**PERIODS**

Each horizontal row in the Modern Periodic table is called a

**. In all there are seven periods denoted by numbers 1 to 7. Each period starts with filling of ns orbitals and ends when ‘np’ orbitals are complete. The number of period represents the principle quantum number of valence shell of element present in it.***Period*
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*1*^{st}Period
The first period corresponds to the filling of electrons in first energy shell (i.e. n = 1). This energy level has only one orbital (i.e. 1s) and therefore, it can accommodate two electrons. This means that there are only two elements in the first period.

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*2*^{nd}Period
The second period starts with electrons beginning to enter the second energy shell (n = 2). Since there are only four orbitals in this shell (2s, 2p

_{x}, 2p_{y}, 2p_{z}) to be filled which can accommodate eight electrons. Thus, second period has eight elements.
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*3*^{rd}Period
In this period, filling of third shell (n = 3) takes place. Third shell consists of nine orbitals. But the five 3d orbitals have higher energy than 4s orbitals. As such only four orbitals (one 3s & three 3p) of third shell are filled before filling of fourth shell. Hence there are only eight elements in third shell).

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*4*^{th}Period
The fourth period corresponds to n = 4. It starts with the filling of 4s – orbitals & 4p orbitals. However, after 4s orbitals but before 4p orbitals, there are five 3d orbitals to be filled. Thus in all nine orbitals (one 4s, five 3d & three 4p) are to be filled and as such there are Eighteen elements in fourth period.

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*5*^{th}Period
The fifth period corresponds to n = 5. It is similar to fourth period. There are nine orbitals (one 5s, five 4d and three 5p) to be filled. Therefore there are Eighteen elements as well in fifth period.

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*6*^{th}Period
This period starts with filling of 6s orbitals and ends with 6p. But in between that 4f-orbitals and 5d–orbitals are to be filled. So, there are sixteen orbitals in all (one 6s, seven 4f, five 5d & three 6p) to be filled. Thus, there are Thirty-two elements in sixth period.

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*7*^{th}Period
This period starts with filling of 7s and ends with 7p. But in between 5f, 6d are to be filled. So, this too consists of sixteen orbitals to be filled. So, it should contain thirty two elements. But it is still incomplete.

It may be noted that periods 2 and 3 contain 8 elements each and are called

**. There are 18 elements each in 4***Short Periods*^{th}& 5^{th}period and they are called**. Sixth period containing 32 elements is called***Long Periods**Longest Period**while seventh is incomplete.*
The sixth & seventh periods contains a group of 14 elements called

**&***Lanthanides***respectively. If all Lanthanides and actinides are placed horizontally in the same group, there will be undue expansion of the periodic table. Hence they are placed in the separate horizontal row at the bottom of Main periodic table.***Actinides*
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