An atom is the basic building block of matter. It consists of a tiny, dense nucleus made of protons (positive) and neutrons (neutral), surrounded by a cloud of electrons (negative). The atomic number \$Z\$ is the number of protons, while the neutron number \$N\$ can vary. The total number of nucleons (protons + neutrons) is the mass number \$A\$:
\$A = Z + N\$
Think of an atom like a family: the family name (element) stays the same, but the number of siblings (neutrons) can differ, giving each family member a slightly different identity.
• Protons give the atom its charge and identity.
• Neutrons add mass but no charge.
• Electrons orbit the nucleus and determine chemical behaviour.
• The nucleus is extremely small (≈10⁻¹⁵ m) yet contains almost all the atom’s mass.
Isotopes are atoms of the same element (same \$Z\$) that have different numbers of neutrons (\$N\$). They share chemical properties but differ in physical properties such as mass and stability.
Analogy: Imagine two siblings with the same last name but different birth weights – they belong to the same family but have distinct characteristics.
| Isotope | Protons (\$Z\$) | Neutrons (\$N\$) | Mass Number (\$A\$) | Stability |
|---|---|---|---|---|
| \$^{12}\$C | 6 | 6 | 12 | Stable |
| \$^{13}\$C | 6 | 7 | 12 | Stable |
| \$^{14}\$C | 6 | 8 | 14 | Radioactive |
Radioactive isotopes are unstable and transform over time, emitting radiation. For example, \$^{14}\$C decays by beta emission:
\$^{14}\text{C} \rightarrow ^{14}\text{N} + e^- + \bar{\nu}_e\$
The half‑life of \$^{14}\$C is about 5,730 years, making it useful for dating ancient organic material (radiocarbon dating).
Tip: Remember that decay reduces the neutron number by one while increasing the proton number by one (beta minus decay).