The periodic table
Explain H1 Chemistry Period 3, Group 1 and Group 17 trends and reactions, then use property data to deduce unfamiliar elements.
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Before you begin
The Periodic Table arranges the elements so that their properties repeat in a pattern. This topic explains those patterns using electronic structure: trends in size, ionisation energy and electronegativity, the structures and properties of the Period 3 elements and their oxides and chlorides, and the chemistry of Group 1 and Group 17. It ends by using the patterns to identify unknown elements.
Be comfortable with: electron configurations, ionisation energies, structure and physical properties, and redox and half-equations.
Learning goals
- Recognise how electronic configurations change across a period and down a group
- Explain trends in atomic radius, ionic radius, first ionisation energy and electronegativity across a period using shielding and nuclear charge
- Explain trends in atomic radius, ionic radius, first ionisation energy and electronegativity down a group using electron shells, shielding and nuclear charge
- Explain how melting point and electrical conductivity vary across a period using the structure and bonding of the elements
- Explain the trend in volatility of the Group 17 elements using instantaneous dipole–induced dipole attractions
- Explain how the highest oxidation number varies in the Period 3 oxides and chlorides
- Explain how bonding in the Period 3 oxides and chlorides varies using electronegativity
- Describe how the Period 3 oxides react with water
- Explain the acid–base behaviour of the Period 3 oxides and hydroxides, including amphoteric behaviour
- Explain how the Period 3 chlorides react with water
- Suggest the structure and bonding in Period 3 oxides and chlorides from their properties
- Explain the reactivity of the Group 1 elements as reducing agents by how easily they lose electrons
- Explain the reactivity of the Group 17 elements as oxidising agents by how easily they gain electrons
- Explain the trend in thermal stability of the Group 17 hydrides using bond energies
- Predict the properties of an element in a given group from periodic trends
- Deduce the nature, likely position and identity of an unknown element from its properties
Syllabus statements covered
- recognise variation in the electronic configurations across a Period and down a Group
- describe and explain qualitatively the general trends and variations in atomic radius, ionic radius, first ionisation energy and electronegativity: — across a Period in terms of shielding and nuclear charge
- describe and explain qualitatively the general trends and variations in atomic radius, ionic radius, first ionisation energy and electronegativity: — down a Group in terms of increasing number of electron shells, shielding and nuclear charge
- interpret the variation in melting point and in electrical conductivity across a Period in terms of structure and bonding in the elements (metallic, giant molecular, or simple molecular)
- describe and explain the trend in volatility of the Group 17 elements in terms of instantaneous dipole-induced dipole attraction
- state and explain the variation in the highest oxidation number of the elements in oxides (for Na2O; MgO; Al2O3; SiO2; P4O10; SO3) and chlorides (for NaCl; MgCl2; AlCl3; SiCl4; PCl5)
- state and explain the variation in bonding in oxides and chlorides in terms of electronegativity (with the exception of AlCl3)
- describe the reactions of the oxides with water (for Na2O; MgO; Al2O3; SiO2; P4O10; SO3)
- describe and explain the acid/base behaviour of oxides (for Na2O; MgO; Al2O3; SiO2; P4O10; SO3) and hydroxides (for NaOH; Mg(OH)2; Al(OH)3), including, where relevant, amphoteric behaviour in reaction with sodium hydroxide (only) and acids
- describe and explain the reactions of the chlorides with water (for NaCl; MgCl2; AlCl3; SiCl4; PCl5)
- suggest the types of structure and bonding present in the oxides and chlorides from observations of their chemical and physical properties
- describe and explain the relative reactivity of elements of: — Group 1 as reducing agents in terms of ease of loss of electrons
- describe and explain the relative reactivity of elements of: — Group 17 as oxidising agents in terms of ease of gain of electrons
- describe and explain the trend in thermal stability of Group 17 hydrides in terms of bond energies
- predict the characteristic properties of an element in a given Group by using knowledge of chemical periodicity
- deduce the nature, possible position in the Periodic Table, and identity of unknown elements from given information of physical and chemical properties
Lessons
Work through them in order.
- Trends in atomic radius, ionisation energy and electronegativityExplain trends in atomic and ionic radius, ionisation energy and electronegativity from nuclear attraction.
- Period 3 elements: melting point and conductivityRelate the Period 3 melting-point and conductivity pattern to metallic, giant and molecular structures.
- Period 3 oxides and chloridesExplain the bonding, water reactions and acid–base behaviour of the specified oxides, hydroxides and chlorides.
- Group 1 as reducing agentsExplain why Group 1 reducing strength increases down the group through easier outer-electron loss.
- Group 17 chemistry trendsExplain the trends in volatility, oxidising strength and hydrogen-halide stability down Group 17.
- Deducing unknown elements from dataIdentify an unknown element by combining electron, physical, acid–base and redox evidence.
Practise and check
Or choose
Topic reference
Quick reference
| Question | First move | What to show |
|---|---|---|
| A trend across Period 3 | same outer shell, similar shielding, more protons | stronger attraction for the outer electrons |
| A trend down a group | one more shell: greater distance and more shielding | these outweigh the extra protons; weaker attraction |
| Compare isoelectronic ions | same electrons; compare the numbers of protons | more protons, smaller ion |
| Melting point across Period 3 | name the structure of each element | metallic, then giant molecular (Si), then simple molecular |
| Oxides with water | classify the oxide | basic (Na, Mg), amphoteric (Al), acidic (Si to S) |
| Group 1 as reducing agents | ease of losing the outer electron | stronger reducing agents down the group |
| Group 17 as oxidising agents | ease of gaining an electron | weaker oxidising agents down the group |
| Stability of hydrogen halides | H–X bond energy | less stable from HCl to HI |
Core knowledge to remember
- First ionisation energy: the energy needed to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.
- Electronegativity: the ability of an atom in a covalent bond to attract the bonding pair of electrons.
- Across a period, atomic radius decreases and first ionisation energy and electronegativity generally increase. Down a group, the opposite happens.
- Across Period 3, the oxides change from ionic and basic, through amphoteric Al₂O₃, to covalent and acidic.
- Group 1 metals are reducing agents: they lose an electron and are oxidised. Group 17 elements are oxidising agents: they gain electrons and are reduced.
- The volatility of the halogens decreases down Group 17, because the instantaneous dipole–induced dipole attractions get stronger.
Common mistakes
- Stating a trend without explaining it in terms of nuclear charge, distance and shielding.
- Saying shielding increases a lot across a period.
- Explaining a melting point without first naming the structure and the particles involved.
- Mixing up an oxide’s reaction with water and its reactions with acids and bases.
- Saying a reducing agent is reduced.
- Explaining the stability of hydrogen halides with intermolecular forces instead of H–X bond energy.
- Naming an unknown element from one clue without checking the rest of the evidence.