# What is periodicity? What causes the periodicity in the periodic table?

## What is periodicity?

Periodicity basically means something that happens again and again at regular intervals. You can see periodicity in different fields:

• Chemistry: This is where the term is most commonly used. The periodic table organizes elements based on their properties, and these properties tend to repeat at certain intervals across the table. This repeating pattern is periodicity. Elements in the same group (column) have similar properties because their electron configurations are similar.
• Mathematics: In mathematics, periodicity refers to functions or patterns that repeat after a certain interval. For example, the sine and cosine functions are periodic.

## What causes the periodicity in the periodic table?

The magic behind the periodicity in the periodic table lies in the arrangement of electrons around the atom's nucleus, specifically the valence electrons.

These valence electrons occupy an atom's outermost shell and are primarily responsible for an element's chemical behavior. Here's how they cause periodicity:

• Similar electron configuration, similar properties: Elements within a group (column) of the periodic table have the same number of valence electrons. This similarity in electron configuration translates to similar chemical properties.
• Repeating pattern: As you move across a period (row) in the table, the number of valence electrons increases by one. This orderly increase creates a repeating pattern in properties as you go from left to right.
• Effect on properties: Properties like atomic size, ionization energy, and electronegativity are all influenced by valence electrons. So, the repeating pattern in their number leads to a periodic trend in these properties across the table.

For example, all elements in Group 1 (the alkali metals) have one valence electron in their outer shell. This makes them all very reactive metals. As you move across Period 2 (from Lithium to Boron), the number of valence electrons goes from 1 to 3. This change is reflected in the decreasing metallic character and increasing ionization energy across the period.

So, the underlying cause of periodicity is the cyclical nature of electron filling in atomic orbitals. This cyclical filling dictates the number of valence electrons, which in turn dictates the recurring patterns in chemical properties observed in the periodic table.

Chemical and physical properties of elements return periodically, in the periodic table after a certain period. This formula is called formula of periodicity.

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