3.2: Periodicity Flashcards
What is periodicity?
a regular repeating pattern of atomic physical and chemical properties
What is the trend in melting points across period 3?
- increases
- due to increase in nuclear charge
- more delocalised eolectrons to increase metallic bond strength
- more energy needed to break these bonds
Na, Mg, Al
What are the trends in melting and boiling point across Period 3?
1.Na, Mg, Al
- giant metallic latice with positive metal ions surrounded by a sea of delocalised electrons
- strong electrostatic forces, increasing nuclear charge
- Mp- 371,922,933 l Bp- 1156,1380,2740
Si
What are the trends in melting and boiling point across Period 3?
2.Si
- giant covalent/ macromolecular molecule
- many strong covalent bonds
- Mp- 1683 l BP- 2628
melting and boiling point fluctuate
P4, S8, Cl2
What are the trends in melting and boiling point across Period 3?
3.P4,S8,Cl2
- simple molecular
- weak intermolecular forces, VDW forces, more molecules, stronger forces as larger molecule
- Mp- 317, 392, 172 l Bp- 553, 718, 238
melting and boiling point fluctuate
Ar
What are the trends in melting and boiling point across Period 3?
4.Ar
- atomic
- very weak forces between atoms
- Mp- 84 l Bp- 87
melting and boiling point fluctuate
What is the trend in ionisation energies across Group 2?
- Melting point decreases down the group (Mg has anomalylously low b.p. due to differnt crystal structure)
- Atomic radius increases
- Solubility of salts increases
- Reactivity with water increases
- Ionisation energy decreases
What is the trend in melting points across Group 2?
- Size of the atoms increase
- Distance between the nucleus and “sea” of delocalised electrons increases
- Strength of metallic bond decreases
- Energy required to overcome the bond decreases
- Melting points decrease down the group
- Magnesium has an unexpectedly low melting point
Why does reactivity increases down Group 2?
- Increased atomic radius/ size of atoms/ number of electron shells
- Greater distance between nucleus and outer shell electrons
- Weaker attraction between outer electrons and nucleus / more shielding
- Easier to remove outer electrons
What happend when you react Group 2 metals with water?
By reacting group 2 metal with water, metal hydroxides and hydrogen are formed.
M(s) + 2H2O (l) → M(OH)2 (aq) + H2 (g)
Metal is oxidised
Hydrogen is reduced
Magnesium also reacts with steam at high temperatures to produce magnesium oxide:
Mg(s) + H2O(g) → MgO(s) + H2(g)
What is the pattern in solubility in Group 2 hydroxides?
Group 2 hydroxides, X(OH)2, are all white solids that become more soluble down the group.
- Magnesium hydroxide- almost insoluble
- Calcium hydroxide- sparingly soluble
- Strontium hydroxide- more soluble
- Barium hydroxide-dissolves to produce strong alkaline solution (because of the OH- ions)
What is the pattern in solubility in Group 2 sulfates?
By reacting group 2 metal with sulphuric acid, metal sulphates are formed.
Reaction equation:
Mg (s) + H2SO4 (aq) → MgSO4 (aq) + H2 (g)
The group 2 sulphates get less soluble down the group (Mg-Ba).
BaSO4 is insoluble.
Acidified BaCl2 solution
How do you test for sulphate ions?
- Sulphate ions in solution, (SO4^2-), are detected using acidified barium chloride solution.
- The test solution is acidified using a few drops of dilute hydrochloric acid, and then a few drops of barium chloride solution are added.
- A white precipitate of barium sulfate forms if sulfate ions are present:
BaCl2(aq) + Na2SO4(aq) → 2NaCl(aq) + BaSO4(s) - The hydrochloric acid is added first to remove any carbonate ions that might be present - they would also produce a white precipitate, giving a false positive result.
What are different uses of Group 2 in medicine, agricuture and titanium extraction?
- Magnesium hydroxide (Mg(OH)2), milk of magnesia, acts as an “antacid”- neutralises excess stomach acid- as a treatment for indigestion.
- Barium sulphate (BaSO4) is used in x-rays to image a patient’s stomach and intestines as it is opaque to x-rays.
- Calcium hydroxide (Ca(OH)2) can be added to solid to raise the pH level to 6-7, the optimum for most plants.
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Magnesium: TiO2 is converted into TiCl4 by heating with carbon and chlorine, TiCl4 is then reduced by Mg
** TiCl4(g) + 2Mg(l) -> Ti(s) + 2MgCl2(l)** - titanium is reduced, magnesium is oxidised
What are Group 7 elements?
- halogens
- highly reactive non-metals of group 7
- Flourine- pale yellow
- Chlorine- green
- Bromine- red-brown
- Iodine- grey