1.4 Atomic notation & electron arrangements · Year 9 Science

1.4

Atomic notation & electron arrangements

1.4 · The atom

Atomic notation & electron arrangements

with ions and isotopes

By the end of this section:

I can say what atomic number (Z) and mass number (A) mean
I can use the atomic number to identify an element and find it in the periodic table
I can write an atom of element X in symbol notation: mass number A top left, atomic number Z bottom left
I can work out and draw (as a Bohr diagram) the electron arrangement of each of the first twenty elements
I can explain that an atom becomes an ion when it loses or gains an electron, and work out the ion’s charge from its atomic number and its number of electrons
I can say what an isotope is and describe how isotopes of one element are alike and how they differ

1 · Read

The notes, the key idea and the words to know

Worksheets Reference: Reading a periodic-table tile Worksheet A: Atoms, ions and isotopes Worksheet C: Lewis diagrams, ions, formulae and equations

Every atom is described by two numbers:

Atomic number, Z: the number of protons. It defines the element — change it and you have a different element.
Mass number, A: the number of protons + neutrons — the count of the heavy particles in the nucleus.

From these two numbers you can find every particle:

neutrons = A − Z
electrons = protons (in a neutral atom)

We write them around the symbol like this: the mass number on top, the atomic number below. Tap a part of the symbol to see what it means, or change the particles to see what you make.

What do the numbers mean?Tap the symbol · change the particles

Lithium-7: 3 protons, 4 neutrons, 3 electrons
Sodium ion Na⁺: 11 protons, 12 neutrons, 10 electrons
Chloride ion Cl⁻: 17 protons, 18 neutrons, 18 electrons
Chlorine-35: 17 protons, 18 neutrons, 17 electrons
Chlorine-37: 17 protons, 20 neutrons, 17 electrons
Carbon-14: 6 protons, 8 neutrons, 6 electrons

Tap the top number, the bottom number, the letters or the charge.

Protonssets the element

Neutronssets the mass

Electronssets the charge

neutrons

charge

arrangement

The worked examples:

Where this comes from: isotopes and half-lives from the Live Chart of Nuclides, International Atomic Energy Agency (IAEA), data downloaded 3 October 2026. Abundances are the share of each element’s atoms found in nature. The usual ions are the simple ions met in this course.

Electrons sit in shells and fill from the inside out. For the first twenty elements (up to calcium) the rule is: 2 electrons in the first shell, 8 in the second, 8 in the third, and then the next two go in the fourth (calcium is 2, 8, 8, 2). We write the electron arrangement as numbers separated by commas. This rule works only up to calcium: after that shells can hold more and fill in a different order, so you are never asked for them.

The electrons in the outermost occupied shell are the outer-shell electrons, also called valence electrons; they decide how an atom reacts.

How to draw a Bohr diagram4 steps · use the arrow keys or the buttons

Find the atomic number, Z. Z is the bottom number on the symbol. It counts the protons.
Draw the nucleus. Protons and neutrons go in the middle. Work out the neutrons from the two numbers.
Fill the shells from the inside out. Each shell must be full before the next one starts.
Check it, then write it down. The electrons you drew must add up to Z.

Try it with:

Try potassium in the guide: it has four shells (period 4) and one outer electron (Group 1).

Isotopes of an element behave the same chemically, because they have the same number of electrons; they differ only in mass.

Isotopes have real uses, for example in nuclear medicine and in radioactive dating: carbon-14 is radioactive, so archaeologists use it to date ancient bones and wood.

The one part everyone mixes upTap to open

This is the idea most students get tangled, so go slowly.

Electrons = protons — always (for a neutral atom). An atom has no overall charge, so the negative electrons must exactly balance the positive protons. If there are 3 protons, there are 3 electrons. This one is guaranteed.

Neutrons are the independent one. Neutrons add mass and help hold the nucleus together, but their number is not fixed by the protons — which is exactly why isotopes exist (same element, different neutrons). So you must work neutrons out with A − Z. Never assume neutrons = protons; sometimes they happen to be equal (carbon-12 has 6 and 6), but often they are not (lithium-7 has 3 protons and 4 neutrons).

So what does each particle control?

change the protons → a different element
change the neutrons → a different isotope (same element)
change the electrons → an ion (same element, now charged)

An ion has lost or gained electrons, so it is charged. If it lost electrons it is a positive ion (cation); if it gained electrons it is a negative ion (anion).

A negative ion. A chlorine atom has 17 protons and 17 electrons (arrangement 2, 8, 7). It gains one electron and becomes a chloride ion, Cl⁻.

Open the atom explorer above and press the worked-example buttons under it (lithium-7, a sodium ion, a chloride ion, chlorine-35 and chlorine-37): for each one it shows the protons, neutrons and electrons, and checks the charge. Then try this one yourself:

Try it first

A chloride ion is ³⁵₁₇Cl⁻. How many electrons does it have?

Your answer

Z = 17, so a chlorine atom has 17 protons and, when neutral, 17 electrons.
The − means it has gained one electron.
17 + 1 = 18 electrons. Check: 17 − 18 = −1 ✓

A positive ion (Na⁺) has lost electrons; a negative ion (Cl⁻) has gained them. Note the name: the ion is chloride, not chlorine.

Chlorine-35 and chlorine-37 both have Z = 17, so both have 17 protons and 17 electrons: they are the same element. But they have 35 − 17 = 18 and 37 − 17 = 20 neutrons. Same element, different neutrons = isotopes.

Ions and how they bond Try it: How ions form

The electrons in the outer shell are also called valence electrons. They are the ones that take part when atoms react.

Atoms lose or gain electrons until they have a full outer shell, like the nearest noble gas. That is the stable arrangement.

Metals lose electrons and become positive ions (cations).
Non-metals gain electrons and become negative ions (anions).

How ions form5 steps · you move the electron

Look at the outer shells. Atoms are stable with a full outer shell. Count what each one has.
Move the electrons. {metal} gives its outer electron to {non}. You move it.
Work out the charges. Charge = protons − electrons. One atom at a time.
Opposite charges attract. The positive and negative ions pull on each other. Ionic bond: the attraction between oppositely charged ions.
Billions of them: a giant lattice. The same ions repeat in every direction, each one pulling on its neighbours.

Try it with:

The pull between a positive ion and a negative ion is an electrostatic attraction: that is what the ionic bond is.

Chlorine and chloride are different things. Chlorine is a poisonous gas made of Cl₂ molecules. Chloride is the ion Cl⁻ found in table salt.

Watch out: 5 common mistakesTraps to avoid and what is true instead

Not thisNeutrons always equal protons.

ThisNot true. Only sometimes (carbon-12: 6 and 6). Lithium-7 has 3 protons but 4 neutrons. Always use A − Z.
Not thisElectrons equal neutrons.

ThisNo — electrons equal protons in a neutral atom. Neutrons are separate and can differ.
Not thisIsotopes are different elements.

ThisNo — same element, same protons. Only the neutron number (and so the mass number) differs.
Not thisThe mass number is the atom’s mass in grams.

ThisNo — it is a count of protons + neutrons (a relative figure), not a mass in grams.
Not thisA “+” ion has gained something.

ThisA positive ion has lost electron(s); a negative ion has gained electron(s). Charge = protons − electrons.

5 key words and what they meanatomic number (Z) · mass number (A) · electron arrangement · ion · isotope

atomic number (Z): the number of protons; also called the proton number — defines which element it is
mass number (A): the number of protons + neutrons; also called the nucleon number (protons and neutrons together are called nucleons)
ion: an atom that has lost or gained electrons, so it carries a charge
isotope: atoms of the same element (same Z) with different numbers of neutrons
relative atomic mass: the average mass of an element’s atoms, allowing for its isotopes
electron arrangement: how the electrons are spread across the shells, e.g. 2, 8, 1 for sodium

2 · Watch

Videos picked for this section

Know the key terms Inside Atoms: Electron Shells and Valence Electrons Free Animated Education

Know the key terms What is an Ion? Why Atoms Lose Their Electrons Free Animated Education

Know the key terms Isotopes: The Siblings of Atoms Free Animated Education

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Videos in other languagesEspañol · Deutsch · Português · العربية · 中文 · Русский

We try to help you in other languages. Learn the idea here in your own language, then learn the science words from the English or French video. Not all of these videos have been independently checked. If something does not make sense, or does not match what we did in class, ask your teacher.

Número atómico, número de masa e isótopos KhanAcademyEspañol

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3 · Now play with this

Things to try yourself, with questions to answer

This is what you will see.

Open the simulation

Simulation · PhET

Build an Atom

Make ions by taking electrons off atoms and adding them on, and see why sodium makes Na⁺ and chlorine makes Cl⁻.

Use the screen called Atom.

Build a neutral sodium atom: 11 protons, 12 neutrons and 11 electrons. Open the Net Charge panel with the plus sign.
Drag the one electron in the outer shell back to the Electrons bucket. Watch the Net Charge and the symbol.
Clear it and build a neutral chlorine atom: 17 protons, 18 neutrons and 17 electrons.
Add one more electron to the outer shell and watch the Net Charge again.
Now try magnesium (12 protons, 12 neutrons): how many electrons do you have to remove to leave full shells?

Check what you found

Sodium lost one electron. What is its charge now, and why is it not −1?

+1. It still has 11 protons (+11) but only 10 electrons (−10). Losing a negative electron leaves the atom positive.

Why does chlorine gain an electron instead of losing seven?

Its arrangement is 2, 8, 7. One more electron fills the outer shell (2, 8, 8); gaining one is far easier than losing seven. It becomes the chloride ion, Cl⁻.

Predict, then check: what ion does magnesium form?

Mg²⁺. Its arrangement is 2, 8, 2, so it loses the 2 outer electrons to leave 2, 8, a full outer shell. 12 protons − 10 electrons = +2.

This is what you will see.

Open the simulation

Simulation · PhET

Isotopes and Atomic Mass

Find out why chlorine's atomic mass on the Periodic Table is about 35.5 when no single chlorine atom has that mass.

Use the screen called Mixtures.

Click Cl (chlorine) in the periodic table at the top right.
Choose Nature's Mix and look at the Percent Composition and the Average Atomic Mass.
Choose My Mix, then drag only chlorine-35 atoms into the box and read the Average Atomic Mass.
Add chlorine-37 atoms to your mix, a few at a time, and keep watching the Average Atomic Mass.

Check what you found

How are chlorine-35 and chlorine-37 different, and how are they the same?

They have the same number of protons (17), so both are chlorine, but chlorine-37 has 2 more neutrons. They are isotopes of chlorine.

Look at Nature's Mix: which isotope is more common, and roughly by how much?

Chlorine-35 is more common, about three times as common as chlorine-37.

Predict: why is the average atomic mass of chlorine about 35.5 and not exactly 35 or 37? Check by changing your mix.

The average takes account of both isotopes. Because chlorine-35 is about three times as common as chlorine-37, the average is pulled towards 35 but is a little above it, at about 35.5.

This is what you will see.

Open the simulation

Simulation · PhET

Build a Nucleus

Find out that the number of protons decides the element and the number of neutrons decides the isotope.

Use the screen called Chart Intro.

Use the arrow buttons under Protons to add 2 protons, and read the element name shown at the top left.
Use the arrow buttons under Neutrons to add neutrons until you have 2 protons and 2 neutrons.
Take away one neutron and read the name again.
Take away one proton and read the name again.

Check what you found

What is the name of the nucleus with 2 protons and 2 neutrons, and what does the number after the name mean?

Helium-4. The number is the mass number: 2 protons plus 2 neutrons.

What changed when you took away a neutron, and what stayed the same?

It became helium-3, a different isotope of the same element. The number of protons stayed at 2, so it is still helium.

Predict: what happens to the element when you take away a proton? Check it.

The element changes, because the number of protons decides the element. With 1 proton the atom is hydrogen.

Also try

See it Electron arrangements

sodium · 2, 8, 1

Slide through the elements: each new electron goes in the lowest shell with room.

4 · Check you've got it

Quiz yourself, explain it in writing, then tick the section off

Quiz me

Five questions, marked straight away.

Electron arrangements Mass number & neutrons Ions Isotopes Counting neutrons Build an ion Build an isotope

Test yourself

Questions one at a time, as many as you like. Your teacher can see your practice.

Explain it

Write the answer in your own words, then compare it with a model answer.

Explain it · 4 marks

Describe Describe how sodium chloride forms from sodium atoms and chlorine atoms. Use electron arrangements and charges in your answer.

Write your answer here first

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Show a model answer

Tick each point your answer makes

Sodium (2, 8, 1) loses its one outer electron and becomes Na⁺ (2, 8).
Chlorine (2, 8, 7) gains one electron and becomes Cl⁻ (2, 8, 8).
The positive and negative ions attract each other strongly.
They form a huge network (a giant lattice) in a 1 : 1 ratio.

Model answer

Sodium (2, 8, 1) loses its one outer electron to become Na⁺ (2, 8). Chlorine (2, 8, 7) gains one electron to become Cl⁻ (2, 8, 8). The positive and negative ions attract each other strongly and form a huge network (a giant lattice) in a 1 : 1 ratio.

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More practice

Atom quiz: build it, name iton jmaths

Booklet pp. 15–18
Knowledge Book pp. 15–18
In your class workbook: Chapter 1.3 The ‘Bohr model’: Atoms, ions and isotopes (pp. 16–26) · Revise this section

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Atoms, the Periodic Table & chemical change

Atomic notation & electron arrangements

By the end of this section:

1 · Read

2 · Watch

Same electrons, different neutrons

Tick each point your answer makes

Model answer

What this page stores about you

Quiz

Simulation