Why does barium (ba) have a larger atomic radius than magnesium (mg)?

Since Ba and Mg are in the same column,

The reason will be Ba has more electrons, i.e. more orbitals than Mg, so apparently it's wider than Mg

If a hydrogen atom is neutral, it must also have  one electron. an oxygen atom (o) has an atomic number of eight (8). this means a neutral oxygen atom has eight protons and eight electrons. the element actium (ac) has an atomic number of 89, so it has 89 protons and 89 electrons in a neutral atom