C3 Chemistry

History of the Periodic Table

1866: John Newlands Table Newlands ordered the elements according to atomic mass. He also put them into groups with similar properties. He called his table the ‘law of octaves’: Problems with his table:  More than one element in some boxes (Co, Ni)  Metals in same groups as non-metals (Ni, Br)  Groups only work for first 17 elements  Elements were still being discovered

1869: Dmitri Mendeleev’s Table Mendeleev also ordered the elements using atomic mass but reversed the positions of certain elements so that they fitted the pattern better. He also left gaps for elements yet to be discovered:

The Modern Periodic Table

The modern periodic table is based on Mendeleev’s table but arranges the elements in order of atomic number.

alkali metals transition metals halogens nobel gases Columns of the Periodic Table Elements are put into groups that have similar chemical properties and the same number of outer shell electrons.

Ar X X XX X X XX X X X X XX XX XX Ne X X XX X X XX X X The number of outer shells determines the period (horizontal row) of an element. Rows of the Periodic Table

Group 1: The Alkali Metals

Li X X X Chemical Properties:  Very reactive (stored in oil)  Lose their outer electron to become 1+ ions Physical Properties:  Low density  Soft and weak  Shiny when cut  Good conductors 1+ Li X X

Reactivity increases as you go down the group:  Outer shell is further from the nucleus  Leaving electron is more shielded from the nucleus by other electrons  Electron is lost more easily Li X X X Na X X X X X X XX X X X K X X X X X X XX X X X X XX X XX X X Reactivity of Group 1

+  metaloxygen metal oxide +  4Na (s) O 2 (g) 2Na 2 O (s) Reaction with Oxygen Observations:  Metals are shiny when first cut  A white oxide layer quickly forms on surface of metal

Reaction with Water Observations:  Floats and moves on surface of water  Fizzing/bubbles of gas  Metal disappears  Produces an alkaline solution which turns Universal indicator purple +  metalwater + hydrogen metal hydroxide +  2K (s) 2H 2 O (l) + H 2 (g) 2KOH (aq)

Group 7: The Halogens

 The halogens have 7 electrons in their outer shell  They exist as covalently bonded diatomic molecules F F XX X X XX X F 2 : Pale yellow gas Cl 2 : Pale green gas Br 2 : Red/brown liquid I 2 : Black/purple solid I I XX X X XX X

+1 Li X X F X  Halogens form ionic bonds with metals  They gain an electron to become halide ions e.g fluoride ions = F - Reaction with metals:

F X X X X X X XX X Cl X X X X X X XX X X X X XX X XX Br X X X X X X XX X X X X XX X XX X XX X X XX X Reactivity of Group 7 Reactivity decreases as you go down the group:  Outer shell is further from the nucleus  Incoming electron is more shielded from the nucleus by other electrons  Electron is less easily gained

A more reactive halogen will displace a less reactive halide ion from a solution of its salt. Examples Cl 2 + 2KBr  Br 2 + 2KCl  Cl 2 displaces Br - because it is more reactive I 2 + 2KBr   Iodine is less reactive than bromine Halogens + Halide ions

The Transition Metals

 Found in middle block of the periodic table  They have 2 electrons in their outer (4 th ) shell  The 3 rd shell is then filled to have a total of 18 electrons!!

Cr (III)Cr (II)Cr (VI) Mn (VI)Mn (VII) Properties:  Excellent conductors of heat & electricity  Hard & strong with high densities & melting points  Less reactive than group 1  Make excellent catalysts  Can form more than one type of ion shown using Roman numerals. e.g. Cu(I) = Cu +, Cu(II) = Cu 2+  Form coloured compounds

What is Hard Water?

Hard water contains dissolved calcium or magnesium ions (Ca 2+ or Mg 2+ ). Hard water leads to solid scum forming when it is mixed with soap instead of a nice lather: sodium stearate + calcium  calcium stearate + sodium (soap) ions (scum) ions This increases the cost of washing because more soap is needed before it becomes effective. hard water soft water SCUM LATHER

Temporary Hard Water 1.CO 2 dissolves in rain water to make it acidic 2.Calcium carbonate (in limestone) dissolves in the acidic water to form soluble calcium ions and hydrogen carbonate ions: CaCO 3 (s) + H 2 O (l) + CO 2 (aq) Ca 2+ (aq) + 2HCO 3 - (aq) limestone CaCO 3 (s) Ca 2+ (aq) + HCO 3 - (aq)

Heating temporary hard water causes the HCO 3 - ions to decompose which reverses the reaction. This softens the water but produces scale (solid calcium carbonate). Scale reduces the efficiency of heating systems and kettles. CaCO 3 (s) + H 2 O (l) + CO 2 (aq) Ca 2+ (aq) + 2HCO 3 - (aq)

Permanent Hard Water This is hard water that cannot be softened by boiling. It forms when calcium sulphate (in gypsum) dissolves in water. CaSO 4 (aq) Gypsum CaSO 4 (s)

Benefits of Hard Water  Calcium ions in drinking water leads to strong bones and teeth  It could also reduce the risk of heart disease

Removing Hardness

Washing Soda Soda crystals are used in washing machines and dish washers and contain sodium carbonate. The carbonate ions react with the ions in hard water to produce insoluble metal carbonates: Ca 2+ (aq) + CO 3 2- (aq)  CaCO 3 (s) This eliminates the need to heat water in order to soften it which saves energy.

Before - HardAfter - Soft Ion-Exchange Columns Dishwashers contain columns which are filled with resin beads which are packed with sodium ions (Na + ). The Ca 2+ and Mg 2+ ions in hard water are exchanged for the Na + ions as water passes through the column. The resin beads can be recharged by washing the column with sodium chloride (salt) solution.

Water Treatment

Filter Jugs These remove some of the impurities found in tap water:  Activated carbon removes toxic chlorine compounds  Ion-exchange resin removes metal ions such as Ca 2+ & Mg 2+  Silver nanoparticles kill bacteria

Filter jugs do not produce completely pure water. 100% pure water can only be achieved by distillation. This would require huge amounts of energy and would be very expensive! 4 solution condenser water out water in thermometer pure liquid HEAT

Water Fluoridation

For fluoridationAgainst fluoridation  Fluoride protects the teeth of those with poor dental care  Fluoride decreases teeth cavities by about 30% by killing bacteria  This bacteria also causes some types of heart disease  Fluoride is only added in tiny amounts (0.0001%)  There is no solid proof that fluoridation harms our health  The benefits of fluoridation are not significant. Toothpaste and mouthwashes are for more important for dental health  Excess fluoride can lead to fluorosis in children  Excess fluoride in adults can lead to weaker bones  Some studies even suggest excess fluoride can lead to learning difficulties and Alzheimer's disease  It is unethical to give treatments to people that they have not consented to