# According to atomic structure what determines which element will be the first and which will be the last in a period?

### Asked by Tun Tun | 15th Apr, 2017, 11:54: AM

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Consider the following examples.

For Sodium:

- First take the Atomic number of the given element. Say Na. Atomic no. 11
- Then calculate the electronic configuration using the atomic number 2+8+1=11 therefore the electronic configuration will be 2, 8, 1.
- Here we find that the no of valence electrons of element is 1 as the valence shell has 1 electron.
- So know we easily understand that the group is 1 because the no of valence electrons is 1.
- Now to find the period we have to understand that the period is the number of shell the element is having therefore here sodium has 3 shells (K-2, L-8, and M-1). So the period of the given element is 3.

Thus the position of the given element (here sodium) is ….

Group

Group 1

Valence electrons = 1

Period

3^{rd}

Number of shells = 3

When the valence shell has 1 electron, it indicates the first element of the period.

For Li: 2,1

For Na: 2,8,1

For K: 2,8,8,1

So sodium is placed in Group 1 because it contains 1 electron in the outermost shell and period 3 since it contains 3 shells.

For Chlorine:

- First take the Atomic number of the given element. Say Cl... Atomic no. 17
- Then calculate the electronic configuration using the atomic number 2+8+7=17 therefore the electronic configuration will be 2, 8, 7.
- Here we find that the no of valence electrons of element is 7 as the valence shell has 7 electrons.
- So know we easily understand that the group is 7 because the no of valence electrons is 7.
- Now to find the period we have to understand that the period is the number of shell the element is having therefore here chlorine has 3 shells (K-2, L-8, and M-7). So the period of the given element is 3.

Thus the position of the given element (here chlorine) is …..

Group

VII A (17^{th} )

Valence electrons = 7

Period

3^{rd}

Number of shells = 3

When the valence shell has 7 electrons, it indicates the last element of the period.

For F: 2,7

For Cl: 2,8,7

So chlorine is placed in Group 17 because it contains 7 electrons in the outermost shell and period 3 since it contains 3 shells.

Consider the following examples.

For Sodium:

- First take the Atomic number of the given element. Say Na. Atomic no. 11
- Then calculate the electronic configuration using the atomic number 2+8+1=11 therefore the electronic configuration will be 2, 8, 1.
- Here we find that the no of valence electrons of element is 1 as the valence shell has 1 electron.
- So know we easily understand that the group is 1 because the no of valence electrons is 1.
- Now to find the period we have to understand that the period is the number of shell the element is having therefore here sodium has 3 shells (K-2, L-8, and M-1). So the period of the given element is 3.

Thus the position of the given element (here sodium) is ….

Group |
Group 1 |
Valence electrons = 1 |

Period |
3 |
Number of shells = 3 |

When the valence shell has 1 electron, it indicates the first element of the period.

For Li: 2,1

For Na: 2,8,1

For K: 2,8,8,1

So sodium is placed in Group 1 because it contains 1 electron in the outermost shell and period 3 since it contains 3 shells.

For Chlorine:

- First take the Atomic number of the given element. Say Cl... Atomic no. 17
- Then calculate the electronic configuration using the atomic number 2+8+7=17 therefore the electronic configuration will be 2, 8, 7.
- Here we find that the no of valence electrons of element is 7 as the valence shell has 7 electrons.
- So know we easily understand that the group is 7 because the no of valence electrons is 7.
- Now to find the period we have to understand that the period is the number of shell the element is having therefore here chlorine has 3 shells (K-2, L-8, and M-7). So the period of the given element is 3.

Thus the position of the given element (here chlorine) is …..

Group |
VII A (17 |
Valence electrons = 7 |

Period |
3 |
Number of shells = 3 |

When the valence shell has 7 electrons, it indicates the last element of the period.

For F: 2,7

For Cl: 2,8,7

So chlorine is placed in Group 17 because it contains 7 electrons in the outermost shell and period 3 since it contains 3 shells.

### Answered by Vaibhav Chavan | 15th Apr, 2017, 01:23: PM

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