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# Main structure of to the periodic table

## Organisation of the periodic table

A simplified version of the periodic table. The group numbers are shown above and period numbers shown on the left.

The periodic table arranges the chemical elements according to the atomic numbers and recurring chemical properties.

Atomic mass increases across the period and down the group.

The elements are arranged from left to right and top to bottom in the order of increasing atomic number (the number of protons in an atom).

Elements found between groups 2 and 3 are classified as transition elements and usually do not follow the trends in chemical and physical properties discussed in this section.

## Periods

The horizontal rows of the periodic table are called periods. The atomic number increases from left to right in a period.

Atoms in the same period have the same number of electron shells.

Nitrogen ($\ce{N}$), oxygen ($\ce{O}$) and fluorine ($\ce{F}$) all belong to the second period. All three elements have their valence electrons found in the second electron shell.

The period in which an element is found reflects the number of the electron shell holding the valence electrons.

Beryllium is found in the second period, and so its valence electrons are found on the second electron shell. Magnesium and calcium are found in the third and fourth period respectively.

## Groups

The columns (vertical) of the periodic table are called groups. Groups can be labelled by using Arabic numbers (1, 2, 3, etc.) or Roman numerals (I, II, III, etc.).

The atomic number increases as one moves from top to bottom in a group.

Elements in the same group have atoms with the same number of valence electrons. These elements cannot be found in the same period and their valence electron shells are never of the same number.

$\ce{Li}$, $\ce{Be}$, $\ce{N}$ and $\ce{O}$ belong to groups 1, 2, 5 and 6 respectively.

The number of valence electrons of each atom corresponds to their group numbers. Atoms of group 0 do not follow this, as they each have eight valence electrons.

As valence electrons are responsible for chemical bonding, atoms in the same group have similar reactions and bonding patterns.