Composition of Nucleus: Protons, Neutrons and Nuclides

The nucleus is the tiny, dense centre of an atom that contains almost all of its mass. It is made of two kinds of particles: protons, which carry positive charge, and neutrons, which have no charge. Together, protons and neutrons are called nucleons.

Understanding the nucleus matters because nuclear reactions, radioactivity, and the identity of every element depend on how many protons and neutrons are present. Changing the proton number creates a completely different element.

Key Concepts

• Atomic number (Z): the number of protons in the nucleus. It decides which element the atom is. Unitless count.
• Mass number (A): total number of nucleons, so A = Z + N, where N is the neutron number.
• Nuclide: a specific nucleus described by its Z, N, and energy state, written as ᴬZX, for example ¹⁴₆C.
• Isotopes: nuclides with the same Z but different N, like ¹¹H, ²₁H, and ³₁H.
• Isobars: nuclides with the same A but different Z, like ¹⁴₆C and ¹⁴₇N.
• Isotones: nuclides with the same N but different Z.

How It Works

1. Count the protons to get Z; this names the element on the periodic table.
2. Count protons plus neutrons to get A.
3. Subtract: N = A − Z to find neutrons.
4. Compare Z, A, and N with other nuclides to classify isotopes, isobars, or isotones.

Protons repel each other because of their positive charges, yet the nucleus stays together because the strong nuclear force attracts nucleons at very short distances of about 1 to 2 femtometres (1 fm = 10⁻¹⁵ m). This force is much stronger than electric repulsion at that range but vanishes at larger distances.

Worked Example

For the nuclide ²³⁸₉₂U, find Z, N, and the number of electrons in a neutral atom.

1. Z is the lower left number: 92 protons.
2. A = 238, so N = A − Z = 238 − 92 = 146 neutrons.
3. A neutral atom has electrons equal to protons, so 92 electrons.

Common Misconceptions

• Students think neutrons have negative charge. They are neutral; their name tells this.
• Many think changing neutrons changes the element. Only changing protons changes the element; changing neutrons makes an isotope.
• Some believe the nucleus contains electrons. In modern understanding, electrons are outside the nucleus; beta decay creates an electron at the moment of decay.

Real-World Applications

Nuclide knowledge is used in nuclear medicine for tracers like technetium-99m, in carbon dating, in nuclear power fuel selection such as uranium-235, and in smoke detectors using americium-241.

In short, a nucleus is defined by its protons (Z) and neutrons (N), A = Z + N, and this simple counting explains isotopes, isobars, and all of nuclear chemistry.