Periodic Trend
Periodic trends are systematic patterns in elemental properties that arise from the regular variation in nuclear charge and electron configuration across periods and down groups of the periodic table. Key periodic trends include atomic radius, ionisation energy, electron affinity, electronegativity, and metallic character, all of which change predictably as atomic number increases. Understanding periodic trends allows chemists to predict chemical reactivity, bond types, and physical properties of elements and their compounds without needing to memorise individual data for every element.
Major periodic trends and their directions
| Property | Across a Period (→) | Down a Group (↓) | Reason |
|---|---|---|---|
| Atomic Radius | Decreases | Increases | More protons pull electrons in; more shells add distance |
| Ionisation Energy | Increases | Decreases | Harder to remove electron from smaller, more nuclear atom |
| Electronegativity | Increases | Decreases | Greater nuclear attraction for bonding electrons |
| Electron Affinity | Generally increases | Generally decreases | Easier to add electron to compact outer shell |
| Metallic Character | Decreases | Increases | Easier to lose electrons → more metallic |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Periodic Table
The periodic table is a tabular arrangement of all known chemical elements ordered by increasing atomic number, with elements having similar chemical properties placed in vertical columns called groups. It was developed by Dmitri Mendeleev in 1869 and serves as the foundational reference for all of chemistry. The table reveals periodic trends in elemental properties such as atomic radius, ionisation energy, and electronegativity, enabling scientists to predict the behaviour of elements and their compounds.
Electronegativity Trend
Electronegativity is a measure of the tendency of an atom to attract a shared pair of electrons towards itself in a covalent bond, and its periodic trend describes how this property changes systematically across the periodic table. Electronegativity increases across a period (left to right) because increasing nuclear charge pulls bonding electrons more strongly, and decreases down a group because the bonding electrons are further from the nucleus and shielded by additional inner electron shells. On the Pauling scale, fluorine is assigned the highest electronegativity value of 3.98, making it the most electronegative element, while caesium and francium have the lowest values near 0.79.
Chemical Period
A chemical period is a horizontal row in the periodic table, in which all elements have the same number of electron shells (principal quantum levels). There are seven periods in the modern periodic table, corresponding to the seven principal energy levels occupied by electrons in known elements. Moving across a period from left to right, the atomic number increases by one with each step, leading to systematic changes in properties such as metallic character, atomic radius, and ionisation energy.
From Greek "periodos" (recurring cycle) + Latin "tendere" (to stretch, to tend toward). The concept of periodic trends emerged from Mendeleev's 1869 periodic law, which stated that the properties of elements are a periodic function of their atomic weights (now understood as atomic number).