WJEC AS Chemistry Unit 1.6 - The Periodic Table

Structure of the Periodic Table

• Elements arranged according to increasing atomic number

• Vertical columns; groups. All elements contain same outer electron configuration. Groups number; number of electrons in outer shell, therefore elements within a group have similar chemical properties

• Horizontal rows; periods. All elements in a period have same number of quantum shells containing electrons

• Table is divided into blocks; Gps 1 and 2=s-block as elements’s outer electrons are in an s orbital. Groups 3-8=in p-block. Elements between 2+3=in the d-block

• Usually metals ok left and middle and non-metals on right

Oxidation and Reduction

• gain oxygen=oxidised

• Lost oxygen=reduced

• Electron transfer; oxidation is loss of electrons, reduction is gain of electrons. Explained using half equations. Oxidising agent oxidised other reaction and is itself reduced. Reducing agent reduces other reactant and is itself oxidised.

• Work out oxidation numbers of atoms/ions; increases=oxidised, decreases=reduced

Trends in ionisation energy

• General increase across a period; increase in nuclear charge in same energy level so little extra shielding and therefore a greater attraction between nucleus and outer electrons

• Decrease between Group 2 and Group 3; G3 elements’ outer electrons is in a new sub shell of slightly higher energy level and partly shielded by the s electrons

• Decrease between G5 and G6; in G6, the electron is removed from an orbital containing a pair of electrons. The repulsion between these electrons makes the electron easier to remove. In G5, the electron is removed from a singly occupied orbital

• Decrease down a group; outer electron has increased shielding from inner electrons and is further from nucleus. This outweighs increase in nuclear charge

Trends in electronegativity

• Increases across a period; increase in nuclear charge but bonding electrons are always shielded by same inner electrons so there is a greater attraction between the nucleus and the bonding pair

• Decrease down the group; bonding electrons have increased shielding from the nucleus, so attraction between nucleus and bonding electrons decreases

• Most en elements at top right

• Least en elements on bottom left

Trends in melting and boiling temperatures

• decrease down group 1; increasing size of ions, strength of metallic bonding weakens, reducing energy required to break bonds and melt the metal

• Decrease up group 7; decreasing strength of VdW as size decreases

• Increase up to group 4 and decrease down to group 7; changes in electronegativity and strength of intermolecular forces. 1-4 en increases meaning atoms more likely to attract electrons and form stronger VdW bonds. 4-7: increased electronegativity and the strength of the chemical bonds within the molecules start to offset the effect of the stronger van der Waals forces

Reactions of Group 1 elements with water/steam

• React vigorously with cold water to form the hydroxide and hydrogen

• Reaction increases in vigour as you go down the group;

  • Lithium floats on water, gently fizzing

  • Sodium melts into a ball that dashes around the surface

  • Potassium melts into a ball and catches fire

  • Caesium explodes and shatters the glass container

Reactions of Group 1 elements with oxygen

• form solid white oxides

• Burn with a characteristic flame

Reactions of Group 2 elements with water/steam

• react less vigorously than G2

• The hydroxide and hydrogen are formed

• Magnesium reacts very slowly

• Reactivity increases as you go down the group

  • Calcium produced a steady stream of bubbles and the liquid becomes cloudy as a white precipitate of calcium hydroxide forms

  • Barium produces greater effervesce and the solution is clearer as barium hydroxide is more soluble

Reactions of Group 2 elements with Oxygen

• Apart from Mg, all G2 metals tend to burn with a characteristic flame

• All burn to form solid white oxides

Reactions of Group 1 elements with acids

• too reactive to be added directly to acids

Reactions of Group 2 elements with acids

• All G2 metals react vigorously with HCl to produce a colourless solution of the metal oxide and bubbles of hydrogen

• Reactivity increases down the group

• Only Mg reacts with sulfuric acid as others have insoluble sulfates

Reactions of aqueous cations

Characteristic flame colours of s-block cations

• All s-block elements apart from Mg may be identified by a flame test

• A clean metal wire/splint is moistened with HCl, dipped in the compound and held in a non-luminous Bunsen flame

• Li+ = red

• Na+ = orange-yellow

• K+ = lilac

• Mg2+ = (no colour)

• Ca2+ = brick red

• Sr2+ = crimson

• Ba2+ = apple green

Trend in reactivity of Group 1 metals

• Increases down the group due to increased atomic radius and decreased forces of electrostatic attratction ebtween electrons and nucelus

Trend in reactivity of Group 2 metals

Increases down the group due to decrease in IE and increase in shielding

Trend in thermal stability of Group 2 carbonates

• All G2 carbonates decompose on heating to the oxide and carbon dioxide

• Thermal stability increases down the group

• Shown by heating the carbonate and seeing how long it take the carbon dioxide to turn the limewater cloudy

Trend in thermal stability of Group 2 hydroxides

• All G2 hydroxides decompose on heating to the oxide and steam

• The thermal stability decreases down the group

• The hydroxides have to be heated more strongly before they will decompose

Trends in solubility in water of Group 1 compounds

All group 1 compounds are soluble

Trends in solubility in water of Group 2 compounds

• many G2 compounds are in soluble

  • All nitrates are soluble

  • All carbonates are insoluble

  • Hydroxides become more soluble down the group

  • Sulfate become less soluble down the group

Basic characteristics of the oxides of Group 1

• General; metal oxides are basic and non-metal oxides are acidic. All s-block metal oxides are strong basic and neutralise acids to form a salt and water

• G1 oxides and barium oxide react with water to form a soluble hydroxide

• The hydroxides are soluble so are alkalis

Basic characteristics of the hydroxides of Group 1

• Formed when G1 oxides or barium oxide react with water to form a soluble hydroxide

• Hydroxides are soluble so = alkalis

Basic characteristics of the oxides of Group 2

• General; metal oxides are basic

• All s-block metal oxides are strong bases

• Neutralise acids to form a salt and water

Basic characteristics of the hydroxides of Group 2

• Barium oxide and water forms a soluble hydroxide. Since hydroxide = soluble, they’re alkali

• Other G2 hydroxides are not very soluble so saturated solutions of these hydroxides are only weakly basic as conc of hydroxide ion is very low

Trend in volatility of Group 7

• decreases down the group

• Number of electrons increases with atomic number, there’s an increase in the induced dipole-induced dipole intermolecular forces folding the diatomic molecule together

• Melting and boiling temps increase down the group

• Volatile; substances that form vapours easily

• Substance with low bt has high volatility, high bt has low volatility

Reactions of the halogens with metals

• React directly with most metals to form the halide

Trend in reactivity of the halogens in terms of relative oxidising power

• Both reactivity and oxidising power decrease down the group

• Halogens react by gaining electrons to form negative halide ions. Gain electrons during reactions so are reduced and oxidise the here substance

• Down the group, the outer electrons are shielded more and are further from the nucleus

• Harder to attract electrons

Test for halide ions

• Silver nitrate test

• The test has to be done in solution. If start with solid, must first dissolve in water

• Add a few drops of nitric acid to neutralise and ensure any other anions are removed as they would also form precipitates

• Aqueous silver nitrate solution is added to give;

  • Cl-; White precipitate

  • Br-; Cream precipitate

  • I-; (pale) Yellow precipitate

• Precipitate formed is the insoluble silver halide

• Difficult to differentiate between colours of precipitates in a single test where only one precipitate is seen

• Aqueous ammonia is added to the precipitate;

  • AgCl; precipitate dissolves in dilute ammonia

  • AgBr; Precipitate does not dissolve much in dilute ammonia but dissolves in conc ammonia

  • AgI; Precipitate insoluble in dilute and conc ammonia

Displacement reactions of halogens

• A halogen in a higher position in the group will oxidise a halide ion from lower in the group as oxidising powers decrease down the group

• When a halogen is added to an aqueous solution containing a halide ion;

  • Chlorine displaces bromide and iodide

  • Bromine displaces only iodide

  • Iodine does not displace either chloride or bromide

• When these displacement reactions happen, there are colour changes

Use of chlorine in water treatment and the related health and ethical issues

• Chlorine is commonly added to water as the gaseous element and the equilibrium is established

• The chlorate ion, ClO-, kills bacteria and other microbes so adding chlorine to water makes it safe to drink/swim in

• Chlorination is used to prevent the outbreak of serious diseases

• Risks;

  • Highly toxic

  • Can react with naturally occurring organic compounds found in the water supply to form chlorinated hydrocarbons which can cause liver and kidney cancer

• Risks are small compared to risks of untreated water

• Appear to be only beneficial effects below 1ppm

• Some people object to water chlorination as forced mass medication

Use of fluoride ions in water treatment and the related health and ethical issue

• Generally added to water to reduce tooth decay by preventing cavities

• Water fluoridation reduces cavities in children

• Effectiveness in adults is less clear

• Can cause dental fluorosis which leads to tooth discolouration

• No clear evidence of other adverse affects from water fluoridation

• Appears to only have beneficial affects below 1ppm

• Many people invested to water fluoridation as forced mass medication

• Given the Prevalence in dental products - many people think adding fluoride to water supplies/bottled water can be detrimental to long-term dental health

A solution is suspected to contain CO₃^2-(aq), SO₄^2-(aq) and OH^-(aq). Describe the actions you would take to confirm this.

• Carbonate; add a G2M salt and effervesence observed

• Sulfate; add barium and a thick white ppt is formed

• Hydroxide; add magnesium or calcium and a thin white ppt formed

  1. Add an acid (nitric acid) and observe if an effervsence occurs. If carbonate ions are present, effervesecence will occur due to production of Carbon Dioxide; CO₃^2-(aq) + H⁺(aq) →H₂O + CO₂(g)

  2. Until effervesence occurs, add carbonate ions removed from solution

  3. Divide solution in half

  4. Add aqueuous barium ions (e.g. barium nitrate) to one half of the solution. Oberseve if a thick white ppt is formed.