Question

Identify the odd one out.
Barium, Radium, Calcium, Francium.

Answer 1

The element which is the odd one out is a member of the first group of the periodic table. In other words, the odd element is an alkali metal. It has the atomic number 87.

Complete step by step answer:
We know that the elements in the periodic table are divided into four blocks. These four blocks of the periodic table are known as s, p, d, and f blocks. This classification of the periodic table into four blocks is on the basis of the type of orbital (s or p or d or f) into which the last electron of an atom of the element enters.
Now, the s – block elements is the block which is situated on the extreme left of the periodic table. The elements of this block are called the s – block elements as when their electronic configurations are written, the last electron enters the s orbital. This block comprises two groups. These two groups are group 1 (or group IA) and group 2 (or group IIA).
The first group of the periodic table consists of six elements and they are lithium, sodium, potassium, rubidium, cesium and francium. These elements are also called alkali metals as their hydroxides are very strong bases or alkalis. The atomic numbers of lithium, sodium, potassium, rubidium, cesium and francium are 3, 11, 19, 37, 55 and 87 respectively. They all have only 1 electron in their outermost s orbital.
The second group of the periodic table consists of six elements and they are beryllium, magnesium, calcium, strontium, barium and radium. The atomic numbers of beryllium, magnesium, calcium, strontium, barium and radium are 4, 12, 20, 38, 56 and 88 respectively. They all have 2 electrons in their outermost s orbital. These elements are also known as alkaline earth metals as their oxides are alkaline in nature.
Hence, francium is an alkali metal and the rest are alkaline earth metals. So, the odd one out is francium.

Note:
Among the alkali metals, lithium is found to be widely distributed in nature. But the amount present in minerals is always small. Some important ores of lithium are lepidolite and petalite. On the other hand, sodium being highly reactive is not found in the native state but is widely distributed in nature in the combined state. Among the alkaline earth metals, only magnesium and calcium occurs in considerable amounts in the earth’s crust and radium is radioactive in nature.