Periodicity

Chapter 7

Periodic table

• Elements are arranged by increasing atomic (proton) number

• Periods (rows) show repeating patterns in physical and chemical properties (periodicity)

• Groups (columns) contain elements with similar chemical properties

• Elements in the same group have the same number of outer-shell electrons, giving similar reactions and bonding behaviour

• Elements in the same period have same number of shells

Periodic trends: electron configuration across Periods 2 and 3

• Across a period, proton number increases by 1 for each element

• Electrons are added to the same principal energy level (shell)

• Number of outer-shell electrons increases by 1 across the period

• Period 2 fills: 2s → 2p

• Period 3 fills: 3s → 3p

• This repeating pattern in electron configuration causes periodic trends in chemical and physical properties

Classification of elements into blocks

• Elements are classified by the sub-shell being filled by the highest-energy electron

• s-block: highest-energy electron enters an s-sub-shell (Groups 1 and 2)

• p-block: highest-energy electron enters a p-sub-shell (Groups 13–18)

• d-block: highest-energy electron enters a d-sub-shell (transition elements)

• Block position is determined by electron configuration.

D block vs Transition elements

• d-block elements → elements with the highest-energy electron entering a d-sub-shell

• Transition elements → d-block elements that form at least one ion with an incomplete d-subshell

First ionisation energy & successive ionisation energy

First ionisation energy → energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1⁺ ions

Successive ionisation energies → energy required to remove further electrons one after another from gaseous ions

Trends in first ionisation energy

Across a period:
increases because nuclear charge increases, electrons added to same shell thus similar shielding and stronger attraction between greater psoitive nucleus and outer electrons and smaller atomic radius so more nuclear attractions and thus less energy needed to become ionised

Down a group:
Decreases, more shells added increases shielding larger atomic radius otuer electorn further from nucleus weaker attraction lower ionisation energy

Exceptions to trends in ionisation energy

• Be → B: decrease because electron enters higher-energy p-subshell

• N → O: decrease due to electron repulsion in paired p orbitals

Using successive ionisation energies

• Look for a large jump in ionisation energy values

• Large jump means an electron is removed from a new shell closer to the nucleus

• Number of electrons removed before the jump = number of outer-shell electrons

• Number of outer-shell electrons can be used to determine the group of the element

Ionisation energy link to bohr model

• The Bohr model proposes electrons occupy fixed energy levels (shells) around the nucleus

• Trends in ionisation energy show electrons are not all the same distance from the nucleus

• Large jumps in successive ionisation energies indicate removal of electrons from a new inner shell

Mettalic bonding

All metals are formed by a giant mettalic lattice with mettalic bonds

This is the strong froces of electrostatic attraction between a sea of delocalised electrons and positive metal ions (cations)

Giant covalent lattices:

• Giant covalent lattices are networks of atoms joined by strong covalent bonds throughout the structure

• Require large amounts of energy to break many covalent bonds → very high melting points

Diamond

• Each carbon bonded to 4 others in a tetrahedral structure

• No delocalised electrons → does not conduct electricity

• Very hard due to strong covalent bonding throughout

Graphite

• Each carbon bonded to 3 others in layers

• One delocalised electron per carbon atom

• Conducts electricity

• Weak forces between layers allow layers to slide → soft/slippery

Graphene

• Single layer of graphite

• Strong, lightweight and flexible

• Conducts electricity due to delocalised electrons

• 3 carbon bonded and 1 delocalised electron

Silicon

Giant covalent lattice mad eof 4 silicon atoms bonded to each other in tetrahedral shape

Graphene: properties, applications & benefits

Potential applications:

electronics and touchscreens
batteries and solar cells
composite materials
sensors and conductive coatings

Benefits:

high strength-to-weight ratio
excellent electrical conductivity
flexible and durable material for new technologies

Physical properties of giant covalent lattices

Structure: network of atoms joined by strong covalent bonds throughout the lattice

Melting/boiling points: very high because many strong covalent bonds must be broken

Solubility: insoluble in water and most solvents because solvent hydrogen bonds interactions cannot overcome covalent bonds

Electrical conductivity: usually do not conduct because there are no mobile charged particles apart from graphite and graphene conduct due to delocalised electrons

Physical properties of giant metallic lattices

Structure: positive metal ions in a lattice surrounded by delocalised electrons

Melting/boiling points: generally high due to strong electrostatic attraction between ions and delocalised electrons

Solubility: generally insoluble because metallic bonding is not broken by solvents

Electrical conductivity: conduct in solid and liquid states because delocalised electrons are free to move

Malleability/ductility: layers of ions can slide while metallic bonding remains intact through attraction to delocalised electrons