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Inside the Atom: Protons, Neutrons and Electrons

What are protons, neutrons and electrons?

BeginnerAtomic Structure & PeriodicityLast reviewed 3 October 2026
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What is it?

Every atom is built from three kinds of particle:

ParticleChargeMass (approx.)Where it is
Proton (pX+\ce{p+})+11 uIn the nucleus
Neutron (n\ce{n})01 uIn the nucleus
Electron (eX−\ce{e-})−1about 1/1836 of a protonAround the nucleus

The nucleus is a tiny, dense centre containing the protons and neutrons. The electrons occupy the much larger space around it.

Key idea

The number of protons decides which element an atom is. Every carbon atom has 6 protons; any atom with 6 protons is carbon.

Why does it matter?

The arrangement of these particles explains almost everything in chemistry:

  • Protons give each element its identity and its place in the periodic table.
  • Electrons take part in chemical reactions and bonding, so they decide how an element behaves.
  • Neutrons help hold the nucleus together and explain why atoms of the same element can have different masses (isotopes), which matters in fields such as radiocarbon dating and medical imaging.

How does it work?

1. Atomic number and mass number

  • Atomic number (ZZ) = number of protons.
  • Mass number (AA) = number of protons + number of neutrons.
  • So the number of neutrons = A−ZA - Z.

These are written as a nuclide symbol, with the mass number at the top left and the atomic number at the bottom left:

X1123X211223Na\ce{^{23}_{11}Na}

Sodium-23 has 11 protons and 23 − 11 = 12 neutrons.

2. Neutral atoms and ions

In a neutral atom, the number of electrons equals the number of protons, so the charges cancel.

An ion forms when an atom gains or loses electrons (never protons):

  • Losing electrons gives a positive ion (cation): NaX+\ce{Na+} has 11 protons and 10 electrons.
  • Gaining electrons gives a negative ion (anion): ClX−\ce{Cl-} has 17 protons and 18 electrons.

3. Isotopes

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, and therefore different mass numbers:

IsotopeProtonsNeutrons
Carbon-1266
Carbon-1367
Carbon-1468

Isotopes of an element have essentially the same chemical properties, because they have the same number of electrons.

Think of it like this

If an atom were the size of a large sports stadium, its nucleus would be about the size of a small marble at the centre, yet it would hold almost all of the stadium’s mass. The rest of the “stadium” is the space where the electrons are found.

More precisely

A proton (1.007 u) and a neutron (1.009 u) are not exactly 1 u, and the electron’s mass is about 0.000549 u. Electrons don’t travel in neat circular orbits like planets. Quantum mechanics describes them by orbitals, regions where an electron is likely to be found. Protons and neutrons are themselves made of smaller particles called quarks, which is beyond what chemistry needs here.

Visualise it

Diagram of a carbon-12 atom, not to scale. A small central nucleus contains 6 protons, marked plus, and 6 neutrons, with no charge. Six electrons, marked minus, are shown in a shaded cloud around the nucleus, where electrons are likely to be found. The real nucleus is tens of thousands of times smaller than the atom.
A carbon-12 atom (not to scale).

Worked example

Worked example: Counting particles in an ion

Question: How many protons, neutrons and electrons are in the ion X2656X226256FeX3+\ce{^{56}_{26}Fe^{3+}}?

  1. Protons = atomic number = 26
  2. Neutrons = A−ZA - Z = 56 − 26 = 30
  3. A charge of 3+ means 3 electrons have been lost: 26 − 3 = 23 electrons

Worked example: Writing the symbol from particle counts

Question: A particle has 17 protons, 20 neutrons and 18 electrons. Write its symbol.

  1. 17 protons means the element is chlorine (Z = 17).
  2. Mass number = 17 + 20 = 37.
  3. 18 electrons is one more than 17 protons, so the charge is 1−.

Answer: X1737X217237ClX−\ce{^{37}_{17}Cl-}, a chloride ion of chlorine-37.

Common mistake

Common mistake: Changing protons to make an ion

Ions form by gaining or losing electrons. If the number of protons changed, it would be a different element.

Common mistake: Finding neutrons from electrons

Neutrons = mass number − protons (atomic number), not mass number − electrons. For ions the two give different answers.

Common mistake: Confusing mass number with atomic mass

The mass number is a whole-number count of protons and neutrons in one particular isotope (chlorine-35 has A = 35). The atomic mass on the periodic table (Cl = 35.45) is an average over the element’s naturally occurring isotopes, so it is usually not a whole number.

Notation note

  • An isotope can be written in several ways: X614X26214C\ce{^{14}_{6}C}, X14X2214C\ce{^{14}C}, carbon-14, or C-14. The atomic number is often left out because the symbol already tells you the element.
  • The particles inside the nucleus (protons and neutrons) are together called nucleons.
  • “u” (unified atomic mass unit) is also written amu or Da (dalton).

Remember this

Remember this

  • Protons (+) and neutrons (0) are in the nucleus; electrons (−) are around it.
  • Atomic number Z = protons; mass number A = protons + neutrons.
  • Ions: change the number of electrons, never protons.
  • Isotopes: same element, different numbers of neutrons.

Test yourself

Check your understanding before moving on.

Flashcards

11 cards

  1. Question
    What are the charges of the proton, neutron and electron?
    Answer

    Proton +1, neutron 0, electron −1.

  2. Question
    Where are protons, neutrons and electrons found?
    Answer

    Protons and neutrons in the nucleus; electrons around the nucleus.

  3. Question
    What is the atomic number, Z?
    Answer

    The number of protons in the nucleus. It identifies the element.

  4. Question
    What is the mass number, A?
    Answer

    The number of protons + neutrons in the nucleus.

  5. Question
    How do you find the number of neutrons?
    Answer

    Neutrons = mass number − atomic number (A−ZA - Z).

  6. Question
    How many neutrons are in sodium-23 (Z = 11)?
    Answer

    23 − 11 = 12 neutrons

  7. Question
    How does an atom become a positive ion?
    Answer

    By losing electrons. The number of protons does not change.

  8. Question
    How many electrons are in CaX2+\ce{Ca^{2+}} (Z = 20)?
    Answer

    20 − 2 = 18 electrons

  9. Question
    What are isotopes?
    Answer

    Atoms of the same element (same number of protons) with different numbers of neutrons.

  10. Question
    Where is almost all of an atom's mass?
    Answer

    In the nucleus. Electrons have very little mass (about 1/1836 of a proton).

  11. Question
    Why is the atomic mass of chlorine (35.45) not a whole number?
    Answer

    It is an average over chlorine's naturally occurring isotopes (chlorine-35 and chlorine-37).

Quiz

7 questions

  1. Question 1EasyWhich particle has no electric charge?
    Show answer

    Answer: Neutron

    Neutrons are neutral (charge 0). Protons are +1 and electrons are −1. The nucleus has an overall positive charge because it contains protons.

  2. Question 2EasyThe atomic number of an element is the number of…
    Show answer

    Answer: protons in the nucleus

    The atomic number (Z) counts protons and identifies the element. Protons plus neutrons is the mass number.

  3. Question 3EasyHow many neutrons are in an atom of X1123X211223Na\ce{^{23}_{11}Na}?
    Show answer

    Answer: 12

    Neutrons = A − Z = 23 − 11 = 12. The answer 34 adds instead of subtracting.

  4. Question 4MediumHow many electrons are in a calcium ion, CaX2+\ce{Ca^{2+}}? (Ca: Z = 20)
    Show answer

    Answer: 18

    A 2+ charge means two electrons have been lost: 20 − 2 = 18. The answer 22 adds the electrons instead.

  5. Question 5EasyCarbon-12 and carbon-14 are isotopes. How do their atoms differ?
    Show answer

    Answer: Number of neutrons

    Both have 6 protons (so both are carbon) and, as neutral atoms, 6 electrons. Carbon-12 has 6 neutrons and carbon-14 has 8.

  6. Question 6MediumA particle has 17 protons, 20 neutrons and 18 electrons. Which symbol represents it?
    Show answer

    Answer: X1737X217237ClX−\ce{^{37}_{17}Cl-}

    17 protons makes it chlorine; mass number = 17 + 20 = 37; 18 electrons is one more than the protons, so the charge is 1−.

  7. Question 7MediumUranium-235 and uranium-238 both have 92 protons. How many neutrons does uranium-238 have?
    Show answer

    Answer: 146

    Neutrons = 238 − 92 = 146. Uranium-235 has 235 − 92 = 143 neutrons.

Notes and downloads

  • Worksheet

    Inside the Atom Worksheet

    7 questions on subatomic particles, atomic and mass numbers, ions and isotopes. Answer key included.

    BeginnerFree

References

  1. Brown, T. L.; LeMay, H. E., Jr.; Bursten, B. E.; Murphy, C. J.; Woodward, P. M.; Stoltzfus, M. W. Chemistry: The Central Science, 15th ed.; Pearson, 2022.

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