Question

a.In the modern periodic table, which are the metals the first ten elements
b.What is the significance of atomic number in the modern classification of elements, Explain with the help of an example.

Answer 1

First we know that the modern periodic table is a tabular arrangement of chemical elements according to their atomic numbers, configuration of electrons and chemical properties. In this table there are $7$ periods (rows) and $18$ groups(columns) in the table.
A metal is an element that forms positive ions (cations) and has metallic bonds. Hence, an element is said to be a metal if it loses electrons.

Complete answer:
(a)
The first ten elements in the periodic table are Hydrogen, Helium, Lithium, Beryllium, Boron, Carbon, Nitrogen, Oxygen, Fluorine, Neon.
The atomic number of lithium is $3$. Lithium can lose one electron, making it a lithium cation. It now has more positive protons than electrons so it has an overall positive charge. Hence lithium is a cation (positive ion). Similarly, beryllium has atomic number $4$ and it forms $+ 2$ cations by losing two electrons to other elements.
Hence out of the first ten elements Lithium and Beryllium are two elements which are metals in the modern periodic table.
(b)
Using the atomic number of an atom we can easily identify the element of an atom. The main significance of atomic number is that the elements in the modern periodic table are organized according to increasing atomic number (or electronic configuration). Also, it gives the periodicity of the properties of the elements and relates them to their electronic configurations.
Example: The atomic number increases from $3$ of Lithium to $11$ of sodium, there is a repetition of electronic configuration from $2,1$ to $2,8,1$. It means both having one valence electron.

Note:
Note that the elements lying on the left side of the periodic table tend to lose electrons and those lying on the right side tend to gain electrons. Loss of an electron is called oxidation, whereas gain of an electron is called reduction.