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The atomic number is the number of protons in the nucleus of an atom, which defines the element. Free GCSE AQA Physics notes and 15 questions answered.
Atomic Number, Mass Number, and Neutral Atoms is part of Atoms and isotopes in GCSE AQA Higher Physics.
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The atomic number is the number of protons in the nucleus of an atom, which defines the element. Free GCSE AQA Physics notes and 15 questions answered.
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Use this lesson to stay inside Atoms and isotopes while connecting the detail back to the wider Atomic structure topic.
Read the core explanations from Atomic Number, Mass Number, and Neutral Atoms before testing yourself from memory.
Every atom contains a tiny, dense nucleus surrounded by electrons. The nucleus holds protons (positive charge) and neutrons (no charge). The electrons (negative charge) orbit around this nucleus in shells.
Here's the crucial point: in any neutral atom, the number of electrons exactly equals the number of protons. This balance means the positive and negative charges cancel out completely. The atom has no overall electrical charge.
Think of it like a perfectly balanced see-saw. Three protons on one side need exactly three electrons on the other side to stay level. This balance is what keeps atoms stable and electrically neutral.
Every element has its own identity card - the atomic number. This number tells you exactly how many protons are in the nucleus of every atom of that element. Carbon always has 6 protons, oxygen always has 8 protons, and gold always has 79 protons.
The atomic number is like a fingerprint for elements. It never changes. If you find an atom with 6 protons, it's carbon - no matter what else is going on. If it had 7 protons instead, it would be nitrogen, not carbon.
This is why the periodic table is arranged by atomic number, not by mass. The atomic number increases by one as you move from left to right across each period. Hydrogen (1 proton) comes first, then helium (2 protons), then lithium (3 protons), and so on.
Atoms have mass, and most of that mass comes from the nucleus. Protons and neutrons each have roughly the same mass - about 1 atomic mass unit. Electrons are much lighter, so we usually ignore their mass when calculating.
The mass number tells you the total count of particles in the nucleus. It's protons plus neutrons added together. If carbon has 6 protons and 6 neutrons, its mass number is 12. If the same carbon atom had 8 neutrons instead, the mass number would be 14.
Notice that mass number can vary for the same element. The number of protons stays the same (that's what makes it carbon), but neutrons can change. This gives us different versions of the same element with different masses.
Worked example: Calculating Mass Number
An oxygen atom has 8 protons and 8 neutrons. What is its mass number?
Answer: Mass number = 16
Scientists use a standard way to show atomic information. The element symbol sits in the middle, with the mass number as a superscript (top left) and atomic number as a subscript (bottom left). For example, carbon-12 is written as $^{12}C_6$.
This notation packs in all the key information. The 12 tells you the mass number (protons + neutrons). The 6 tells you the atomic number (protons only). Since it's carbon, you know there are 6 protons, and therefore 6 electrons in a neutral atom.
From this notation, you can work out everything else. If mass number is 12 and atomic number is 6, then neutrons = 12 - 6 = 6. The atom is electrically neutral, so electrons = protons = 6.
Worked example: Interpreting Atomic Notation
What can you tell about the atom $^{23}Na_{11}$?
Answer: Sodium atom with 11 protons, 12 neutrons, 11 electrons
Exam questions often test whether you can move between different ways of representing atoms. You might be given a description and asked to write the notation, or given the notation and asked to find the number of each particle.
The key relationships to remember are: atomic number = protons, mass number = protons + neutrons, and in neutral atoms, electrons = protons. These three rules solve almost every atomic structure question.
Watch out for common traps. Students often confuse mass number with atomic number, or forget that neutrons = mass number - atomic number. Always double-check your arithmetic, especially when calculating neutrons.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The atomic number is the number of protons in the nucleus of an atom, which defines the element. Carbon always has 6 protons, oxygen always has 8 and gold always has 79, and the periodic table is arranged by atomic number.
The atomic number of carbon is 6, because every carbon atom has 6 protons. An atom with 7 protons would be nitrogen, and oxygen always has 8 protons.
The mass number is the total number of protons and neutrons in an atom's nucleus. An oxygen atom with 8 protons and 8 neutrons has a mass number of 8 + 8 = 16; electrons are not counted because they are much lighter.
Atoms are neutral because in any neutral atom the number of electrons exactly equals the number of protons. The positive charges of the protons and the negative charges of the electrons cancel out, so the atom has no overall electrical charge.
An oxygen atom with 8 protons and 8 neutrons has a mass number of 16. Mass number = protons + neutrons = 8 + 8 = 16; electrons do not count because they are too light.
Protons have a positive charge, neutrons have no charge and electrons have a negative charge. Protons and neutrons are in the nucleus, and electrons orbit it in shells.
The number of neutrons equals the mass number minus the atomic number. A sodium atom with mass number 23 and atomic number 11 has 23 − 11 = 12 neutrons.
An atom with 7 protons is nitrogen, because the atomic number, the number of protons, defines the element. An atom with 6 protons is always carbon, and gold always has 79 protons.
A sodium-23 atom with atomic number 11 has 11 protons, 12 neutrons and 11 electrons. Neutrons = mass number − atomic number = 23 − 11 = 12, and a neutral atom has as many electrons as protons.
In atomic notation, the mass number is the top number and the atomic number is the bottom number next to the element symbol. For sodium-23 with atomic number 11, there are 11 protons, 23 − 11 = 12 neutrons and, in a neutral atom, 11 electrons.
A neutral atom has the same number of electrons as protons, so the positive and negative charges cancel out completely. A neutral carbon atom with 6 protons has 6 electrons.
The periodic table is arranged by atomic number, not by mass, because the atomic number, the number of protons, defines the element and never changes. It increases by one moving left to right across each period: hydrogen 1, helium 2, lithium 3.
Electrons do not count in the mass number because they are much lighter than protons and neutrons, so their mass is usually ignored. Protons and neutrons each have a mass of about 1 atomic mass unit.
In atomic notation, the top number is the mass number (protons + neutrons) and the bottom number is the atomic number (protons only). Carbon-12 has a mass number of 12 and an atomic number of 6, so it has 6 neutrons.
The mass number can vary for the same element because the number of neutrons can change while the number of protons stays the same. Carbon with 6 neutrons has a mass number of 12; with 8 neutrons it would be 14.