GCSE · Chemistry · Edexcel · Spec 1CH0
Atomic number defines the element
Carbon and nitrogen atoms are built from the same three particles, so what makes them different elements? One number, and one extra proton flips it.
Chemistry · Periodic table
Every element has its own proton number
Tap an element, then read its Atomic no. row. That number is how many protons sit in the nucleus of every atom of it. Hop from carbon (6) along to nitrogen (7). Is any number used twice?
Tap any cell to load its data into the panel. Tap a lens to highlight a chemical family.
Predict, then check
Carbon atoms have 6 protons. Imagine adding exactly one more proton.
An atom with 6 protons is carbon. Give that atom 7 protons instead. What is it now?
Chemistry · Elements and compounds
Element or compound?
Sort each substance by reading its formula: how many different types of atom does it contain?
Still to sort
Element (0)
Only one type of atom.
Where the line is: One kind of symbol in the formula, however many atoms there are.
Compound (0)
Two or more different elements chemically bonded together.
Where the line is: Two or more different symbols in the formula.
The trick is to look at the symbols in the formula, not at how many atoms there are. Each symbol starts with a capital letter, so Na is one symbol, not two.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
What you need to know
- Atoms are built from three particles: protons, neutrons and electrons.
- Every element is made of its own atoms, and a unique number of protons is what makes them its own.
- The atomic number is the number of protons in the nucleus of every atom of that element, so call it the proton number.
- Change the number of protons and you get a different element. That number is the element's identity.
Have a goYour friend says: 'An atom with 12 protons and an atom with 13 protons are basically the same element, they are only one proton apart.' Are they right?
No. 12 protons and 13 protons are different elements.
The number of protons is the identity, so even a difference of one proton makes a different element.
- An atom has as many electrons as it has protons.
Have a goAn atom has 14 protons. How many electrons does it have?
14 electrons.
An atom has as many electrons as protons, so the two numbers match.
- The mass number is the total number of protons and neutrons in the nucleus, which makes it a different number from the atomic number.
Have a goAn atom has 3 protons and 4 neutrons. What are its atomic number and its mass number?
Atomic number 3, mass number 7.
The atomic number counts protons only (3), while the mass number adds protons and neutrons (3 + 4 = 7).
- An element is made of only one type of atom, and the periodic table lists the elements.
- A compound is two or more different elements chemically bonded together.
- Element symbols are a shared language, so a formula such as CO₂ shows which types of atom a substance contains.
The big picture
The atomic number is the number of protons in every atom of an element, and it is what fixes which element it is. An atom has as many electrons as protons. An element has only one type of atom; a compound has two or more different elements chemically bonded.
Key points
Worked example
Problem
An atom has 11 protons and 12 neutrons. Find its atomic number and its mass number, say how many electrons it has, and name the element using the periodic table.
⚠ Watch out
Giving 'protons and neutrons' as the atomic number. That total is the mass number. The atomic number counts protons only, and it is the one that names the element.
Memory hook
Think of the atomic number as the element's ID number. Same ID, same element. New ID, new element. And the mass number is only the head-count of protons plus neutrons.
Check yourself
An atom has 17 protons. Which element is it, and how many electrons does the atom have? (Check: chlorine, Cl, with 17 electrons.)
Flashcards
(10)What is the atomic number of an element?
What decides which element an atom belongs to?
Which three particles build atoms?
How does the number of electrons in an atom compare with the number of protons?
What is an element?
What is a compound?
What is the mass number of an atom?
Atomic number or mass number: which counts protons only?
Why do chemists write substances with element symbols and formulae?
Is O₂ an element or a compound, and why?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
Learning with Lightbulb is opening soon
You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.
Keep me postedMore Edexcel GCSE Chemistry topics
- Acids, alkalis and the pH scale
- Alcohols: methanol to butanol
- Balanced chemical equations with state symbols
- Balanced ionic equations
- Biological polymers (DNA, starch, proteins)
- Calculating relative atomic mass from isotopic abundances
- Calculating relative formula mass and percentage by mass
- Carbon monoxide as a toxic gas
- Carboxylic acid -COOH and acidic properties
- Catalysts
- Chemical cells
- Chemical test for ammonia
How this lesson was checked. This Edexcel GCSE Chemistry (specification 1CH0)lesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 9 October 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.