What patterns are observed in the periodic table?

The periodic table exhibits patterns in atomic size, ionisation energy, electronegativity, and chemical reactivity.

The periodic table is a systematic arrangement of elements, designed to illustrate recurring ('periodic') trends in the properties of elements. One of the most noticeable patterns is the increase in atomic number from left to right and top to bottom. This reflects the addition of protons and electrons as you move through the elements.

The atomic size, or atomic radius, generally decreases across a period (from left to right) and increases down a group (from top to bottom). This is due to the increasing positive charge of the nucleus, which pulls the electron cloud closer, making the atom smaller across a period. However, as you move down a group, new energy levels are added, increasing the size of the atom.

Ionisation energy, the energy required to remove an electron from an atom, generally increases across a period and decreases down a group. This is because as you move across a period, electrons are added to the same energy level, increasing the positive charge of the nucleus and making the electrons harder to remove. Conversely, as you move down a group, electrons are added to higher energy levels, further from the nucleus, making them easier to remove.

Electronegativity, the ability of an atom to attract a bonding pair of electrons, also shows a pattern. It increases across a period and decreases down a group. This is due to the increasing positive charge of the nucleus across a period, which attracts electrons more strongly, and the increasing atomic size down a group, which decreases the attraction for electrons.

Lastly, the chemical reactivity of elements also shows a pattern. Metals (on the left of the table) generally become more reactive as you move down a group, while non-metals (on the right) become less reactive. This is due to the decreasing ionisation energy down a group for metals, and the increasing ionisation energy across a period for non-metals.