Higher Chemistry SQA

Chemical Changes and Structures: Periodicity

Chemical Changes and Structures: Periodicity

Understanding Periodicity

• Periodicity refers to the repeating patterns seen in the chemical and physical properties of elements arranged in the periodic table.
• Periodicity arises due to the repeating pattern of outer, or valence, electron configurations of elements.
• Trends in the periodic table are often described as being 'across a period' (left to right) or 'down a group' (top to bottom).

Trends Across a Period

• Moving across a period, atomic size decreases due to increased nuclear charge pulling electrons closer to the nucleus.
• Ionisation energy (the energy required to remove an electron from an atom) generally increases across a period. This arises from decreasing atomic size and increasing nuclear charge.
• Electronegativity (ability of an atom to attract a bonding pair of electrons) generally increases across a period due to increased nuclear charge and decrease in atomic size.

Trends Down a Group

• Moving down a group, atomic size increases due to the addition of electron shells.
• Ionisation energy decreases down a group due to the increased atomic size and shielding effect.
• Electronegativity decreases down a group since the bonding pair of electrons is further from the nucleus and more shielded.
• Down a group, there is generally an increase in reactivity for metals and a decrease in reactivity for nonmetals.

Specific Group Trends

• Group 1 elements (alkali metals) are characterised by a single electron in their outermost energy level, making them highly reactive metals.
• Group 17 elements (halogens) have seven electrons in their outer shell, making them highly reactive non-metals.
• Group 18 elements (noble gases) have full outer electron shells, making them very stable and unreactive.

Interpretation of Periodic Trends in Reactivity and Bonding

• The reactivity of elements and the types of bonding they form (ionic, covalent or metallic) can often be predicted through understanding periodic trends.
• The repetitive trends observed in the periodic table are immensely useful in predicting the behaviour of undiscovered or rarely encountered elements.