Periodic Properties and Variations of Properties - Physical and Chemical

The One Thing

Periodic properties recur because electron configurations recur when elements are arranged in increasing atomic number. Changes in nuclear charge, electron shells, shielding effect, and effective nuclear charge explain the principal trends across periods and down groups.

Reactions, Processes and Experiments

Key Terms

• Modern periodic law: The physical and chemical properties of elements are periodic functions of their atomic numbers.
• Period: A horizontal row in the periodic table. The period number indicates the number of occupied electron shells in an atom.
• Group: A vertical column in the periodic table. Elements in a group generally have similar valence-shell electronic configurations and related chemical properties.
• Valence electrons: Electrons present in the outermost shell of an atom; they largely determine chemical reactivity and bonding.
• Atomic radius: A measure of atomic size, commonly considered as half the distance between the nuclei of two bonded identical atoms.
• Ionisation potential: The minimum energy required to remove the most loosely held electron from an isolated gaseous atom in its ground state.
• Electron affinity: The energy change when an isolated neutral gaseous atom accepts an electron to form a negative ion.
• Electronegativity: The tendency of an atom in a chemical bond to attract the shared pair of electrons towards itself.
• Shielding effect: The reduction in the attractive force between the nucleus and outer electrons because inner-shell electrons partially block the nuclear pull.
• Effective nuclear charge: The net positive charge experienced by an outer electron after considering the shielding effect of inner electrons.
• Metallic character: The tendency of an atom to lose electrons and form positive ions.
• Non-metallic character: The tendency of an atom to gain or share electrons and form negative ions or covalent bonds.
• Atomic number: The number of protons in the nucleus of an atom; it determines the identity of the element.
• Valency: The combining capacity of an atom, usually related to the number of electrons lost, gained, or shared to achieve a stable outer shell.

Easily Confused

• Period and group: A period is a horizontal row, and its number indicates occupied electron shells; a group is a vertical column whose elements generally have similar outer-shell configurations.
• Atomic number and mass or size: Atomic number is the number of protons and determines element identity; atomic radius measures atomic size.
• Ionisation potential and electron affinity: Ionisation potential concerns removing an electron from a gaseous atom; electron affinity concerns the energy change when a gaseous atom accepts an electron.
• Electronegativity and electron affinity: Electronegativity is an atom’s attraction for shared electrons in a bond; electron affinity concerns acceptance of an electron by an isolated gaseous atom.
• Metallic and non-metallic character: Metallic character is the tendency to lose electrons, whereas non-metallic character is the tendency to gain or share electrons.
• Cations and anions: Cations are positive ions formed by electron loss and are smaller than their parent atoms; anions are negative ions formed by electron gain and are larger than their parent atoms.
• Across a period and down a group: Across a period, effective nuclear charge generally increases while shell number remains constant; down a group, shell number and shielding effect increase.
• Group 1 and Group 17 reactivity: Group 1 reactivity generally increases down the group because electron loss becomes easier; Group 17 reactivity generally decreases because electron gain becomes less favourable.
• Basic, amphoteric, and acidic oxides: Basic oxides occur generally on the left, acidic oxides on the right, and amphoteric oxides between them across a period.
• Valence electrons and valency: Valence electrons are outer-shell electrons; valency is the combining capacity related to electrons lost, gained, or shared.

What Gets Asked

• Questions may ask for the modern periodic law, the structure of the modern periodic table, or the meanings of periods and groups. Marks are lost by confusing horizontal periods with vertical groups or by omitting that the table is arranged by atomic number.
• Trend questions may require comparison of atomic radius, ionisation potential, electron affinity, electronegativity, metallic character, or non-metallic character across a period and down a group. Marks are lost by reversing the two directions.
• Explanations may require reference to nuclear charge, electron shells, shielding effect, and effective nuclear charge. A trend is inadequately explained if these controlling factors are replaced by a bare statement of increase or decrease.
• Questions may compare the sizes of atoms, cations, and anions. Marks are lost by stating that cations are larger than their parent atoms or that anions are smaller.
• Chemical-property questions may ask why group members show similar reactions or why Group 1 and Group 17 reactivities change differently down their groups. The key distinction is shared valence-electron configuration, together with the ease of electron loss or gain.
• Questions may ask about oxide character or representative-element valency across a period. Marks are lost by omitting amphoteric oxides between basic and acidic oxides or by failing to state the pattern from 1 to 4 and then from 4 to 0.