Periodicity and Period 3 Trends

15 question-and-answer flashcards covering definitions, explanations, exceptions, and trend summaries for Period 3 and general periodicity concepts.

What is the periodic table?

A tabular arrangement of the chemical elements in order of increasing atomic number that displays repeating (periodic) trends in their physical and chemical properties.

What is meant by “periodicity” in chemistry?

The recurring patterns or trends in physical and chemical properties that appear at regular intervals when elements are arranged by increasing atomic number.

List the four main periodic trends observed across Period 3 (Na → Ar).

1) Atomic radius decreases. 2) First ionisation energy generally increases. 3) Electronegativity increases. 4) Melting/boiling points and electrical conductivity change according to structure and bonding.

Why does atomic radius decrease from Na to Ar?

Nuclear charge increases while added electrons enter the same shell, so shielding remains almost constant; the greater nuclear attraction pulls electrons closer, reducing atomic radius.

Why does the first ionisation energy generally increase across a period?

Greater nuclear charge with no extra shielding means stronger attraction between the nucleus and outer electron, so more energy is required to remove an electron.

Which two elements in Period 3 deviate from the ionisation-energy trend and why?

Aluminium (electron removed from a higher-energy p-orbital) and sulfur (paired p-electrons repel each other) both have slightly lower first ionisation energies than expected.

How and why does electronegativity change across a period?

It increases because atoms have smaller radii and higher nuclear charge, allowing them to attract bonding electrons more strongly; metals like Na are least electronegative, non-metals like Cl are most.

Explain the overall pattern of melting and boiling points across Period 3.

They rise from Na to Si (stronger metallic then giant covalent bonding) and fall sharply from P to Ar (simple molecules or atoms held by weak van der Waals forces).

Why do Na, Mg and Al show increasing melting points?

All are metallic; higher charge density and more delocalised electrons from Na (+1) to Al (+3) strengthen metallic bonding, raising melting points.

Why is the melting point of S₈ higher than that of P₄ and Cl₂?

S₈ molecules are larger, so their van der Waals forces are stronger than those in P₄ or Cl₂, requiring more energy to melt.

How does electrical conductivity vary across Period 3?

Good conductors: Na, Mg, Al (delocalised electrons). Silicon: semiconductor. Poor conductors: P, S, Cl, Ar (no free electrons).

Contrast the chemical reactivity of metals versus non-metals in Period 3.

Metals (Na, Mg, Al) are more reactive with water/oxygen and form basic oxides; non-metals (P, S, Cl) form acidic oxides and react with metals or hydrogen; Si is relatively unreactive due to its giant covalent structure.

What is a periodic group, and what do its elements have in common?

A vertical column in the periodic table whose elements possess the same number of outer-shell electrons, giving them similar chemical properties; the group number equals the number of these electrons.

Give three ways the periodic table helps chemists predict element behaviour.

It allows prediction of chemical reactivity, identification of trends in physical properties, and determination of likely oxidation states (also helps recognise isoelectronic species and bonding types).

Provide a concise summary of five key trends across Period 3.

Atomic radius decreases; first ionisation energy increases (with small drops at Al and S); electronegativity increases; melting point rises to Si then falls sharply; electrical conductivity is high for Na–Al and very low from Si onwards.