How to find the position of the elements in the modern periodic table

a) Finding Period of Elements:

Period of the element is equal to highest energy level of electrons or principal quantum number. Look at following examples for better understanding;

₁₆S: 1s²2s²2p⁶3s²3p⁴ 3 is the highest energy level of electrons or principal quantum number. Thus period of S is 3.

₂₃Cr: 1s²2s²2p⁶3s²3p⁶4s²3d⁴ 4 is the highest energy level of electrons or principal quantum number. Thus period of Cr is 4.

b) Finding Group of Elements:

Group of element is equal to number of valence electrons of element or number of electrons in the highest energy level of elements. Another way of finding group of element is looking at sub shells. If last sub shell of electron configuration is "s" or "p", then group becomes A.

₁₉K: 1s²2s²2p⁶3s²3p⁶4s¹ Since last sub shell is "s" group of K is A.

₃₅Br: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁵ Since last sub shell is "p" group of Br is A.

Elements in group B have electron configuration ns and (n-1)d, total number of electrons in these orbitals gives us group of element. Look at following examples.

₂₆Fe: 1s²2s²2p⁶3s²3p⁶4s²3d⁶ 6+2=8 B group

Here are some clues for you to find group number of elements.

Last Orbital Group

ns¹ 1A

ns² 2A

ns²np¹ 3A

ns²np² 4A

ns²np³ 5A

ns²np⁴ 6A

ns²np⁵ 7A

ns²np⁶ 8A

Last Orbital Group

ns²(n-1)d¹ 3B

ns²(n-1)d² 4B

ns²(n-1)d³ 5B

ns²(n-1)d⁴ or ns¹(n-1)d⁵ 6B

ns²(n-1)d⁵ 7B

ns²(n-1)d⁶ 8B

ns²(n-1)d⁷ 8B

ns²(n-1)d⁸ 8B

ns²(n-1)d⁹ or ns¹(n-1)d¹⁰ 1B

ns²(n-1)d¹⁰ 2B

Example: Find period and group of ₁₆X.

₁₆X: 1s²2s²2p⁶3s²3p⁴

3. period and 2+4=6 A group

Example: Find period and group of ₂₄X.

₂₄X:1s²2s²2p⁶3s²3p⁶4s²3d⁴

4. period and 4+2=6 B group

Groups and Periods of elements are found according to their neutral states. Ions and isotopes of elements are not shown in periodic table.

Example: If X⁺² ion has 10 electrons, find its group and period number.

Number of protons=10 + 2=12

In neutral element, number of proton is equal to number of electrons. Thus, X has 12 electrons in neutral state. We write electron configuration according to neutral state of element.

₁₂X=1s²2s²2p⁶3s²

Period number is 3

Group number is 2 and group is A(last orbital is "s")

Example: If electron configuration of X+5 is 1s²2s²2p⁶3s²3p⁶, which one of the following statements are true for X element.

I. Period number of X is 4 and it is transition element

II. X is metal

III. Valence electrons of X are in "s" and "d"

Neutral X element has electron configuration;

X: 1s²2s²2p⁶3s²3p⁶4s²3d³

X is in 4. period and 3+2=5 B group.

Thus, it is metal and all the statements I. II. and III. are true.