Neutrons & Protons

At the heart of every atom lies a tiny, dense region called the nucleus, which contains two types of subatomic particles: protons and neutrons. Together these particles are called nucleons, and they account for virtually all of an atom's mass. The number of protons in the nucleus defines which element an atom belongs to — this is the atomic number (Z) — while the total number of protons plus neutrons gives the mass number (A). Understanding the properties of these particles is fundamental to all of chemistry, from explaining periodic trends to understanding nuclear reactions. Protons carry a single positive electric charge of +1 (in elementary charge units, exactly +1.602 × 10⁻¹⁹ C), while neutrons are electrically neutral — they carry no charge at all. Despite this difference in charge, protons and neutrons have nearly identical masses: a proton has a mass of approximately 1.6726 × 10⁻²⁷ kg, and a neutron is slightly heavier at 1.6749 × 10⁻²⁷ kg. Both are assigned a relative atomic mass of approximately 1 atomic mass unit (u), which makes the mass number a convenient whole-number approximation of an atom's actual mass. Neutrons play a crucial role in nuclear stability. Without neutrons, the positively charged protons in the nucleus would repel each other through electrostatic (Coulomb) repulsion and the nucleus would fly apart. The strong nuclear force, which acts between all nucleons at extremely short range (about 10⁻¹⁵ m), holds the nucleus together. Neutrons contribute to this attractive force without adding to the repulsive proton–proton interactions, effectively acting as a 'nuclear glue.' The ratio of neutrons to protons (N/Z ratio) in a nucleus determines whether it is stable or radioactive.

Key takeaways

Location: The Nucleus
Charge
Mass Comparison
Atomic Number (Z) and Mass Number (A)
Internal Structure: Quarks
Nuclear Stability and the N/Z Ratio

Location: The Nucleus

Protons and neutrons are confined to the nucleus, which has a radius on the order of 1–10 femtometres (1 fm = 10⁻¹⁵ m). By contrast, the atom's electron cloud extends to roughly 10⁻¹⁰ m — about 100,000 times larger. This means the nucleus occupies an incredibly small fraction of the atom's volume yet contains more than 99.97% of its mass. The nuclear density is extraordinarily high, approximately 2.3 × 10¹⁷ kg/m³.

Charge

A proton carries a charge of +1 elementary unit (+1.602 × 10⁻¹⁹ C). A neutron carries zero charge. Because atoms are electrically neutral overall, the number of protons must equal the number of electrons in a neutral atom. When an atom gains or loses electrons it becomes an ion, but the proton count — and therefore the element's identity — never changes in ordinary chemical reactions. Only nuclear reactions (fission, fusion, radioactive decay) can change the proton number.

Mass Comparison

The proton mass is 1.6726 × 10⁻²⁷ kg, and the neutron mass is 1.6749 × 10⁻²⁷ kg — the neutron is only about 0.14% heavier than the proton. In relative terms, both are assigned 1 atomic mass unit (1 u = 1.6605 × 10⁻²⁷ kg). By comparison, an electron's mass is only 9.109 × 10⁻³¹ kg, which is roughly 1/1836 of a proton's mass. This is why electrons contribute negligibly to atomic mass.

Atomic Number (Z) and Mass Number (A)

The atomic number Z equals the number of protons in the nucleus and uniquely identifies the element. For example, all carbon atoms have Z = 6. The mass number A equals the total number of nucleons: A = Z + N, where N is the neutron number. Isotopes of the same element have the same Z but different N (and therefore different A). For example, carbon-12 (¹²C) has 6 protons and 6 neutrons, while carbon-14 (¹⁴C) has 6 protons and 8 neutrons. Carbon-14 is radioactive and used in radiocarbon dating.

Internal Structure: Quarks

Protons and neutrons are not truly fundamental — they are each composed of smaller particles called quarks, held together by the strong force mediated by gluons. A proton consists of two up quarks and one down quark (uud), giving a net charge of +1. A neutron consists of one up quark and two down quarks (udd), giving a net charge of 0. This internal quark structure, described by quantum chromodynamics (QCD), explains why the neutron is slightly heavier than the proton despite having the same number of quarks.

Nuclear Stability and the N/Z Ratio

For light elements (Z ≤ 20), stable nuclei tend to have a neutron-to-proton ratio close to 1:1. As Z increases, more neutrons are needed to offset proton–proton repulsion, so the N/Z ratio for stable heavy nuclei rises toward ~1.5. Nuclei with too many or too few neutrons are unstable and undergo radioactive decay: beta-minus decay (n → p + e⁻ + antineutrino) increases Z, and beta-plus decay (p → n + e⁺ + neutrino) decreases Z, both moving the nucleus toward stability.

The Key Equation

A = Z + N

A — Mass number (dimensionless (integer)): Total count of nucleons (protons + neutrons) in the nucleus; approximately equal to the atomic mass in atomic mass units (u).
Z — Atomic number (proton number) (dimensionless (integer)): Number of protons in the nucleus; uniquely identifies the chemical element and equals the number of electrons in a neutral atom.
N — Neutron number (dimensionless (integer)): Number of neutrons in the nucleus; determines the isotope. N = A − Z.

Worked Example

Problem. An atom of iron (Fe) has an atomic number of 26 and a mass number of 56. (a) How many protons does it have? (b) How many neutrons does it have? (c) How many electrons does the neutral atom have? (d) Write the nuclide notation for this isotope.

^{56}_{26}\text{Fe}: \; Z=26 \text{ protons}, \; N=30 \text{ neutrons}, \; 26 \text{ electrons}

Answer. Iron-56 has 26 protons, 30 neutrons, and 26 electrons.

Figure

Diagram

Origins · 1 minJames Chadwick and how this was discovered

James Chadwick

1891–1974 · British

James Chadwick discovered the neutron in 1932, one of the most important discoveries in 20th-century physics. Working at the Cavendish Laboratory in Cambridge, he correctly interpreted experiments in which beryllium was bombarded with alpha particles. The reaction $\ce{^{9}_{4}Be + ^{4}_{2}He -> ^{12}_{6}C + ^{1}_{0}n}$ produced a penetrating, electrically neutral radiation that he identified as a stream of neutral particles with mass approximately equal to the proton. This discovery completed the picture of the atomic nucleus and earned him the 1935 Nobel Prize in Physics. Chadwick's neutron discovery also made nuclear fission and the development of nuclear technology possible. Chadwick spent several years as a prisoner of war in Germany during World War I, but managed to conduct basic scientific experiments in the internment camp using improvised equipment and chemicals from toothpaste and fertiliser.

Electrons and Electron Configuration — Electrons are the third type of subatomic particle; understanding all three particles together gives a complete picture of atomic structure and determines chemical behaviour.
Isotopes and Relative Atomic Mass — Isotopes differ only in neutron number; the concept builds directly on understanding what neutrons are and how N = A − Z works.
The Periodic Table and Atomic Number — The periodic table is organised by atomic number (proton count), making knowledge of protons essential to understanding elemental identity and periodicity.
Radioactivity and Nuclear Decay — Radioactive decay involves changes to proton and neutron numbers in the nucleus; nuclear stability concepts (N/Z ratio) introduced here are directly applied in this topic.
Atomic Mass and the Mole — The atomic mass unit is defined relative to proton and neutron masses; this concept is foundational to stoichiometric calculations involving molar mass.