{Al}$ (aluminium-27).

Protons: $13$ .
Neutrons: $27 - 13 = 14$ .
Electrons (neutral): $13$ .

Protons and neutrons sit in the tiny dense nucleus; electrons occupy shells around it.

Ion. An atom that has gained or lost electrons.

Cation ( $+$ ): lost electrons.
Anion ( $-$ ): gained electrons.

Worked. Mg²⁺ ion. Magnesium has $Z = 12$ .

Protons: $12$ .
Electrons: $12 - 2 = 10$ (lost 2 electrons to form $2+$ ).

Losing the two outer electrons (red) leaves 12 protons and 10 electrons — a 2+ ion with a full outer shell.

Proton +1, neutron 0, electron -1.
$^{A}_{Z}\text{X}$ : $Z$ protons, $A$ nucleons.
Neutrons = $A - Z$ .
Electrons (neutral atom) = $Z$ .
Ion: $\pm$ charge from gain/loss of electrons.

Electronic configurations (2, 8, 8, 2)

Electrons fill shells from inside out: 2 in first, 8 in second, 8 in third (for elements 1-20).

Shell capacities (for the first 20 elements).

Shell 1: max 2 electrons.
Shell 2: max 8.
Shell 3: max 8 (for IGCSE; technically 18 in full theory).
Shell 4: starts filling after shell 3 has 8.

Filling order. Fill the lowest shell first, then move outwards.

Worked. Sodium ( $Z = 11$ ).

Configuration: $2, 8, 1$ .
Shell 1: 2. Shell 2: 8. Shell 3: 1 outer electron.

Worked. Chlorine ( $Z = 17$ ).

Configuration: $2, 8, 7$ .

Worked. Calcium ( $Z = 20$ ).

Configuration: $2, 8, 8, 2$ .

Electrons fill shells from the inside out — 2 in the first shell, then 8, then the rest.

Outer electrons (valence) determine reactivity. Group 1 (1 outer e⁻) and Group 17 (7 outer e⁻) are highly reactive. Group 18 (8 outer e⁻ — full shell) are inert (noble gases).

Drawing electronic structure. Cambridge accepts dot diagrams (concentric circles with × or • marks) — show the right number of electrons in each shell.

2, 8, 8, 2 capacity for shells 1-4 (first 20 elements).
Fill lowest shell first.
Outer-shell electrons = chemistry / reactivity.
Sodium: 2,8,1. Chlorine: 2,8,7.

Linking electronic structure to the Periodic Table

Period = number of shells. Group = number of outer electrons.

Period number = number of OCCUPIED shells.

Period 1: 1 shell (H, He).
Period 2: 2 shells (Li to Ne).
Period 3: 3 shells (Na to Ar).
etc.

Group number (for the main groups, 1-2 and 13-18) = number of OUTER-SHELL electrons.

Group 1 (alkali metals): 1 outer e⁻ (Li, Na, K).
Group 17 (halogens): 7 outer e⁻ (F, Cl, Br).
Group 18 (noble gases): full outer shell (He: 2; Ne: 8; Ar: 8).

Why the Periodic Table is the SHAPE it is. Elements in the same group have the SAME outer-shell configuration — that's why they have similar chemical properties. Sodium and potassium both have 1 outer electron → both react vigorously with water.

An element's position is read straight from its electronic configuration.

Worked. A neutral element has electronic configuration $2, 8, 6$ .

Period 3 (3 shells).
Group 16 (6 outer electrons).
Element: sulfur ( $Z = 16$ ).

Cambridge tip. Most properties (atomic radius, ionisation energy, reactivity) follow trends DOWN groups and ACROSS periods. See the Periodic Trends study note for details.

Period number = shells occupied.
Group number = outer-shell electrons.
Same group → same outer config → similar chemistry.
Configuration $2, 8, 6$ : Period 3, Group 16.

Quick recap

Proton +1, neutron 0, electron -1.
Notation $^{A}_{Z}\text{X}$ : protons $Z$ , neutrons $A-Z$ .
Shell capacities (1-20): 2, 8, 8, 2.
Period = shells. Group = outer electrons.
Same group → similar chemistry.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Proton number — number of protons in a nucleus; defines the element.
Nucleon number — total of protons and neutrons in a nucleus.
Electronic configuration — distribution of electrons across shells, e.g. 2, 8, 1.
Period — horizontal row in the Periodic Table.
Group — vertical column; elements with the same outer-shell electron count.

How it’s examined

Atomic structure appears every Paper 2 (4-5 marks: identify particles, write electronic configuration) and every Paper 4 (5-7 marks: link configuration to Periodic Table position, ions). Examiner reports flag confusing $A$ with $Z$ and forgetting that ions have a different number of electrons from the neutral atom.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (1.4); 0620/42 Oct/Nov 2024 — Q3 (electronic configurations); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

Take this whole topic with you

Step-by-step worked examples — Atomic Structure and the Periodic Table

Step-by-step solutions to past-paper-style questions on atomic structure and the periodic table, written exactly the way a tutor would explain them at the board.

1Find subatomic particles in an atom
Getting started• nuclide
Question
How many protons, neutrons and electrons are in $^{23}_{11}\text{Na}$ (neutral atom)?
Step-by-step solution
1. Step 1
  The bottom number is the proton (atomic) number $Z = 11$ , so there are $11$ protons.
2. Step 2
  The top number is the nucleon (mass) number $A = 23$ . Neutrons $= A - Z = 23 - 11 = 12$ .
3. Step 3
  An atom is neutral, so electrons $=$ protons $= 11$ .
Answer
$11$ protons, $12$ neutrons, $11$ electrons.
Examiner tip
Neutrons are always $A - Z$ , never $A$ . The top number counts protons AND neutrons together.
2Relative charges and masses of the three particles
Getting started• subatomic particles
Question
State the relative charge and relative mass of a proton, a neutron and an electron.
Step-by-step solution
1. Step 1
  Proton: relative charge $+1$ , relative mass $1$ . Found in the nucleus.
2. Step 2
  Neutron: relative charge $0$ , relative mass $1$ . Also in the nucleus.
3. Step 3
  Electron: relative charge $-1$ , relative mass $\frac{1}{1840}$ (≈ negligible). Found in shells around the nucleus.
Answer
Proton: $+1$ , mass $1$ . Neutron: $0$ , mass $1$ . Electron: $-1$ , mass $\frac{1}{1840}$ (negligible).
Examiner tip
Because electrons have negligible mass, almost all of an atom's mass is in the nucleus (protons + neutrons).
3Write an electronic configuration
Building confidence• Adapted from 0620/22 May/Jun 2024 Q3• configuration
Question
Write the electronic configuration of phosphorus, $Z = 15$ .
Step-by-step solution
1. Step 1
  There are $15$ electrons to place. Fill the inner shells first: shell 1 holds $2$ , shell 2 holds $8$ . That uses $10$ electrons.
2. Step 2
  Place the remaining $15 - 10 = 5$ electrons in the third shell.
  $2,8,5$
Answer
$2,8,5$
Examiner tip
Always check the digits sum to $Z$ : $2 + 8 + 5 = 15$ . ✓
4Group and Period from a configuration
Building confidence• periodic table
Question
An element has the electronic configuration $2,8,7$ . State its Group, its Period, and name it.
Step-by-step solution
1. Step 1
  The number of outer-shell electrons gives the Group: $7$ outer electrons → Group VII.
2. Step 2
  The number of occupied shells gives the Period: $3$ shells → Period 3.
3. Step 3
  Total electrons $= 2 + 8 + 7 = 17 = Z$ , so the element is chlorine.
Answer
Group VII, Period 3 — chlorine.
Examiner tip
Group = OUTER electrons only; Period = number of shells. Don't use the total electron count for the Group.
5Particles in an ion
Stretch• ions
Question
Work out the numbers of protons, neutrons and electrons in the ion $^{27}_{13}\text{Al}^{3+}$ .
Step-by-step solution
1. Step 1
  Protons $= Z = 13$ (the bottom number is unchanged by the charge).
2. Step 2
  Neutrons $= A - Z = 27 - 13 = 14$ .
3. Step 3
  A $3+$ charge means the atom has lost 3 electrons: electrons $= 13 - 3 = 10$ .
Answer
$13$ protons, $14$ neutrons, $10$ electrons.
Examiner tip
Charge changes only the electron count. A positive ion has FEWER electrons than protons; a negative ion has MORE. The neutral atom $\text{Al}$ has configuration $2,8,3$ .
6Link configuration to position and behaviour
Stretch• periodic table, reasoning
Question
Element X has $19$ electrons. Write its electronic configuration, give its Group and Period, and predict the charge of the ion it forms.
Step-by-step solution
1. Step 1
  Fill the shells: $2$ , then $8$ , then $8$ (the third shell takes $8$ before the fourth starts in this simple model), leaving $1$ for the fourth shell.
  $2,8,8,1$
2. Step 2
  Outer shell has $1$ electron → Group I. There are $4$ occupied shells → Period 4. (X is potassium, K.)
3. Step 3
  A Group I metal loses its single outer electron to reach a stable noble-gas configuration, forming a $1+$ ion.
Answer
$2,8,8,1$ ; Group I, Period 4; forms $\text{K}^{+}$ .
Examiner tip
For the first 20 elements the third shell is treated as holding $8$ before the fourth begins, so K is $2,8,8,1$ not $2,8,9$ .

Model Answers — Atomic Structure and the Periodic Table

High-scoring sample answers for atomic structure and the periodic table on the Cambridge IGCSE 0620 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
Paper 4 short-answer style1 mark
State the relative charge of an electron. (1 mark)
Model answer
$-1$ (a single negative charge).
Why this scores
One mark for $-1$ . The proton is $+1$ and the neutron is $0$ ; only the electron is negative.
Question 2
Paper 4 structured style2 marks
An atom is represented as $^{35}_{17}\text{Cl}$ . State the number of protons and the number of neutrons in this atom. (2 marks)
Model answer
Protons $= 17$ (the proton number $Z$ ). Neutrons $= A - Z = 35 - 17 = 18$ .
Why this scores
Two marks: one for $17$ protons, one for $18$ neutrons. The most common error is writing $35$ neutrons — that is the total of protons AND neutrons.
Question 3
Paper 4 structured style3 marks
An atom has $16$ electrons. Write its electronic configuration and name the element. (3 marks)
Model answer
Fill the shells in order: $2$ in the first, $8$ in the second, leaving $6$ for the third, giving $2,8,6$ . The total is $2 + 8 + 6 = 16$ , so the proton number is $16$ — the element is sulfur.
Why this scores
Three marks: correct first/second shells ( $2,8$ ); correct outer shell ( $6$ ); correct element (sulfur). Check the digits add up to the electron number.
Question 4
Paper 4 (Extended) structured style4 marks
An element is in Group II and Period 3 of the Periodic Table. Deduce its electronic configuration and explain how you used its position to work it out. (4 marks)
Model answer
The Period tells you the number of occupied electron shells, so Period 3 means three shells. The Group tells you the number of outer-shell electrons, so Group II means two electrons in the outer shell. Filling the inner shells fully ( $2$ then $8$ ) and placing $2$ in the third shell gives the configuration $2,8,2$ . (The element is magnesium.)
Why this scores
Four marks: Period → number of shells; Group → number of outer electrons; inner shells filled $2,8$ ; final configuration $2,8,2$ . Stating the link in words is what scores the explanation marks.
Question 5
Paper 4 (Extended) structured style5 marks
Calculate the number of protons, neutrons and electrons in the ion $^{24}_{12}\text{Mg}^{2+}$ . (5 marks)
Model answer
The proton number is $12$ , so there are $12$ protons. The nucleon number is $24$ , so the number of neutrons is $A - Z = 24 - 12 =$ $12$ neutrons. A neutral magnesium atom would have $12$ electrons, but the $2+$ charge shows it has lost two electrons, so it has $12 - 2 =$ $10$ electrons. The lost electrons came from the outer shell, leaving the stable configuration $2,8$ .
Why this scores
Five marks: $12$ protons; using $A - Z$ ; $12$ neutrons; recognising a $2+$ ion has lost $2$ electrons; $10$ electrons. The charge never changes the proton or neutron count.
Question 6
Paper 4 (Extended) structured style6 marks
Describe the structure of an atom, and explain why almost all of its mass is concentrated in the nucleus. (6 marks)
Model answer
An atom has a small, central nucleus that contains protons and neutrons. The electrons move around the nucleus in shells (energy levels). A proton has a relative charge of $+1$ and a relative mass of $1$ ; a neutron has a relative charge of $0$ and a relative mass of $1$ ; an electron has a relative charge of $-1$ and a relative mass of only $\frac{1}{1840}$ , which is negligible. An atom is electrically neutral because the number of protons equals the number of electrons, so the positive and negative charges balance. Because the electrons have negligible mass, almost all of the atom's mass comes from the protons and neutrons in the nucleus, which is why the mass is concentrated there.
Why this scores
Six marks across: nucleus contains protons and neutrons; electrons in shells around it; relative charges ( $+1$ , $0$ , $-1$ ); relative masses ( $1$ , $1$ , $\frac{1}{1840}$ ); atom neutral because protons $=$ electrons; mass in nucleus because electron mass is negligible.

Key Formulae — Atomic Structure and the Periodic Table

The formulae you need to memorise for atomic structure and the periodic table on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

Nuclide notation
$^{A}_{Z}\text{X}$
$A$
nucleon (mass) number = protons + neutrons
$Z$
proton (atomic) number = number of protons
When to use
Reading off particle numbers: protons $= Z$ , neutrons $= A - Z$ , electrons $= Z$ for a neutral atom.
Shell-filling order (first 20 elements)
$2,\ 8,\ 8,\ 2$
When to use
Building electronic configurations of H to Ca: shell 1 holds $2$ , shell 2 holds $8$ , shell 3 holds $8$ , then the fourth shell begins.

Key Definitions and Keywords — Atomic Structure and the Periodic Table

Definitions to memorise and the exact keywords mark schemes credit for atomic structure and the periodic table answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Proton (atomic) number $Z$
Examiner keyword
The number of protons in the nucleus of an atom. It defines the element and (in a neutral atom) equals the number of electrons.
Nucleon (mass) number $A$
Examiner keyword
The total number of protons plus neutrons in the nucleus of an atom.
Electronic configuration
Examiner keyword
How the electrons of an atom are arranged in shells around the nucleus, e.g. $2,8,7$ for chlorine.
Group
Examiner keyword
A vertical column in the Periodic Table. Elements in the same Group have the same number of outer-shell electrons.
Period
Examiner keyword
A horizontal row in the Periodic Table. Elements in the same Period have the same number of occupied electron shells.

Common Mistakes and Misconceptions — Atomic Structure and the Periodic Table

The traps other students keep falling into on atomic structure and the periodic table questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Confusing $Z$ (proton number) with $A$ (nucleon number).
0620/42 — recurring
Why it happens
Two numbers sit close together in the nuclide symbol and look similar.
How to avoid it
$Z$ is at the BOTTOM = protons. $A$ is at the TOP = protons + neutrons. Neutrons $= A - Z$ .
✕Writing the number of neutrons as $A$ instead of $A - Z$ .
Why it happens
The top number is called the 'mass number', so students read it as the neutron count.
How to avoid it
Neutrons are always nucleon number MINUS proton number: neutrons $= A - Z$ .
✕Putting more than $8$ in a shell (e.g. writing K as $2,8,9$ ).
Why it happens
Forgetting the simple shell-capacity model for the first 20 elements.
How to avoid it
For H to Ca: shell 1 holds $2$ , shell 2 holds $8$ , shell 3 holds $8$ , then the fourth shell starts. So K is $2,8,8,1$ .
✕Forgetting to change the electron count for an ion (or changing the proton count).
Why it happens
Students treat ions exactly like neutral atoms, or adjust the wrong particle.
How to avoid it
Charge changes ONLY electrons. Positive ion = fewer electrons (lost); negative ion = more electrons (gained). Protons and neutrons are unchanged.

Atomic Structure and the Periodic Table — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Particle

Charge

Relative mass

Location

Proton

+1

1

Nucleus

Neutron

0

1

Nucleus

Electron

-1

\sim 1/1836

Shells around nucleus

Period number = number of OCCUPIED shells.

Period 1: 1 shell (H, He).
Period 2: 2 shells (Li to Ne).
Period 3: 3 shells (Na to Ar).
etc.

Group number (for the main groups, 1-2 and 13-18) = number of OUTER-SHELL electrons.

Group 1 (alkali metals): 1 outer e⁻ (Li, Na, K).
Group 17 (halogens): 7 outer e⁻ (F, Cl, Br).
Group 18 (noble gases): full outer shell (He: 2; Ne: 8; Ar: 8).

An element's position is read straight from its electronic configuration.

Worked. A neutral element has electronic configuration $2, 8, 6$ .

Period 3 (3 shells).
Group 16 (6 outer electrons).
Element: sulfur ( $Z = 16$ ).

Cambridge tip. Most properties (atomic radius, ionisation energy, reactivity) follow trends DOWN groups and ACROSS periods. See the Periodic Trends study note for details.

Atomic Structure and the Periodic Table

Short Notes - Atomic Structure and the Periodic Table

Detailed Study Notes

At a glance

What you’ll learn

Subatomic particles and notation

Electronic configurations (2, 8, 8, 2)

Linking electronic structure to the Periodic Table

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Find subatomic particles in an atom

2Relative charges and masses of the three particles

3Write an electronic configuration

4Group and Period from a configuration

5Particles in an ion

6Link configuration to position and behaviour

Nuclide notation

Shell-filling order (first 20 elements)

Proton (atomic) number ZZZ

Nucleon (mass) number AAA

Electronic configuration

Group

Period

✕Confusing ZZZ (proton number) with AAA (nucleon number).

✕Writing the number of neutrons as AAA instead of A−ZA - ZA−Z.

✕Putting more than 888 in a shell (e.g. writing K as 2,8,92,8,92,8,9).

✕Forgetting to change the electron count for an ion (or changing the proton count).

Atomic Structure and the Periodic Table — frequently asked questions

Atomic Structure and the Periodic Table

Short Notes - Atomic Structure and the Periodic Table

Detailed Study Notes

At a glance

What you’ll learn

Subatomic particles and notation

Electronic configurations (2, 8, 8, 2)

Linking electronic structure to the Periodic Table

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Find subatomic particles in an atom

2Relative charges and masses of the three particles

3Write an electronic configuration

4Group and Period from a configuration

5Particles in an ion

6Link configuration to position and behaviour

Nuclide notation

Shell-filling order (first 20 elements)

Proton (atomic) number ZZZ

Nucleon (mass) number AAA

Electronic configuration

Group

Period

✕Confusing ZZZ (proton number) with AAA (nucleon number).

✕Writing the number of neutrons as AAA instead of A−ZA - ZA−Z.

✕Putting more than 888 in a shell (e.g. writing K as 2,8,92,8,92,8,9).

✕Forgetting to change the electron count for an ion (or changing the proton count).

Atomic Structure and the Periodic Table — frequently asked questions

Proton (atomic) number $Z$

Nucleon (mass) number $A$

✕Confusing $Z$ (proton number) with $A$ (nucleon number).

✕Writing the number of neutrons as $A$ instead of $A - Z$ .

✕Putting more than $8$ in a shell (e.g. writing K as $2,8,9$ ).

Proton (atomic) number $Z$

Nucleon (mass) number $A$

✕Confusing $Z$ (proton number) with $A$ (nucleon number).

✕Writing the number of neutrons as $A$ instead of $A - Z$ .

✕Putting more than $8$ in a shell (e.g. writing K as $2,8,9$ ).