As we move down the group, the ionisation energy decreases, due to increase in atomic size, thereby, increasing the distance of the electrons from the nucleus. The first ionisation energy of lead is slightly higher than that of tin. The reason is due to lanthanide contraction. The atomic size of tin and lead are very close, and hence, the charge density is more in case of lead, due to more charge. Due to higher charge density, the outer electrons are attracted more by the nucleus in lead and hence, the ionisation energy is more in the case of lead.