What is the trend in atomic mass within Group I alkali metals?

The atomic mass within Group I alkali metals increases as you move down the group.

Group I alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). As you move down the group from lithium to francium, the atomic mass of these elements increases. This is due to the increase in the number of protons and neutrons in the nucleus of the atoms.

Each subsequent element in the group has an additional electron shell compared to the one before it. This means that each subsequent element has more protons in its nucleus and more electrons surrounding the nucleus. The increase in the number of protons and neutrons (which together make up the atomic mass) is greater than the increase in the number of electrons. This is because protons and neutrons are much heavier than electrons. Therefore, the overall atomic mass increases as you move down the group.

For example, lithium has an atomic mass of about 7, while sodium has an atomic mass of about 23. This trend continues with potassium (approximately 39), rubidium (approximately 85), caesium (approximately 133), and francium (approximately 223).

Understanding this trend is important as it helps to predict the properties of these elements. For instance, heavier alkali metals are more reactive than lighter ones. This is because as the atomic mass increases, the outermost electron is further from the nucleus and is therefore less tightly held. This makes it easier for the atom to lose this electron and participate in chemical reactions. Therefore, understanding the trend in atomic mass can help you to understand the chemical behaviour of the alkali metals.