Year 7 · Chemistry · C2 Atoms

Atoms, but bitesize.

A revision booklet — six short topics, from what an atom is to comparing atoms using the periodic table.

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Take it one topic at a time. There are six topics. Each one is short — about 10 minutes. Do one or two a day.

Topic 01 · C2 · Atoms

Atoms and elements

By the end of this topic you'll know what an atom is, what an element is, and how the periodic table organises them.

Part 1What is an atom?

Everything you can see, touch, smell or taste is made of atoms. The chair you're sitting on. The screen you're reading. The air you're breathing. You.

An atom is the smallest unit of matter that can exist on its own. They are unbelievably tiny — a typical human body contains around 7 octillion atoms (that's 7 followed by 27 zeros).

For a long time scientists thought atoms were the smallest thing possible. We now know atoms are made of even smaller particles called subatomic particles — but you'll meet those in the next topic.

Keywords for Part 1

Atom
The smallest unit of matter that can exist on its own. The basic building block of everything.
Matter
Anything that has mass and takes up space — solids, liquids, and gases.

Part 2Elements: one type of atom

There are many different types of atom. Hydrogen atoms. Oxygen atoms. Gold atoms. Each type is different.

An element is a substance made of only one type of atom. So oxygen gas is an element — it's just oxygen atoms. Pure gold is an element — only gold atoms. Iron is an element — only iron atoms.

If you mix two or more elements together, you've made something else — a compound or a mixture. But a single element is just one type of atom, on its own.

16 O oxygen 8 12 C carbon 6 56 Fe iron 26 three elements — each made of just one type of atom
Three elements from the periodic table

⚠ Watch out — element vs compound

An element is ONE type of atom. Water is NOT an element — it's hydrogen and oxygen joined together. That makes it a compound. Don't confuse the two. If a substance is made of more than one type of atom, it's not an element.

Keywords for Part 2

Element
A substance made of only one type of atom.
Compound
A substance made of two or more different types of atom, chemically joined together.

Part 3The periodic table

The periodic table is a chart that lists every known element. It's arranged in a special order — not alphabetically, but so that elements with similar properties end up near each other.

The columns running down are called groups. The rows running across are called periods. Elements in the same group behave in similar ways — for example, all the elements in group 1 are soft, shiny metals that react with water.

For now, just remember the purpose: the periodic table is how scientists organise the elements so it's easier to understand and predict how they behave.

Keywords for Part 3

Periodic table
A chart that lists all the known elements, organised so similar elements are near each other.
Group
A vertical column in the periodic table. Elements in the same group have similar properties.
Period
A horizontal row in the periodic table.
Quick check

Which of these is an element?

  • AWater (H₂O) — it contains hydrogen and oxygen
  • BSalt (NaCl) — it contains sodium and chlorine
  • CPure iron (Fe) — it contains only iron atoms
  • DAir — it contains nitrogen, oxygen, and a few others
Show answer
C — pure iron. An element is made of only ONE type of atom. Water and salt are compounds (two types of atom joined). Air is a mixture (different molecules mixed but not joined). Only pure iron is just iron atoms.

Test yourself

5 questions · click to reveal each answer

  1. What is an atom?
    The smallest unit of matter that can exist on its own. The basic building block of everything.
  2. What is an element?
    A substance made of only one type of atom.
  3. Give two examples of elements.
    Any two from: oxygen, hydrogen, carbon, iron, gold, copper, sodium, helium... — any single element from the periodic table.
  4. What is the purpose of the periodic table?
    To organise the elements so scientists can understand how they behave and how they react with each other.
  5. What's the difference between a group and a period on the periodic table?
    A group runs vertically (a column). A period runs horizontally (a row). Elements in the same group have similar properties.
Topic 02 · C2 · Atoms

The atomic model

By the end of this topic you'll know how our idea of the atom has changed, the three subatomic particles, and where each one is found.

Part 1How the model changed

Scientists couldn't actually see atoms for most of history — they're far too small. So they built models based on the evidence they had at the time. As scientists discovered more, the model kept changing.

John Dalton (early 1800s). Dalton imagined atoms as tiny, solid spheres — like billiard balls. You couldn't break them up. They were just solid lumps of matter. We call this the solid sphere model.

Niels Bohr (1913). Over 100 years of new experiments showed Dalton was wrong about atoms being solid. Bohr came up with the planetary model: a central nucleus, with electrons moving around it in fixed paths called shells. A bit like planets around the Sun.

SOLID SPHERE Dalton · 1800s 100+ years of evidence PLANETARY Bohr · 1913
How our model of the atom changed

⚠ Watch out — models change, that's how science works

Both Dalton and Bohr were doing good science. Dalton wasn't "wrong" — he made the best model he could with the evidence available in 1800. When new evidence came in, the model was updated. Even Bohr's model isn't the full story today — modern physicists use a more complex "quantum" model. A scientific model is the best explanation for the evidence we have right now.

Part 2The three subatomic particles

Atoms are made of three smaller particles. We call them subatomic particles ("sub" means "smaller than"). A handy way to remember them: PEN.

P — Proton. Found in the nucleus (the centre of the atom).
E — Electron. Found in shells around the nucleus.
N — Neutron. Also found in the nucleus, alongside the protons.

The space inside an atom is mostly empty. The nucleus is tiny but very dense. The electrons whizz around it in shells, with huge gaps in between. If an atom were the size of a football stadium, the nucleus would be a pea in the middle.

+ + + N N N − − − − − − NUCLEUS protons + neutrons ELECTRON found in shells SHELL distance from nucleus EMPTY SPACE most of an atom
Inside an atom: nucleus, shells, electrons

⚠ Watch out — shells aren't tracks, they show distance

A common mistake is to think electrons "travel along" their shell like a train on tracks. They don't. The shell shows the distance the electron is from the nucleus. Different shells = different distances. We draw them as circles because it's a clear way to show this on paper, not because the electrons actually move in circles.

Keywords for Part 2

Subatomic particle
A particle smaller than an atom. There are three: proton, neutron, electron.
Nucleus
The centre of an atom. Contains protons and neutrons.
Shell
The distance an electron is from the nucleus. Electrons are found in shells around the nucleus.
Quick check

Where in an atom are the protons and neutrons found?

  • ABoth in the shells, around the nucleus
  • BProtons in the nucleus, neutrons in the shells
  • CBoth in the nucleus
  • DSpread throughout the atom, not in any particular place
Show answer
C — both in the nucleus. Protons and neutrons live together in the nucleus at the centre of the atom. Electrons are the ones in shells around the outside. (Remember PEN: Proton in nucleus, Electron in shell, Neutron in nucleus.)

Test yourself

6 questions · click to reveal each answer

  1. Who came up with the solid sphere model of the atom?
    John Dalton, in the early 1800s.
  2. What did Niels Bohr's model add to our understanding?
    A nucleus in the centre with electrons in shells around it — the planetary model. (Before Bohr, atoms were thought of as solid balls.)
  3. Name the three subatomic particles.
    Protons, electrons, and neutrons (PEN).
  4. Which subatomic particles are found in the nucleus?
    Protons and neutrons.
  5. Where are electrons found?
    In shells around the nucleus.
  6. What is between the electrons and the nucleus?
    Empty space. Most of an atom is empty.
Topic 03 · C2 · Atoms

Atomic number & mass number

By the end of this topic you'll be able to read a periodic table tile, and work out how many protons, neutrons, and electrons any atom has.

Part 1Reading a periodic table tile

Every box on the periodic table tells you four things: the name of the element, its chemical symbol, the atomic number, and the mass number (also called atomic mass).

9 Be beryllium 4 MASS NUMBER protons + neutrons SYMBOL used in equations NAME of the element ATOMIC NUMBER number of protons REMEMBER: mass number is on top atomic number is below
A periodic table tile, labelled

Keywords for Part 1

Atomic number
The number of protons in an atom. The smaller number on a periodic table tile.
Mass number
The total number of protons + neutrons. The bigger number on a periodic table tile.
Chemical symbol
One or two letters used to represent an element (e.g. O for oxygen, Fe for iron).

Part 2Working out the particles

Once you can read the tile, three simple rules tell you everything about the atom's particles:

protons = atomic number

electrons = atomic number   (in a neutral atom)

neutrons = mass number − atomic number

Why these work:

· The atomic number is the count of protons. Every element has a unique number of protons — that's what makes it that element.
· A normal atom is electrically neutral, so the number of electrons matches the number of protons.
· The mass number counts protons AND neutrons. So to find just the neutrons, subtract the atomic number.

⚠ Watch out — mass number isn't just neutrons

Mass number = protons plus neutrons, not just the neutrons. To get the neutrons alone, you have to subtract the atomic number from the mass number. If a question asks "how many neutrons?", make sure you've done the subtraction.

Part 3Worked examples

Worked example 1 — Beryllium (Be)

Beryllium has a mass number of 9 and an atomic number of 4. Find the number of protons, electrons, and neutrons.

Protons
= atomic number = 4
Electrons
= atomic number = 4
Neutrons
= mass − atomic = 9 − 4
Answer
P = 4 · E = 4 · N = 5

Worked example 2 — Oxygen (O)

Oxygen has a mass number of 16 and an atomic number of 8. Find P, E, and N.

Protons
= 8
Electrons
= 8
Neutrons
= 16 − 8
Answer
P = 8 · E = 8 · N = 8

Worked example 3 — working backwards

An atom has 5 electrons and a mass number of 11. Name the atom and find its number of neutrons.

Step 1
Electrons = protons, so protons = 5
Step 2
Atomic number = number of protons = 5
Step 3
Look up atomic number 5 on the periodic table → Boron (B)
Step 4
Neutrons = 11 − 5
Answer
Boron · N = 6
Quick check

Sodium has a mass number of 23 and an atomic number of 11. How many neutrons does it have?

  • A11 (used the atomic number)
  • B12 (mass − atomic = 23 − 11)
  • C23 (used the mass number)
  • D34 (added them together)
Show answer
B — 12. Neutrons = mass number − atomic number = 23 − 11 = 12. The trap here is reading the question carelessly — A gives protons or electrons (both also 11), and C gives mass number, not neutrons alone.

Test yourself

7 questions · click to reveal each answer

  1. What does the atomic number tell us?
    The number of protons in an atom (and, since atoms are neutral, also the number of electrons).
  2. What does the mass number tell us?
    The total number of protons + neutrons.
  3. How do you work out the number of neutrons?
    Mass number − atomic number.
  4. Potassium (K): mass 39, atomic number 19. P, N, E?
    P = 19, E = 19, N = 39 − 19 = 20.
  5. Helium (He): mass 4, atomic number 2. P, N, E?
    P = 2, E = 2, N = 4 − 2 = 2.
  6. Iron (Fe): mass 56, atomic number 26. P, N, E?
    P = 26, E = 26, N = 56 − 26 = 30.
  7. An atom has 3 protons, 4 neutrons, and 3 electrons. Name it. What's the atomic number and mass number?
    3 protons means atomic number 3 → Lithium (Li). Mass number = P + N = 3 + 4 = 7.
Topic 04 · C2 · Atoms

Electron configuration

By the end of this topic you'll know how many electrons each shell can hold, and how to draw the electron configuration of any atom up to calcium.

Part 1How many electrons in each shell

Electrons don't all crowd into one big group. They sit in shells around the nucleus, and each shell can only hold a limited number.

For Year 7, you need to know the first three shells:

shell 1: holds up to 2

shell 2: holds up to 8

shell 3: holds up to 8

So the most you can fit in the first three shells is 2 + 8 + 8 = 18 electrons.

nucleus SHELL 1 up to 2 electrons SHELL 2 up to 8 electrons SHELL 3 up to 8 electrons fill inner shells first — only move outward when full
Shell capacities (2, 8, 8 for shells 1, 2, 3)

⚠ Watch out — fill shells from the inside out

Always fill shell 1 completely (with 2 electrons) before starting shell 2. Fill shell 2 completely (with 8 electrons) before starting shell 3. You never leave an inner shell half-empty.

Part 2Writing electron configurations

An electron configuration is a shorthand for which shells have how many electrons. It's written with commas between the shells.

For example, the electron configuration of oxygen (8 electrons) is 2, 6. That means 2 electrons in shell 1, and 6 in shell 2.

Here's the method:

1. Find the total number of electrons (= atomic number).
2. Put 2 in shell 1. Subtract 2.
3. Put up to 8 in shell 2. Subtract.
4. Put any leftovers in shell 3.

Worked example 1 — Lithium (Li)

Lithium has atomic number 3. What is its electron configuration?

Total e⁻
= 3
Shell 1
fill with 2 (max). 3 − 2 = 1 left.
Shell 2
put the last 1 in.
Answer
2, 1

Worked example 2 — Carbon (C)

Carbon has atomic number 6. What is its electron configuration?

Total e⁻
= 6
Shell 1
2. Remaining: 4.
Shell 2
put all 4 in (room for 8).
Answer
2, 4

Worked example 3 — Magnesium (Mg)

Magnesium has atomic number 12. What is its electron configuration?

Total e⁻
= 12
Shell 1
2. Remaining: 10.
Shell 2
8 (full). Remaining: 2.
Shell 3
put the last 2 in.
Answer
2, 8, 2
Cl × × × × × × × × × × × × × × × × × CHLORINE (Cl) 17 electrons total configuration: 2, 8, 7 OUTER SHELL 7 electrons 2 in shell 1, 8 in shell 2, 7 in shell 3 — written as 2, 8, 7
Chlorine — electron configuration 2, 8, 7
Quick check

An atom has 14 electrons. What is its electron configuration?

  • A14 (all in one shell)
  • B2, 12 (overfills shell 2)
  • C2, 8, 4 (fills correctly)
  • D7, 7 (splits evenly)
Show answer
C — 2, 8, 4. Fill shell 1 first (2), then shell 2 (8), leaving 4 left over for shell 3. B is wrong because shell 2 can only hold 8 — you can't squeeze 12 in.

Test yourself

7 questions · click to reveal each answer

  1. How many electrons can the first shell hold?
    Up to 2.
  2. How many electrons can the second shell hold?
    Up to 8.
  3. How many electrons can the third shell hold?
    Up to 8.
  4. Write the electron configuration of oxygen (atomic number 8).
    2 in shell 1, 6 in shell 2 → 2, 6.
  5. Write the electron configuration of neon (atomic number 10).
    2 in shell 1, 8 in shell 2 → 2, 8.
  6. Write the electron configuration of phosphorus (atomic number 15).
    2, 8, 5 (2 + 8 + 5 = 15) → 2, 8, 5.
  7. Write the electron configuration of argon (atomic number 18).
    2 in shell 1, 8 in shell 2, 8 in shell 3 → 2, 8, 8. (All three shells full.)
Topic 05 · C2 · Atoms

The periodic table tells us about electrons

By the end of this topic you'll know how to use an element's position on the periodic table to find its electron configuration — without doing the working out.

Part 1Group number = outer shell electrons

Here's the magic of the periodic table. The way it's organised tells you about electrons directly. No counting needed.

The group number (the column an element is in) tells you how many electrons are in the outer shell of an atom.

· Lithium is in group 1 → 1 electron in outer shell.
· Beryllium is in group 2 → 2 electrons in outer shell.
· Fluorine is in group 7 → 7 electrons in outer shell.

This is why elements in the same group behave similarly: they have the same number of outer-shell electrons, and outer-shell electrons are what control how an atom reacts.

Part 2Period number = number of shells

The period number (the row an element is in) tells you how many shells an atom has.

· Hydrogen is in period 1 → 1 shell.
· Carbon is in period 2 → 2 shells.
· Sodium is in period 3 → 3 shells.
· Potassium is in period 4 → 4 shells.

1 2 3 4 5 6 7 0 GROUP NUMBER 1 2 3 4 PERIOD NUMBER H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca . . . . . . group 0 (right) = full outer shell · groups 1–7 = 1–7 outer electrons going down a column adds a new shell
A simplified periodic table — groups (vertical) and periods (horizontal)

⚠ Watch out — group 0 is the exception

Group 0 (the far right column: He, Ne, Ar, Kr...) is called the noble gases. Their outer shells are completely full, not zero. The "0" doesn't mean they have 0 outer electrons — it means they have no spare ones to give away or share. That's why they barely react with anything.

Part 3Using both numbers together

Put the two rules together and you can read the electron configuration of any atom straight from the periodic table:

period number = how many shells

group number = electrons in the outer shell

Worked example 1 — Beryllium (Be)

Beryllium is in group 2 and period 2. What is its electron configuration?

Period
2 → so 2 shells
Group
2 → so 2 electrons in the outer (2nd) shell
Inner
shell 1 is full → 2 electrons
Answer
2, 2

Worked example 2 — Bromine (Br)

Bromine is in group 7 and period 4. How many electrons are in its outer shell? How many shells does it have?

Group
7 → outer shell has 7 electrons
Period
4 → bromine has 4 shells
Quick check

Aluminium is in group 3 and period 3. What's its electron configuration?

  • A3, 3 (group + period)
  • B2, 8, 3 (fills inner shells first, then 3 in outer)
  • C2, 3 (only 2 shells)
  • D3, 8, 2 (in reverse)
Show answer
B — 2, 8, 3. Period 3 = 3 shells. Group 3 = 3 electrons in the outer (3rd) shell. Inner shells fill up first: shell 1 has 2, shell 2 has 8, leaving 3 for shell 3.

Test yourself

7 questions · click to reveal each answer

  1. What does the group number on the periodic table tell us?
    The number of electrons in the outer shell.
  2. What does the period number on the periodic table tell us?
    The number of shells the atom has.
  3. Caesium is in group 1. How many electrons are in its outer shell?
    1.
  4. Barium is in period 6. How many shells does a barium atom have?
    6.
  5. Iodine is in group 7, period 5. How many shells does it have? How many outer-shell electrons?
    5 shells · 7 outer-shell electrons.
  6. Why do all the group 1 elements behave similarly?
    Because they all have the same number of outer-shell electrons (one). The outer-shell electrons control how an atom reacts, so atoms with the same outer configuration react in similar ways.
  7. Why don't the noble gases (group 0) react much with anything?
    Their outer shells are already full, so they have no need to give away, take, or share electrons with other atoms.
Topic 06 · C2 · Atoms

Comparing atoms

By the end of this topic you'll be able to describe any atom in full, and compare two atoms — the kind of question that comes up in 6-mark exam answers.

Part 1Describing an atom in full

You now have all the tools to describe any atom completely. To do it properly, you need to cover four things:

1. The number of protons, neutrons, and electrons.
2. Where each subatomic particle is (nucleus or shells).
3. The electron configuration (how many in each shell).
4. Where the atom sits on the periodic table (group and period).

Worked example — Describing sodium (Na)

Sodium has mass number 23, atomic number 11. It is in group 1 and period 3. Describe it fully.

Protons
= atomic number = 11 (in the nucleus)
Electrons
= 11 (in shells)
Neutrons
= 23 − 11 = 12 (in the nucleus)
Shells
period 3 → 3 shells
Outer
group 1 → 1 outer electron
Config
2, 8, 1

Part 2Comparing two atoms

A common exam question asks you to compare two atoms. To get full marks, you have to talk about what's the same and what's different — not just describe each one separately.

Useful things to compare:

· number of protons, neutrons, electrons
· location of subatomic particles (which are in the nucleus, which are in shells)
· electron configuration
· group and period (and what this means about their reactivity)

Worked example — Compare sodium (Na) and chlorine (Cl)

Sodium (mass 23, atomic 11, group 1, period 3) and chlorine (mass 35, atomic 17, group 7, period 3). Compare the two atoms.

Same
Both have 3 shells (both period 3).
Both have protons + neutrons in the nucleus, electrons in shells.
Protons
Na = 11 · Cl = 17 (chlorine has more)
Neutrons
Na = 12 · Cl = 18
Electrons
Na = 11 · Cl = 17
Config
Na = 2, 8, 1 · Cl = 2, 8, 7
Outer
Na has 1 outer · Cl has 7 outer

⚠ Watch out — comparing means BOTH similarities and differences

If a question says "compare", the examiner expects you to mention things that are the same AND things that are different. Just writing two separate descriptions won't get full marks. Use linking words like "both", "however", "while", and "in contrast".

Part 3Why this all matters

The point of all this — atomic number, mass number, electron configuration, the periodic table — is to predict how atoms behave.

Atoms with the same outer-shell configuration react similarly. That's why the periodic table is organised the way it is. Every element in group 1 has one outer electron, and they all react with water in similar ways. Every element in group 0 has a full outer shell, and they all barely react with anything.

When you draw an atom or write its electron configuration, you're not doing busy work — you're working out what it's likely to do.

Quick check

Lithium and sodium are both in group 1. Why do they behave similarly?

  • AThey have the same number of protons
  • BThey have the same number of shells
  • CThey have the same number of electrons in their outer shell
  • DThey have the same mass number
Show answer
C — same number of outer-shell electrons. Both have 1 electron in their outer shell (because they're both in group 1). The outer-shell electrons are what control how an atom reacts, so atoms with the same outer configuration react in similar ways. They don't have the same protons (Li = 3, Na = 11) or the same number of shells (Li has 2, Na has 3).

Test yourself

6 questions · click to reveal each answer

  1. List the four things you need to mention to describe an atom fully.
    (1) Number of protons, neutrons, electrons. (2) Where each is found (nucleus or shells). (3) Electron configuration. (4) Group and period on the periodic table.
  2. Describe a carbon atom (mass 12, atomic 6, group 4, period 2) fully.
    Carbon has 6 protons and 6 neutrons in the nucleus, and 6 electrons in shells. Its electron configuration is 2, 4 (2 shells; 4 outer-shell electrons because it's in group 4).
  3. Compare oxygen (mass 16, atomic 8, group 6, period 2) and sulfur (mass 32, atomic 16, group 6, period 3).
    Same: both have 6 outer-shell electrons (both group 6), so they react similarly. Different: oxygen has 8 protons/neutrons/electrons; sulfur has 16 protons, 16 neutrons, 16 electrons. Oxygen has 2 shells (2,6); sulfur has 3 shells (2,8,6).
  4. Two atoms have the same electron configuration in their outer shell but different numbers of shells. Would they behave similarly?
    Yes. Outer-shell configuration controls reactivity. If the outer shells match, the atoms will behave similarly (and they'd be in the same group on the periodic table).
  5. Helium has 2 electrons in its only shell. Why does it barely react with anything?
    Its outer shell is full (shell 1 can only hold 2). Atoms with full outer shells don't need to give away, take, or share electrons, so they don't react. Helium is a noble gas (group 0).
  6. What's the difference between atomic number and mass number?
    Atomic number = the number of protons (smaller number on the periodic table tile). Mass number = protons + neutrons (the larger number).