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Subatomic Particles

Subatomic Particles

9 min read Updated Mar 26, 2026

Although your university chemistry course may have mentioned quarks, leptons, and gluons, the MCAT keeps things simpler. There are three subatomic particles you need to know cold: protons, neutrons, and electrons.

Protons

Protons live in the nucleus - the dense center of the atom. Each proton carries a charge of +1 and has a mass of approximately 1 atomic mass unit (amu).

The number of protons defines the element. Change the number of protons and you change the element entirely. This number is called the atomic number (Z). All carbon atoms have 6 protons. All oxygen atoms have 8. No exceptions.

Neutrons

Neutrons also live in the nucleus. They have no charge (0) and a mass of approximately 1 amu - just barely heavier than a proton.

Neutrons contribute to the atom’s mass but not its charge. The total number of protons plus neutrons is called the mass number (A). The number of neutrons can vary within the same element, creating isotopes - atoms with the same atomic number but different mass numbers. We will cover isotopes in detail in the next section.

Electrons

Electrons are fundamentally different from the particles in the nucleus. They move through the space surrounding the nucleus in regions called orbitals. Each electron carries a charge of -1 and has a mass that is approximately 12000\frac{1}{2000} of a proton’s mass - so small that electrons contribute virtually nothing to the atom’s overall mass.

Because electrons are so light compared to protons and neutrons, the mass of an atom is almost entirely determined by its nucleus. This is why we can effectively ignore electron mass when calculating atomic mass.

Electrons closer to the nucleus are at lower energy levels and are held more tightly. Electrons farther out are at higher energy levels, are held less tightly, and interact most strongly with the surrounding environment. The outermost electrons are called valence electrons, and they are the electrons most involved in chemical bonding and reactions.

Charge and Ions

In a neutral atom, the number of protons equals the number of electrons, so the charges cancel out. But atoms can gain or lose electrons:

  • Losing electrons creates a cation (positive charge). Think: “cations are pawsitive.”
  • Gaining electrons creates an anion (negative charge). Anions are negative.

Summary Table

| Particle | Symbol | Relative Mass (amu) | Charge | Location |
|----------|--------|-------------------|--------|----------|
| Proton | p+ | ~1 | +1 | Nucleus |
| Neutron | n0 | ~1 | 0 | Nucleus |
| Electron | e- | ~12000\frac{1}{2000} | -1 | Orbitals |

Key Relationships

  • Atomic number (Z) = number of protons = defines the element
  • Mass number (A) = protons + neutrons
  • Number of neutrons = A - Z
  • In a neutral atom: protons = electrons
  • In an ion: electrons = protons - charge (for cations) or protons + |charge| (for anions)

Inside an atom, and how to read an isotope symbol

Atomic structure
A schematic atom the electrons occupy all of this volume nucleus p⁺ and n⁰ electrons Not to scale: a real nucleus is ~1/100 000 of the atom's width. The three particles particle charge mass what it decides Proton p⁺ · in the nucleus +1 1.007 amu 1.673 × 10⁻²⁷ kg which element it is Neutron n⁰ · in the nucleus 0 1.009 amu 1.675 × 10⁻²⁷ kg which isotope it is Electron e⁻ · outside, in orbitals −1 0.00055 amu 9.109 × 10⁻³¹ kg the charge, and the chemistry An electron is ~1/1836 of a proton, so an ion weighs what its atom did. Reading the symbol, and the three hydrogens 1 1 H Protium ordinary hydrogen protons (Z) 1 neutrons (A − Z) 0 mass (amu) 1.0078 abundance 99.98 % 2 1 H Deuterium stable, used in D₂O protons (Z) 1 neutrons (A − Z) 1 mass (amu) 2.0141 abundance 0.0156 % 3 1 H Tritium radioactive, t½ ≈ 12.3 y protons (Z) 1 neutrons (A − Z) 2 mass (amu) 3.0160 abundance trace A · mass no. Z · atomic no.
1

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Reading the symbolThe bottom number is Z, the proton count, and it is what makes the element what it is. The top number is A, protons plus neutrons. Neutrons are the difference, A − Z, and they are never written directly.

Isotopes versus ionsChanging the neutron count gives an isotope: same element, same chemistry, different mass. Changing the electron count gives an ion: same element, same mass to three figures, different charge and very different behaviour.

Why atomic masses are not whole numbersThe mass on the periodic table is a weighted average over natural abundances. Chlorine sits at 35.45 because it is roughly three quarters chlorine-35 and one quarter chlorine-37, not because any single atom weighs 35.45 amu.

Mass lives in the nucleus, chemistry lives outside it. A proton and a neutron weigh about the same; an electron weighs about one eighteen-hundredth as much, which is why adding or removing electrons changes an atom's charge and reactivity without meaningfully changing its mass.
A nickel-60 cation has a +2 charge (60Ni2+). How many protons, neutrons, and electrons does it have?
Click to reveal answer

28 protons, 32 neutrons, 26 electrons. Nickel has atomic number 28 (always 28 protons). Neutrons = 60 - 28 = 32. The +2 charge means it lost 2 electrons: 28 - 2 = 26 electrons.

Which subatomic particle determines an atom’s identity as a particular element?
Click to reveal answer

The proton. The number of protons (atomic number, Z) uniquely identifies every element. Change the number of protons and you have a different element. Neutrons can vary (isotopes) and electrons can vary (ions), but protons define what element you have.

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