Periodicity

18 Question-and-Answer flashcards covering definitions, trends, explanations, and exceptions related to periodicity and Period 3.

What is meant by “periodicity” in chemistry?

The repeating trends in the physical and chemical properties of elements observed when they are arranged in order of increasing atomic number across the periodic table.

What fundamental factor causes periodicity?

The recurring pattern of electron configurations as atomic number increases, leading to predictable changes in atomic radius, ionisation energy, electronegativity, and other properties.

Which period is commonly used to illustrate periodicity and why?

Period 3 (Na → Ar) because it includes metals, a metalloid, and non-metals that all add electrons to the same shell and show clear, contrasting trends.

What happens to atomic radius across a period from left to right?

Atomic radius decreases.

Why does atomic radius decrease across a period?

Nuclear charge increases while electrons are added to the same energy level, so shielding is constant and the stronger attraction pulls electrons closer to the nucleus.

What is the general trend in first ionisation energy across a period?

First ionisation energy generally increases from left to right.

Why do first-ionisation energies of Al and S deviate from the general upward trend?

Al’s outer electron is in a higher-energy p-orbital (easier to remove), while S has paired p-electrons that repel each other, lowering the energy needed for removal.

How does electronegativity change across a period?

Electronegativity increases from left to right.

Give two reasons electronegativity increases across a period.

Atoms become smaller (shorter bond distance) and have greater nuclear charge, so they attract shared electrons more strongly.

Describe the melting-point pattern across Period 3.

Metallic Na→Al rise due to stronger metallic bonds; Si is very high (giant covalent); P→Cl drop as simple molecules with van der Waals forces; Ar is the lowest (monatomic).

How does electrical conductivity vary across Period 3?

Na, Mg, Al are good conductors (many delocalised electrons); Si is a semiconductor; P, S, Cl are insulators; Ar is an insulator.

What is the trend in metallic character across a period?

Metallic character decreases; elements become less willing to lose electrons and more likely to gain them.

How do the oxides of Period 3 elements change in behaviour from Na to Cl?

They shift from basic (Na2O, MgO) to amphoteric (SiO2) to acidic (P4O10, SO3, Cl2O7).

Which Period 3 oxides react vigorously with water and why?

Na2O and MgO (metal oxides) because they are basic and contain highly ionic O2− ions that readily form hydroxides in water.

How does oxidation number flexibility change for non-metals across Period 3?

Maximum oxidation states increase (P, S, Cl) as the number of outer-shell electrons rises, allowing higher positive oxidation numbers in compounds.

Define a ‘group’ in the periodic table.

A vertical column whose elements share the same number of outer-shell electrons, giving them similar chemical properties.

List three reasons the periodic table is an important tool for chemists.

It lets chemists predict element properties, understand bonding/reactivity trends, compare physical properties, and classify elements into s, p, d, and f blocks.

Quick recap: what three key factors drive all periodic trends?

Increasing nuclear charge, constant (or gradually changing) shielding within a period, and the resulting changes in attraction between nucleus and electrons.