Describe the composition of the nucleus in terms of protons and neutrons.
Key Concepts
The nucleus is the dense central part of an atom.
It contains two types of sub‑atomic particles:
Protons – positively charged, mass ≈ 1 u.
Neutrons – neutral, mass ≈ 1 u.
Electrons orbit the nucleus and contribute negligibly to the atomic mass.
Notation and Symbols
Symbol
Meaning
Charge
Mass (u)
\$p\$
Proton
\$+1e\$
1.0073
\$n\$
Neutron
0
1.0087
\$e^{-}\$
Electron
\$-1e\$
0.0005
Atomic Number, Mass Number and Notation
The composition of a nucleus is expressed using the symbols:
\$Z\$ – atomic number = number of protons.
\$N\$ – number of neutrons.
\$A\$ – mass number = total nucleons, \$A = Z + N\$.
Standard nuclear notation places \$A\$ and \$Z\$ as superscript and subscript to the left of the chemical symbol:
\$\,^{A}_{Z}\text{X}\$
where “X” is the element’s chemical symbol.
Examples
Carbon‑12: \$^{12}_{6}\text{C}\$
\$Z = 6\$ (6 protons)
\$N = 12 - 6 = 6\$ neutrons
Uranium‑235: \$^{235}_{92}\text{U}\$
\$Z = 92\$ (92 protons)
\$N = 235 - 92 = 143\$ neutrons
Isotopes
Atoms of the same element (same \$Z\$) that have different numbers of neutrons are called isotopes. They share chemical properties but differ in mass and nuclear stability.
Stability and Binding Energy (brief)
Protons repel each other electrically, but the strong nuclear force, mediated by neutrons, holds the nucleus together. The balance of these forces determines nuclear stability.
Suggested diagram: Cross‑section of an atom showing a dense nucleus (protons and neutrons) surrounded by electron shells.
Summary
The nucleus consists of protons and neutrons, collectively called nucleons.
Atomic number \$Z\$ identifies the element; mass number \$A\$ gives the total nucleons.
Isotopes have the same \$Z\$ but different \$N\$ and \$A\$.
Understanding nuclear composition is fundamental for topics such as radioactivity, nuclear reactions, and atomic structure.