CHEMISTRY
2010
GROUP 18 ELEMENTS: Noble Gases (Monoatomic gas) 1. These noble gases in group 18 of the Periodic Table make up almost 1% of the air. 2. All noble gases do not: (a) Dissolve in water, (b) Conduct electricity (c) Conduct heat (d) Monoatomic gases at room temperature. Element/ Electron Symbol config Helium, 2 He Increasing Neon, 2.8 boiling Ne point, Argon, 2.8.8 melting Ar point, Krypton, density 2.8.18.8 Kr and radius Xenon, 2.8.18.18.8 Xe Radon, 2.8.18.32.8 Ra 3. Except for He, all of the noble gas atoms have 8 electrons in their outer most shells. This arrangement called the octet configuration. Thus, the arrangement of He (with 2 electrons in outer most shells ± extremely stable electron arrangement) is duplex electron configuration. 4. Boiling point and melting point are low because they have very low intermolecular force of attraction (Van der Waals force). 5. Density is low because the molecules are far apart from each other (big size molecules will have bigger intermolecular forces of attraction). 6. Low reactivity of of noble noble gases due to unusual high large ionization energies and unusual low electron affinity. 7. Uses: (a) Helium ± super conductors, fill airships and weather balloons, divers¶ tank (80% He, 20% O 2), cool metals down. (b) Neon ± advertising advertising light light (glow red in low pressure tubes), fill television tubes. (c) Argon ± fill light bulbs (does not react with tungsten filament), welding (prevent hot metal from reacting with O2 from the air). (d) Krypton ± laser surgery, photographic photographic flash lamps. (e) Xenon ± lighthouse lamps (blue light), making electron tubes. (f) Radon ± cancer treatment.
4.1
GROUP 1 ELEMENTS: Alkali 4.2 metals 1. They are all metals which react with with water water to form alkaline solution. 2. All alkali metals are, (a) Solid (b) Silvery and shiny surfaces (c) Soft and melt easily (d) Conductors of electricity
Element/ symbol
Electron config
Lithium, Li
2.1
Sodium, Na
2.8.1
*only density and radius increasing. The melting points, boiling points and hardness of the atoms decrease down the group because the size of the atoms increases down the group causes the metallic bonding between atoms become weaker. The radius of the atoms increases down the group because the lower members have more shells of electrons. Thus, mass of atom increases. The density increases down the group due to its increasing mass and will produce stronger intermolecular attraction. The atoms are closer, resulting in their densities being higher. Electropositivity is a measure of the ability of an atom to lose its valence electrons. The valence electrons of the lower members are further away from the nucleus, and can be easily donated. Thus, electropositivity increases down the group. Chemical properties: (a) Reactivity depends on their ability to give away the valence electron (increase down the group). (b) Alkali metals metals react with oxygen gas to form metal oxides. Example: Na(s) O2(g) Na2O (c) Alkali metals react with water to form alkaline metal hydroxide solution and hydrogen gas. Example: 2K(s) 2H2O( ) 2KOH(aq) H2(g) (d) Alkali metals react with with halogen (fluorine, chlorine and bromine) to form colourless, crystalline ionic salts called halides. Example: 2Li(s) Cl2(g) 2LiCl(s) 2Na(s) Br 2( ) 2NaBr(s) 2K(s) I2(s) 2KI(s) Safety precautions: (a) Lithium, sodium, and potassium are highly reactive to water and air, thus must be kept under non reactive liquid (paraffin oil). (b) Reaction of potassium, rubidium, caesium and francium are explosive. Thus, a small piece should be used always during experiment. (c) Never handle those alkali metals with your fingers because it can react with moist which will form a corrosive hydroxide.
Potassium, K 3.
4.
5.
6.
7.
8.
Decreasing boiling point, melting point, electropositivity and hardness.
2.8.8.1
2.
At room temperature, chlorine is a gas, bromine is a liquid, and iodine is a solid. Element/ Electron symbol config Increasing Fluorine, boiling 2.7 F point, Chlorine, melting 2.8.7 Cl point, density and Bromine, 2.8.8.7 radius Br Iodine, 2.8.18.8.7 *reactivity I decreases. Astatine, 2.8.18.18.8.7 At 3. Physical properties of halogen: All are coloured Have pungent & irritating smell (liquid)
Non-metal
Chlorine Bromine Iodine Do not conduct electricity
Poisonous
Cl (g), Br (l), I (s)
4.
5.
Chemical properties of halogen: (a) Halogens react with heated aluminium to produce powdery solid halides. Example: 3Cl2(g) 2Al(s) 2AlCl3(s) 3Br 2( ) 2Al(s) 2AlBr 3(s) (b) Halogens react with phosphorus to form phosphorus halides. (c) Halogens react with H2 to form halides. Example: Cl2(g) H2(s) 2HCl(s) Br 2( ) H2 (s) 2HBr(s) (d) Halogens react among themselves. Example: I2(s) Cl2(g) 2HCl(s) Safety precautions: (a) Vapour of fluorine, chlorine, and bromine are poisonous. (b) Iodine affects negatively the respiration of all living things. (c) Astatine is radioactive. (d) All experiments of halogens should be done inside a fume chamber. (e) Safety goggles and and gloves should be used. TRANSITION ELEMENTS: Group 3 to group 12. All elements in this block block of the Periodic Table have similar properties. Physical properties: (a) Hard, shiny and dense, (b) Good conductors of heat and electricity, (c) Are malleable and ductile, (d) High tensile strength,
4.4 1. 2.
GROUP 17 ELEMENTS: Halogens 4.3 (diatomic molecules) 1. Halogens are not conductors of heat and electricity.
THEME: Matter Around Us
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CHEMISTRY
2010
(e)
3.
4.
Radius of all transition elements elements is almost constant, (f) Decreasing from left to right ± boiling point and melting point (g) Increasing from left left to right ± proton number and density, electronegativity (increase slowly). Below are some of the transition elements: Proton Elements Symbol number Scandium Sc 21 Titanium Ti 22 Vanadium V 23 Chromium Cr 24 Manganese Mn 25 Iron Fe 26 Cobalt Co 27 Nickel Ni 28 Copper Cu 29 Zinc Zn 30 Special characteristic: (a) Majority of the transition elements elements have more than one oxidation number in their compounds.
Elements
Compound
Chromium
Potassium dichromate (IV) Chromium (II) chloride Manganese (II) chloride Manganese (IV) oxide Potassium manganate (VI) Potassium manganate (VII) Iron(II) chloride Iron(III) chloride Nickel(II) sulphate Nickel(III) bromide Copper(I) chloride Copper(II) oxide
Manganese
Iron
Nickel
Copper
Formula
+6
CrCl3
+3
MnCl2
+2
MnO2
+4
K2MnO4
+6
KMnO4
+7
FeCl 2
+2
FeCl 3
+3
NiSO 4
+2
NiBr 3
+3
CuCl
+1
CuO
+2
1.
2.
FORMATION OF COMPOUNDS Conditions for the formation of chemical bonds include the following: (a) Electrons in completely filled shells do not take part in bond formation. (b) Only valence electrons are involved in bond formation. (c) The combining atoms will change their electron arrangements to achieve the stable noble gas electron arrangements. The octet rule ± 2.8.18.32.18.8.
Charge of 7 protons Charge of 7 electrons Total charge
5.
=+7 =-7 =0
Charge of 7 protons Charge of 10 electrons Total charge
=+7 =-10 = -3
Formation of ionic bonds:
A. Formation of potassium chloride, KCl (a) Figure below shows the transfer of of one electron from a potassium atom to a chlorine atom to form the ionic compound potassium chloride.
IONIC BONDS (Electrovalent bonds) Ionic bond is the chemical bond formed from the transfer of electron from metal atoms to non-metal atoms. Formation of ionic bond are as follows:
5.2
2.
(b)
3.
Formation of cations (positive ions): An atom that loses their valence electrons to achieve stable duplet or octet electron arrangement similar to noble gases is called cation (fewer electrons than protons ± metal atoms). Metal atoms are more electropositive ± group 1, 2 and 13. + Example: Li Li e y
(b)
y
5.1
1.
Most transition elements form coloured ions as shown below: Formula of Colour Ions the ions (aqueous) 2Chromate(VI) CrO4 Yellowish 2Dichromate(VI) Cr 2O7 Orange 2+ Iron(II) Fe Greenish 3+ Iron(III) Fe Brownish 2+ Copper(II) Cu Bluish 2+ Cobalt(II) Co Pale reddish Manganate(VII) MnO 4 Purple 3+ Chromium(III) Cr Greenish (c) Many of the transition elements elements are able to from complex ions. (d) Many of the transition elements elements can act act as catalyst. Catalysts are used in chemical reaction to speed up the rate of a reaction. A few industrial processes that use these elements or their compounds as catalysts are: Haber process (manufacture ammonia) ± iron.
y
gases is called anions (more electrons than protons ± non-metal atoms). Non-metal atoms are electronegative ± group 15, 16 and 17. 3Example: N e N
CHAPTER 5: Chemical Bonds
Oxidation number
K2Cr 2O7
Contact process (manufacture sulphuric acid) ± Vanadium(V) oxide. Ostwald process (manufacture nitric acid) ± Platinum. Hydrogenation of vegetable oil (manufacture margarine) - nickel,
+
-
The oppositely-charged ions, K and Cl , formed are then strongly attracted to each other by strong electrostatic forces in the crystal lattice, called ionic bonds or electrovalent bonds. Hence, the ionic compound potassium chloride with the formula KCl is formed.
B. Formation of calcium sulphide, CaS (a) Figure below shows the transfer of of electrons electrons from a calcium atom to a sulphur atom to form the ionic compound calcium sulphide.
y
y
Charge of 3 protons Charge of 3 electrons Total charge
4.
=+3 =-3 =0
Charge of 7 protons Charge of 10 electrons Total charge
=+3 =-2 = +1
Formation of anions (negative ions): An atom that accept electrons into their valence shells to achieve the stable octet electron arrangement similar to noble y
THEME: Matter Around Us
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