Question

Why does barium react more rapidly with cold water than magnesium does?
A. Barium atoms are larger and form ions more easily than magnesium ions.
B. Barium floats on the surface of the water but magnesium sinks in the water.
C. Barium hydroxide is less soluble than magnesium hydroxide.
D. The sum of the 1st and 2nd ionization energies of barium is more than that for magnesium.

Answer 1

In this question, we need to comment on the reason behind the rapid reaction of Barium in cold water in comparison with the reaction of magnesium in cold water. For this, we will go through the atomic size and arrangements of the atomic structure.

Complete answer:
On going down the group the size of the atom decreases. The increase in size is due to the addition of one extra orbit in that atom. This addition of extra orbit increases the energy level for that atom. And thus, it results in an increase in the atomic size of that atom. Since barium is placed lower than magnesium i.e., at the bottom in the second group of periodic table, therefore barium atom are larger in size than magnesium atom and hence barium atoms loses its valence electron more readily than magnesium atom and forms positive ions i.e., cation. This is because in going down the group, the metals become more likely to lose an electron because the electron being removed lies increasingly farther from the positive nucleus. That is, the electron lost from the barium atom to form barium ion lies at a much greater distance from the attractive positive nucleus, and is thus easier to remove than the electron that must be removed from a magnesium atom to form magnesium ion. This is the reason why barium reacts more rapidly with cold water than magnesium.

**So option A, is the correct answer.

Note: **
Larger on going down the group the ionization energy decreases and hence greater is the ease in losing electrons for an atom. (Alternative way to do it). On moving down the group, new shells are being added. The nuclear charge is not enough to reduce the size enough. So the radius increases as we move down the group. In other words, we can say that larger the size of an atom, easier is to lose its valence electron for that atom.