Arrange the following cations in order of decreasing size :Ra2+,Sr2+,Ca2+,Ba2+.Also explain it.

As we know that on moving downwards in a group size decreases in periodic table.Therefore the order of decreasing size of the cations is :

Ra2+>Ba2+>Sr2+>Ca2+.

Thanks

Size increases as you go down the periodic table and increases as you go from right to left on the table.

https://en.wikipedia.org/wiki/Periodic_trends#/media/File:Periodic_trends.svg

To arrange the cations in order of decreasing size (or increasing atomic radius), we need to consider the effective nuclear charge and shielding effect.

The effective nuclear charge increases as you move across a period from left to right in the periodic table because the number of protons in the nucleus increases. This stronger positive charge attracts the valence electrons more strongly, resulting in a smaller atomic radius.

On the other hand, the effective nuclear charge remains relatively constant within a group or column of the periodic table. This is because the increase in the number of electrons in the inner energy levels provides more shielding or protection for the valence electrons from the attractive force of the nucleus. Thus, as we move down a group, the atomic radius generally increases.

Now, let's apply this information to arrange the cations in order of decreasing size:

1. Ra2+: Radium (Ra) is the largest among the given cations. It belongs to the same group as Ba2+ but has one more proton in its nucleus, resulting in a slightly stronger attractive force. However, since both Ba2+ and Ra2+ have the same number of valence electrons, the shielding effect dominates, and the larger atomic radius of Ra2+ makes it the largest cation.

2. Ba2+: Barium (Ba) is the next largest cation. It belongs to the alkaline earth metals group, and as mentioned above, the atomic radius generally increases as we move down a group. Despite having the same charge as Sr2+ and Ca2+, Ba2+ is larger due to being lower in the periodic table.

3. Sr2+: Strontium (Sr) is smaller than Ba2+ but larger than Ca2+. It is one period above Ba2+ in the periodic table, so it has a smaller atomic radius.

4. Ca2+: Calcium (Ca) is the smallest cation among the given options. It is smaller than Sr2+ due to being in the same period but having a higher effective nuclear charge.

Therefore, the cations arranged in order of decreasing size are:
Ra2+ > Ba2+ > Sr2+ > Ca2+.