Explain the Modern Periodic Law and the present form of the periodic table.

(N/A) Work of Moseley and Modern Periodic Law:
- When Mendeleev developed his periodic table,chemists knew little about the internal structure of the atom. The beginning of the $20^{th}$ century witnessed profound developments in theories about sub-atomic particles.
- In $1913$,the English physicist Henry Moseley observed regularities in the characteristic $X$-ray spectra of the elements. He established the relation $\sqrt{v} = a(Z - b)$,where $v$ is the frequency of the emitted $X$-ray and $Z$ is the atomic number of the element.
$(i)$ $A$ plot of $\sqrt{v}$ versus $Z$ yields a straight line.
$(ii)$ $A$ plot of $v$ versus $Z$ does not yield a straight line.
- He thereby showed that the atomic number is a more fundamental property of an element than its atomic mass.
- Modern Periodic Law: "The physical and chemical properties of the elements are periodic functions of their atomic numbers."
$(B)$ Modern Periodic Table: The modern periodic table contains $7$ periods and $18$ groups.
- Period: $A$ horizontal row in the periodic table is known as a period.

$Period$	$1, 2, 3, 4, 5, 6, 7$
$Number$ of elements	$2, 8, 8, 18, 18, 32, 32$

- Groups: Vertical columns in the periodic table are known as groups.
- According to the recommendation of the International Union of Pure and Applied Chemistry $(IUPAC)$,the groups are numbered from $1$ to $18$,replacing the older notation of groups ($IA$ to $VIIIA$,$IB$ to $VIIIB$).