The reason octane, c8h18, is a liquid at room temperature, while methane, ch4, is a gas at the same temperature, is because:

Both molecules are alkanes. But, methane is small molecule with molecular mass M = 16 g/mol (Ar (C) = 12, Ar (H) = 1) and octane is larger molecule with molecular mass, M = 114 g/mol (8·12 + 12·1). The smaller and lighter is the molecule, movement and space between molecules increased, so methane is a gas.