IGCSE Chemistry (1.9 to 2.2 & 2.4 & 2.5)) Revision Notes (1.9) Atomic Structure All substances are made up of atoms - atoms are very small - 1/1,000,000,000 cm (one billionth) Substances with only one type of atom are elements However: Atoms are not the smallest particles Atoms are actually made up of sub-atomic particles arranged in a structure with a nucleus and an outer shell In the Nucleus: Protons - Positively charged (+ve) Neutrons - Not charged (neutral) together these are called Nucleons In the outer shell(s): Electrons - Negatively charged (-ve) LEARN THIS: (1.10 / 1.11 / 2.4) Atomic Number, Mass Number, Relative Atomic Mass & Isotopes: • Atoms are usually neutral - the number of protons (+ve) is equal to the number of electrons (-ve) • The atomic number (or proton number) of any element is the number of protons in the nucleus of the element - it is fixed and unique - only one element has that atomic number • The mass number (or nucleon number) of a particular atom of an element is equal to the number of protons plus the number of neutrons in the nucleus (i.e. the total number of nucleons) - this may vary in different isotopes of the element (see below) • The number of neutrons in the nucleus is roughly equal to the number of protons but this may vary to produce different isotopes of the same element • The relative atomic mass (RAM or Ar) of any element is the weighted average mass of the isotopes of the element - based on the carbon-12 scale, where the isotope carbon-12 has a mass of exactly 12 units (calculations of Ar are based on the percentages of different isotopes in any given sample of the element - see below (see also: Edexcel Chemistry p177) • Electrons orbit at a relatively large distance from the nucleus (atoms are mainly empty space) - their mass is roughly 1/2000 (1/1836) that of a proton (2,000 electrons have the same mass as 1 proton) the mass of the electrons is therefore negligible and can be ignored • The electrons are arranged in concentric shells orbiting the nucleus:o The inner shell can contain a maximum of two electrons this 2, 8, 8 rule only applies o The outer shells can contain a maximum of eight electrons to the first 20 elements • (2.4) The number of electrons in the outer shell (Valence Electrons) gives the element’s group number (vertical column) and explains why elements in the same group have similar properties • The number of shells orbiting the nucleus gives the element’s period number (horizontal row) • (1.13 / 2.1) The discovery of this atomic structure of the elements allows them to be arranged in the Periodic Table, where the vertical columns are Groups and the horizontal rows are Periods IGCSE Chemistry (1.9 to 2.2 & 2.4 & 2.5)) Revision Notes (1.14 / 1. 15) Deductions based on Atomic Configuration & the Periodic Table: 1. Look at the diagram of the atom above: There are two outer ‘shells’ orbiting the nucleus (2, 5) - this element is in period 2 There are five electrons in the outer shell (2, 5) - this element is in group 5 The number of electrons (7) equals the number of protons - the atomic number is 7 The element must be nitrogen (group 5, period 2) 14 Relative Atomic Mass (Ar) N Element symbol 7 And... Because the atomic mass is 14 and the atomic number is 7 there must be 7 neutrons in the nucleus (14 - 7 = 7) Atomic Number 2. Look at the Periodic Table - you should know the symbols of the elements in bold You need to be able to “read” the information about each element as follows: Atomic number: 11 - there are 11 protons in the nucleus (Proton Number: Z) - there are 11 electrons arranged in three shells: 2,8,1 - therefore, it’s in period 3 / group 1 Atomic Mass: 23 - there must be 12 neutrons in the nucleus (23 - 11) 23 Na 11 (Nucleon Number: A) OR: Atomic Number: 17 - there are 17 protons in the nucleus - there are 17 electrons arranged in three shells: 2,8,7 - therefore, it’s in period 3 / group 7 (it’s Chlorine) Atomic Mass: 23 - there must be 18 neutrons in the nucleus (35 - 17) 35 X 17 (2.2) Periodic Table of the Elements - Metals & Non-metals I VIII 1 HALOGENS 2 He II III IV V VI VII 7 9 11 12 14 16 19 20 Li Be B C N O F Ne 3 ALKALI EARTH METALS 4 5 23 TRANSITION METALS Na Mg 11 6 24 12 7 8 9 28 31 32 35 40 Al Si P S Cl Ar 79 17 80 14 15 75 16 40 45 48 51 52 55 56 59 59 63 65 70 73 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 26 101 27 103 28 105 29 108 30 112 31 115 32 119 33 122 34 128 35 127 36 131 20 85 Rb 37 88 Sr 38 21 89 Y 39 22 23 91 Zr 40 24 93 25 96 Nb Mo 41 42 99 Tc 43 Ru 44 Rh 45 Pd 46 Ag 47 Cd 48 In 49 Sn 50 Sb 51 Te 52 53 I 84 Xe 133 137 139 178 181 184 186 190 192 195 197 200 204 207 208 2O9 210 222 Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 55 223 56 226 57 227 72 261 73 262 74 266 75 264 76 277 77 268 78 281 79 272 Fr Ra Ac Rf Sg Bh Hs Mt Ds Rg 87 ALKALI METALS 88 89 104 Db 105 Metals 106 107 108 109 110 81 82 83 84 85 5 6 86 NOBLE GASES 111 ‘Between’ Metals (Metaloids) 4 54 Cs 80 3 18 39 19 2 10 27 13 1 Non-metals The elements are arranged in order of their Atomic Number - i.e. the number of Protons in the nucleus, which is equal to the total number of Electrons in the outer shells - You need to know the element families: Alkali Metals (group 1), Alkali Earth Metals (group 2), Transition Metals (between groups 2&3), Halogens (group 7) & Noble Gases (group 8) – These are also called the Inert Gases (inert = unreactive – because they have a ‘full’ outer shell – 2.5) 7 PERIODS 1 4 GROUPS H IGCSE Chemistry (1.9 to 2.2 & 2.4 & 2.5)) Revision Notes (1.11) More on Isotopes and their Uses Isotopes are varieties of an element that have different numbers of neutrons in the nucleus They have the same Atomic Number but different Mass Numbers In other words, they have different atomic masses and may be unstable or radioactive However, the atomic number and the number of protons in the nucleus are the same For Example: 6 7 Li 3 8 Li 3 Li 3 - different atomic masses: 3 Isotopes of Lithium - 4, 3, & 5 neutrons in the nuclei (‘Lithium-7’, ‘Lithium-6’ - same atomic number & ‘Lithium-8’) (‘Common’ Lithium) Different isotopes of an element may have different properties and therefore different uses For Example: Carbon dating: Carbon-14 (14C) is a radioactive isotope (radioisotope) - its nucleus is unstable radiation is released from the nucleus as it changes to carbon-13 - (12C & 13C nuclei are stable and not radioactive) Nuclear Power Generation, Nuclear Bombs and Nuclear Reactors in submarines and aircraft carriers make use of radioactive Uranium-235 (235U) and Plutonium-238 (238Pu) Radioisotopes are widely used in medicine for the diagnosis and treatment of diseases (notably cancer) and injected or ingested before body scans to make certain tissues more easily visible Radioactive tracers are widely used in industry to trace blockages and leaks; to show cracks in metals and faults in engines; in mining, oil and gas industries to search for deposits In Agriculture radiation is used to kill bacteria, moulds and other microorganisms in strawberries, onions, potatoes, meats, and spices, preventing the food from spoiling, and making it safer to eat. Radiation in Consumer Products: smoke detectors / photocopiers (to prevent static buildup) / sterilisation of contact lenses, baby powder and cosmetics See also: http://www.raesystems.com/sites/default/files/downloads/ap-232-radioisotopes-in-everyday-life.pdf (1.12) Calculations of Relative Atomic Mass (Ar / RAM) Based on the percentage abundance of an element’s isotopes in any sample of the element For example: A typical sample of Chlorine contains: 75% 35Cl (chlorine-35) + 25% 37Cl (chlorine-37) i.e. in 100 atoms there would be: 75 35Cl atoms + 25 37Cl atoms The total mass of the 100 atoms would be: (75 x 35) + (25 x 37) = 3550 (Notice: No units it’s all relative) The average mass of one atom would be: 3550 / 100 = 35.5 (the RAM of chlorine) Or: Magnesium: 78.6% 24Mg + 10.1% 25Mg + 11.3% 26Mg 100 atoms total mass: (78.6 x 24) + (10.1 x 25) + (11.3 x 26) = 2432.7 RAM: 2432.7 / 100 = 24.3 (to 3 significant figures)