24. Exercises XV

 

The modern periodic table is based on the atomic massatomic particleatomic volumeatomic number, which is directly related to the electronic configuration. One can find the group number and period number of an element based on electronic configuration. For example, if an element has $6$ or $7$$3$ or $4$$1$ or $2$$4$ or $5$ electrons in its outermost shell, it would belong to group $1$ or $2$. And if it has $3$ or more electrons in its outermost shell, it would belong to group 11101312 for the number of electrons in the outermost shell.

 

All the alkali metals have one electron in their outermost shell, so they are placed in group $1$. Thus, all the group $2$ elements have 1523 electrons in their outermost shell. In group $15$ elements, there are $5$ electrons in their outermost shell. Similarly, the number of shells in an element indicates its period number. For example, the atomic number of sodiumsiliconaluminummagnesium is $12$, and its electronic configuration is $2$, $8$, $2$. Thus it is an element of the $3$rd period.