The periodic table is shaped the way it is because it reflects the periodic law, which states that elements arranged by increasing atomic number show a periodic recurrence of chemical properties. Its shape specifically mimics the filling of electron shells and subshells in atoms, with rows (periods) representing new electron shells starting to fill and columns (groups) containing elements with similar valence electron configurations and thus similar chemical properties.

Electron Configuration Basis

• The shape of the periodic table corresponds directly to the electron configurations of elements. For example, the first period has two elements filling the 1s orbital, the second and third periods fill the 2s and 2p, and 3s and 3p orbitals respectively.
• Transition metals in the middle section fill d orbitals, which is why there is a block of 10 elements in those groups.
• The layout groups elements with the same number of outer (valence) electrons into columns, explaining chemical similarities among groups.

Periods and Groups

• Each horizontal row (period) begins when a new electron shell starts filling, explaining the length and arrangement of periods.
• Each vertical column (group) contains elements with similar valence electron arrangements, resulting in similar chemical behavior, which is why they are grouped vertically.

Practical Arrangement

• Noble gases form the right edge of the table because their outer electron shells are full, leading to chemical inertness.
• The table is organized to visually separate elements with differing chemical properties into distinct columns and blocks (s, p, d, f blocks), leading to its distinctive stepped shape.

Thus, the periodic table's shape is a direct consequence of the atomic structure of elements and the periodic recurrence of their chemical properties as electrons fill atomic orbitals.