Periods and Electronic Configuration

The periodic table is arranged in such a way that elements with similar properties are grouped together. One of the key factors that determine an element’s chemical behaviour is its electronic configuration, the arrangement of electrons in its atomic shells.

New period, new shell

Periods and Shells:

A period in the periodic table consists of elements that have the same number of electron shells. As we move from left to right within a period, the atomic number increases by one, meaning each consecutive element has one more electron than the previous one.

Electronic Configurations of the First Three Periods:

Filling of Electrons in a Period:

Within a period, electrons are added to the same outermost shell as we move from left to right. After reaching the octet (8 electrons in the outer shell), a new shell starts filling in the next period.

Electron Holding Capacity of Shells:

• The K shell (first shell) can hold a maximum of 2 electrons (1st period).
• The L shell (second shell) can hold 8 electrons (2nd period).
• The M shell (third shell) starts filling but follows the octet rule, leading to 8 elements in the 3rd period.

Law of Electron Octet:

Atoms tend to achieve stability by having 8 electrons in their outermost shell, making the octet rule a key factor in chemical reactivity. This explains why the 2nd and 3rd periods contain only 8 elements each, despite the M shell having the capacity for more electrons.

Valence Electrons and Reactivity:

• The number of valence electrons (electrons in the outermost shell) determines how an element reacts with others.
• Elements in the same group have the same number of valence electrons and therefore show similar chemical properties.
• Example: All elements in Group 1 (Li, Na, K, etc.) have 1 valence electron, making them highly reactive metals.

Position in the Periodic Table:

• Metals (left side): Fewer valence electrons, tend to lose electrons easily, making them good conductors.
• Nonmetals (right side): More valence electrons, tend to gain electrons, forming negative ions.
• Noble gases (Group 18): Have full outer shells, making them chemically inert.