Question

What are periods in a periodic table?
What is meant by period number? What does it signify?

Answer 1

As we move from the light elements to the heavy elements, we keep the elements having similar properties across periodically, because of which we call it as a periodic table. The recurring properties are organized so that we can easily see similarities between elements. The periodic table is arranged in periods and groups.

Complete step by step answer:
Periods are the horizontal rows in a periodic table. The periodic table consists of ${\text{7}}$ periods. Elements in the same period all share something in common. They have the same number of energy shells. Each new period row represents a new shell.
The period number of elements represents how many energy levels for the element of that period are possible. It signifies the number of shells of an element.

Additional Information:
First Period: ${\text{n = 1}}$ indicates that there is only one main energy level for the elements of this period.This period has two elements ${\text{H}}$ and {\text{He}}$$$${\text{.}}
Second Period: ${\text{n = 2}}$ there are two shells in the elements of this period. It has ${\text{8}}$ elements starting from ${\text{Li}}$ to {\text{Ne}}$$$${\text{.}}
Third Period: ${\text{n = 3}}$ there are three shells in the elements of this period. It has also ${\text{8}}$ elements starting from ${\text{Na}}$ to {\text{Ar}}$$$${\text{.}}
Fourth Period: ${\text{n = 4}}$ there are four shells in the elements of this period. It has ${\text{18}}$ elements starting from ${\text{K}}$ to {\text{Kr}}$$$${\text{.}}
Fifth Period: {\text{n = 5}}$$$${\text{.}} It has ${\text{18}}$ elements starting from ${\text{Rb}}$ to {\text{Xe}}$$$${\text{.}}
Sixth Period: {\text{n = 6}}$$$${\text{.}} This period has $32$ elements, starting from ${\text{Cs}}$ to ${\text{Rn}}$.
Seventh Period: {\text{n = 7}}$$$${\text{.}} This period is an incomplete period which at present has $19$ elements starting from ${\text{Fr}}$ to ${\text{Ha}}$.

Note:
Periodic tables classify the elements on the basis of the modern periodic law i.e. the elements are arranged in the increasing order of their atomic numbers. The elements are grouped into s-block, p-block, d-block and f-block.