The boiling point of a substance is the temperature at which its vapour pressure equals the pressure surrounding the liquid and the liquid changes into a vapour. The boiling point of a substance increases as the atomic number of its constituent atoms increases. This is because the boiling point of a substance is related to the strength of the intermolecular forces between its molecules. The stronger the intermolecular forces, the higher the boiling point of the substance.

As the atomic number of an atom increases, the number of protons in the nucleus of the atom also increases. This increase in the number of protons leads to a stronger electrostatic attraction between the positively charged nucleus and the negatively charged electrons in the atom. This stronger electrostatic attraction results in stronger intermolecular forces between the molecules of the substance, which leads to a higher boiling point.

For example, the boiling point of methane (CH4), which has one carbon atom, is -161.6 °C. The boiling point of ethane (C2H6), which has two carbon atoms, is -88.6 °C. The boiling point of propane (C3H8), which has three carbon atoms, is -42.1 °C. The boiling point of butane (C4H10), which has four carbon atoms, is -0.5 °C.

As the number of carbon atoms in the molecule increases, the boiling point of the substance increases because the intermolecular forces between the molecules become stronger as the number of protons in the molecule increases.