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THE ATOM • Our modern idea of an atom is a very small nucleus containing protons (positively charged) and neutrons (neutral), surrounded by a cloud of electrons (negatively charged). • • • Protons and neutrons are about the same size Electrons are about 1/1846 the size of a proton. Almost all of the mass (about 99.99%) of an atom is in the nucleus. The rest is empty space. If an atom was the same size as the school oval, the nucleus would be the size of a green pea on the cricket pitch and the electrons would be pinpoints moving around the boundary at rapid speeds. Elements • • In an element, all of the atoms have the same number of protons. There are about 112 different elements, each with its own symbol. The first letter is always capital. If there is a second letter, it is lowercase e.g. Mg, Na. Isotopes • • • Isotopes have the same number of protons but a different number of neutrons. Isotopes of elements have the same chemical properties, as that only depends on the number of protons (and electrons). A different number of neutrons changes the mass and this only affects the physical properties e.g. melting point and boiling point. 12 C 6 Atomic Number Mass Number Atomic Number = number of protons in the nucleus of the atom. Mass Number = number of protons and neutrons in the nucleus of the atom. Name Hydrogen Carbon Carbon Chlorine Potassium • Symbol 1 1H 12 6C 14 6C 35 Cl 17 39 K 19 Atomic No. Mass No. No. of protons No. of neutrons No. of electrons 1 1 1 0 1 6 12 6 6 6 6 14 6 8 6 17 35 17 18 17 19 39 19 20 19 In a neutral atom, the number of protons in the nucleus and the number of electrons orbiting the nucleus is always the same. Ions • Ions are charged atoms. There are a different number of electrons to protons. They are formed when an element either loses or gains electrons. e.g. Na+ has 11protons but only 10 electrons. has 16 protons but 18 electrons. S2- ELECTRON CONFIGURATION The electrons of an atom are not arranged randomly, but exist in shells. They are negatively charged but they do tend to pair up (much like girls going to the toilet!!! ☺). They can pair up because they spin on their axis and thus generate a magnetic field. Two electrons spinning on their axis in the opposite direction will be attracted towards each other due to the magnetism. The maximum number of electrons that each shell can hold increases as you increase the distance from the nucleus (because there is more volume), but the force of attraction on them by the nucleus will decrease the further away they are. The formula used to work out the maximum number of electrons each shell can hold is 2n2 where n is the number of the shell. Shell Number 1 2 3 4 5 6 7 Old Naming System K L M N O P Q Max. no. of electrons 2 8 18 32 50 72 98 These shells never get filled. It actually follows a mathematical progression: 1 pair 1 pair 1 pair + 3 pairs 1 pair + 3 pairs + 5 pairs 2 electrons 8 electrons 18 electrons + 3 pairs + 5 pairs + 7 pairs 32 electrons ELECTRONIC CONFIGURATION OF FIRST 36 ELEMENTS Name Symbol Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulphur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Atomic No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 Elec. Config. 1 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.8.1 2.8.2 2.8.3 2.8.4 2.8.5 2.8.6 2.8.7 2.8.8 2.8.8.1 * 2.8.8.2 * 2.8.9.2 2.8.10.2 2.8.11.2 2.8.12.2 2.8.13.2 2.8.14.2 2.8.15.2 2.8.16.2 2.8.17.2 2.8.18.2 2.8.18.3 2.8.18.4 2.8.18.5 2.8.18.6 2.8.18.7 2.8.18.8 * Potassium and calcium appear to have a ‘funny’ configuration. The third shell is not full before electrons appear in the fourth shell (1 pair). At this stage of your chemical career, remember the rule “you can’t have more than 8 electrons in the outer shell of an element.”