Protons and Neutrons Make Up What? The Atomic Nucleus Explained

The short answer is that protons and neutrons make up the atomic nucleus. This dense core sits at the center of every atom, containing nearly all of the atom’s mass. Understanding the nucleus is central to chemistry and physics, as its composition determines what element an atom belongs to and how it behaves.

What Is the Atomic Nucleus?

The atomic nucleus is a small, dense region at the center of an atom. It is composed of protons and neutrons, which are collectively called nucleons.

Protons carry a positive electric charge. Neutrons have no net electric charge. The number of protons in the nucleus defines the element. For example, any atom with 6 protons is carbon, and any atom with 8 protons is oxygen. This number is the atomic number.

Neutrons add mass to the nucleus without changing the element. Atoms of the same element can have different numbers of neutrons; these variants are called isotopes. For instance, carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons.

The nucleus is held together by the strong nuclear force, one of the four fundamental forces of nature. This force overcomes the electrostatic repulsion between positively charged protons, which would otherwise push the nucleus apart.

Why Protons and Neutrons Stay Together

The electrostatic force causes like charges to repel. Two protons placed near each other experience a strong repulsive force. So why does the nucleus not fly apart?

The answer is the strong nuclear force. This force acts between all nucleons (proton-proton, proton-neutron, and neutron-neutron). It is about 100 times stronger than the electromagnetic force, but its range is limited to about 1 femtometer. At the distances inside a nucleus, the strong force dominates and binds the nucleons together.

Neutrons play a key role here. Because they feel the strong force but add no extra electrostatic repulsion, they help stabilize the nucleus. In larger nuclei, extra neutrons are needed to keep the nucleus stable. This is why heavy elements like uranium have many more neutrons than protons (for example, uranium-238 has 92 protons and 146 neutrons).

If a nucleus has too many or too few neutrons relative to protons, it becomes unstable. Such nuclei undergo radioactive decay, emitting particles or energy to reach a more stable configuration.

How the Nucleus Determines an Element’s Properties

The number of protons in the nucleus (the atomic number) is the single most important property of an atom. It determines:

• The element’s identity. Hydrogen has 1 proton, helium has 2, lithium has 3, and so on. IUPAC maintains the official table of elements, where each element is defined by its atomic number IUPAC.
• The atom’s chemical behavior. The number of protons determines how many electrons a neutral atom will have. Electrons dictate how the atom bonds with other atoms. For example, sodium (11 protons) readily gives up one electron, while chlorine (17 protons) readily accepts one.
• The atom’s mass. While protons and neutrons have nearly the same mass, the total number of nucleons gives the atomic mass. Electrons contribute very little mass.

The Royal Society of Chemistry notes that the arrangement of electrons in shells around the nucleus is directly linked to the nuclear charge, which is set by the proton count Royal Society of Chemistry.

Isotopes and Their Uses

Because neutrons do not affect chemical behavior, isotopes of the same element behave almost identically in chemical reactions. However, their different masses lead to different physical properties.

• Carbon-14 is radioactive and used in radiocarbon dating.
• Uranium-235 is fissile (its nucleus can split when struck by a neutron, releasing energy in nuclear reactors).
• Hydrogen-2 (deuterium) has one neutron and is used in nuclear fusion research.

What About the Electrons?

Electrons orbit the nucleus in regions called orbitals. They are negatively charged and are attracted to the positively charged nucleus. The number of electrons in a neutral atom equals the number of protons.

While electrons are crucial for chemistry, they contribute almost nothing to the atom’s mass. The nucleus, made of protons and neutrons, accounts for more than 99.9% of an atom’s mass. If an atom were the size of a football stadium, the nucleus would be about the size of a pea at the center.

Key Takeaways

• Protons and neutrons make up the atomic nucleus, which sits at the center of every atom.
• The number of protons defines the element; the number of neutrons determines the isotope.
• The strong nuclear force binds protons and neutrons together, overcoming the electrostatic repulsion between protons.
• The nucleus contains nearly all of an atom’s mass, while electrons occupy most of its volume.
• Unstable nuclei undergo radioactive decay to reach a more stable balance of protons and neutrons.