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Number of neutrons = mass number - atomic number. Free GCSE Edexcel Chemistry revision notes with key definitions, plus 6 common questions answered.
Mass Distribution and Atomic Particles is part of A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes in GCSE Edexcel Higher Chemistry.
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Number of neutrons = mass number - atomic number. Free GCSE Edexcel Chemistry revision notes with key definitions, plus 6 common questions answered.
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Use this lesson to stay inside A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes while connecting the detail back to the wider Topic 1: Key concepts in chemistry topic.
Read the core explanations from Mass Distribution and Atomic Particles before testing yourself from memory.
Picture an atom as a football stadium. The nucleus would be a marble at the centre, and the electrons would be tiny specks buzzing around the outer edges. Here's the shocking part: that marble contains 99.97% of the stadium's total mass.
Almost all of the mass of an atom is concentrated in the nucleus. This seems impossible when you first hear it - how can something so small contain nearly everything? The answer lies in density. The nucleus is incredibly dense, packed with protons and neutrons, while electrons are practically weightless and occupy huge amounts of empty space.
This mass distribution explains why most alpha particles pass through atoms in the Geiger-Marsden experiment — atoms are mostly empty space. It also explains why nuclear reactions release so much energy compared to chemical reactions. We're dealing with the dense, massive core of the atom.
Not all atomic particles are created equal when it comes to mass. Think of it like comparing a bowling ball, another bowling ball, and a ping pong ball. The proton and neutron are your bowling balls - nearly identical in mass. The electron is the ping pong ball - so light it barely registers.
The relative masses of the three main atomic particles are crucial for understanding atomic structure:
Notice how the electron mass is often rounded to zero in GCSE calculations. This isn't because electrons don't matter - they're crucial for chemical bonding. But when calculating atomic mass, their contribution is negligible. It's like weighing a car and ignoring the weight of the air freshener hanging from the mirror.
Since electrons contribute virtually nothing to atomic mass, we can ignore them when calculating how heavy an atom is. The mass number is the total count of the heavy particles in the nucleus - protons plus neutrons.
This gives us a simple formula that works every time:
Mass numbers appear in nuclear notation. For example, carbon-12 has 6 protons and 6 neutrons, giving a mass number of 12. Carbon-14 has 6 protons and 8 neutrons, giving a mass number of 14. Same element, different mass number - these are called isotopes.
Worked example: Finding Mass Number from Nuclear Composition
An atom of chlorine has 17 protons and 18 neutrons. Calculate its mass number.
Answer: Mass number = 35
Now you can tackle any nuclear composition problem. The key is remembering what each number tells you and using the relationships between atomic number, mass number, and particle counts.
Worked example: Complete Nuclear Analysis
An isotope is written as $^{23}$Na. The atomic number of sodium is 11. Find the number of protons, neutrons, and electrons in this neutral atom.
Answer: Protons: 11, Neutrons: 12, Electrons: 11
This systematic approach works for any isotope. Remember the key relationships: atomic number = protons, mass number = protons + neutrons, and in neutral atoms, electrons = protons.
Examiners love to test whether you truly understand mass distribution and particle counting. Here are the common traps and how to avoid them.
The biggest mistake students make is including electrons when calculating mass number. Remember: mass number = protons + neutrons only. Electrons are counted separately and only matter for determining charge, not mass.
For exam success, practice identifying what each number in nuclear notation means. When you see $^{A}_{Z}$X, remember: A is the mass number (top-left) and Z is the atomic number (bottom-left), and the difference A-Z gives you the neutron count.
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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.
Number of neutrons = mass number - atomic number. For example, in sodium-23 with atomic number 11, there are 11 protons, 23 - 11 = 12 neutrons, and 11 electrons in the neutral atom.
In the Geiger-Marsden experiment, most alpha particles passed through atoms, showing that atoms are mostly empty space. The mass is concentrated in a tiny, dense nucleus.
Mass number = protons + neutrons, the total number of heavy particles in the nucleus. For example, a chlorine atom with 17 protons and 18 neutrons has a mass number of 17 + 18 = 35, written as chlorine-35. Electrons are not included.
A neutral atom has the same number of electrons as protons. For example, a neutral sodium atom with atomic number 11 has 11 protons and 11 electrons.
Almost all of the mass of an atom is concentrated in the nucleus, which contains protons and neutrons. The nucleus is incredibly dense, while electrons are practically weightless and occupy huge amounts of empty space.
Protons and neutrons have nearly identical mass, like two bowling balls, while the electron is so light it is often rounded to zero in GCSE calculations, like a ping pong ball. This is why electrons are ignored when calculating mass number.
The mass of an electron is often rounded to zero because it is so small compared with protons and neutrons, so its contribution to atomic mass is negligible. Electrons still matter for chemical bonding.
Chlorine-35 has 17 protons and 18 neutrons, giving a mass number of 17 + 18 = 35. It can also be written as ³⁵Cl.
Carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons. They are the same element with different mass numbers, so they are isotopes.
In nuclear notation, A is the mass number (top-left) and Z is the atomic number (bottom-left). The difference A - Z gives the number of neutrons.
A neutral sodium-23 atom has 11 protons, 12 neutrons and 11 electrons. The atomic number gives the 11 protons, neutrons = 23 - 11 = 12, and electrons equal protons in a neutral atom.