In the periodic table the element with atomic number 38 belongs to:a)P...
The answer is very simple, I just calculated it, nothing else. For that I firstly subtracted 18 from it and then 8 and again 8, I got 4 and i very well knew that the element having atomic number 4 belong to the 2 group of the periodic table.I got the group but for the period i calculated how many times i subtracted any number from the given atomic no.,i got 3 and i added this 3 to the period of the element of atomic number 4 i.e. 2 i.e.2+3=5. So finally i got 5.Hence i concluded that the element is of the 2 nd group and 5 th period.
Now the question must be arising in your mind that why i subtracted 18 and then 8 and again 8.Let me tell you that.I knew that the difference between the atomic numbers of the 2nd period and 3rd period is 8 and similarly 3rd & 4th-also 8 , 4th & 5th-18,5th & 6th-18 only for first three groups then 32,6th &7th-again 32.
Now why only subtracted 18 from 38 ,why not 8 or 32?Here the answer is also very simple. Now if you get any number between 1 and 10 to tell its period and group you can say it orally.But the main juice is after 10.If the number comes between then 10 and 20 you easily have to subtract 8 and then tell the answer(but remember you have to 2 times 8 from element having atomic number 19 or 20).Now between 21 to 57 you should subtract 18 and then 2 times 8.Now after 57 you first have to subtract 32 then 18 and then 2 times 8.
This is my way , how i do this type of problem sometimes.
This way is not applicable for the F block.But for this block you should remember that all the elements are in 3rd group and if the atomic number of the required element is between 57 and 71 then it would be of 6th period and if it is between 89 to 103 it would be of 7th period.