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Atomic Structure An atom is the smallest possible particle of an element that can exist. An element is made up of only one type of atom. The Structure of the Atom An atom consists of a tiny, massive central nucleus surrounded by the electrons. The electrons occupy shells around the nucleus which is made up of the protons and neutrons. Sub-atomic particle proton neutron Symbol electron Relative Charge +1 0 p n e- -1 The nucleus contains the protons and neutrons is overall positively charged is the heavy part of the atom is very small compared to the rest of the atom IGCSE TOPIC 9.3: ATOMS AND ELEMENTS 1 Relative Mass 1 1 1/1836 (0.000545) The electron shells (energy levels) electrons are arranged in shells (or ‘energy levels’) around the nucleus the electrons are fast moving each shell can only contain a fixed number of electrons. Once a shell is full, any remaining electrons must go into the next shell. Atoms of different elements contain different numbers of protons. If two atoms have the same number of protons then they must be atoms of the same element. Atoms have no overall charge, therefore: The number of protons is the same as the number of electrons Atomic Number and Mass Number Each element in the Periodic Table has a chemical symbol and has two numbers associated with it e.g. The top number (the ‘mass number’ or ‘nucleon number’) and the bottom number (the ‘atomic number’ or ‘proton number’) give us information about the structure of an atom of that element. It can be seen that the elements are arranged in the Periodic Table in order of atomic number. The atomic number gives the number of protons in the nucleus of an atom The mass number gives the number of protons and neutrons in the nucleus Looking again at the information for chlorine given above: This tells us: Symbol for chlorine = Cl Atomic number of chlorine = 17 Mass number of chlorine = 35 We can work out the number of protons, neutrons and electrons from this information: Number of protons = atomic number = 17 Number of neutrons = mass number – number of protons = 35 – 17 = 18 Number of electrons = number of protons = 17 IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 2 Atomic Structure Worksheet 1 Use your Periodic Table to complete the following table. The last three columns require you to write in the number of protons (p), electrons (e-) and neutrons (n). Element Symbol Mass Atomic Number number silicon argon oxygen beryllium sodium boron potassium fluorine neon lead platinum IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 3 p e- n Isotopes The number of protons in an element is fixed e.g. carbon will always have 6 protons in its nucleus. The number of neutrons, however, can vary slightly. There are three kinds of carbon atom: carbon-12, carbon-13 and carbon-14. They have different mass numbers as each has a slightly different number of neutrons in the nucleus: Atoms of the same element with the same number of protons but different numbers of neutrons are known as isotopes Thus there are 3 isotopes of carbon. This has no effect at all on their chemical properties (how they react), since this is determined by the number and arrangement of electrons and all three of the isotopes have the same number of electrons. Carbon-14 can be known as a radioisotope because it is radioactive. This means its nucleus is unstable and it can break down to form a more stable form, emitting radiation as it does so. This property of carbon-14 is used in ‘carbon dating’ ancient artefacts. The other two isotopes of carbon are not radioactive. Radioisotopes of elements can be dangerous due to the radiation they emit, but they can also be extremely useful in, among many other things, medicine (cancer treatment) and checking oil and gas pipes for leaks in conjunction with a Geiger counter.. IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 4 Complete the ‘p e- n’ numbers for the following sets of isotopes: Isotope Symbol Mass Number Atomic number p e- n uranium235 uranium238 chlorine-35 chlorine-37 Electron arrangements (Electron configurations) Working out the number of electrons an atom has is easy – find the proton number from the Periodic Table, and the number of electrons will be the same: number of electrons in an atom = number of protons Electrons are arranged in energy levels (also known as shells) around the nucleus. The lowest energy levels are always filled first. These are closer to the nucleus and hold the least numbers of electrons. The first energy level can only hold 2 electrons. The second energy level can only hold 8 electrons The third energy level can hold 8 electrons* Potassium has an atomic number of 19. An atom of potassium has 19 protons and therefore has 19 electrons. Arranging the electrons in levels, filling the inner (lower energy) levels first gives 2 in the first level; 8 in the second level; 8 in the third level; 1 in the fourth level. This can be shown on a diagram: IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 5 Or it can be written as: 2,8,8,1. So the electron arrangement of potassium is 2,8,8,1. Exercise: Work out the electron arrangement for chlorine. Draw a diagram and give the number form of the electron arrangement. * The third and fourth shells can expand to hold a maximum of 18 electrons, but not when they are the outer shells. For example Rubidium has 37 electrons arranged 2,8,18,8,1. IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 6 Electron Arrangement Worksheet Use your Periodic Table to complete the following table. Draw the diagram AND give the number version of the electron arrangement. hydrogen helium lithium beryllium 2 boron carbon nitrogen oxygen fluorine neon sodium magnesium 2,8,1 aluminium silicon phosphorus sulphur argon potassium calcium 2,8,3 chlorine IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 7 Link between electron arrangement and the Periodic Table We have already seen that elements are arranged in the Periodic Table in order of their atomic number. Clues about the electron arrangement of an element can also be determined by considering its position in terms of Groups (columns)and Periods (rows) in the Periodic Table. Complete the following to illustrate this (the first one has been started for you): Element Electron Arrangement sodium 2,8,1 Number of occupied shells Number of electrons in OUTER shell Period Group 3 1 aluminium chlorine magnesium carbon oxygen What is the relationship between the electron arrangement and the position of the element in the Periodic Table? The number of electron shells = The number of outer electrons = Using this relationship, the electron arrangement can be worked out very quickly. Try it! P Ca Ar ‘Full’ outer shells Atoms with ‘full’ outer shells appear in Group 0. The Group 0 elements are known as the noble gases because they are almost completely unreactive. This is because of their electronic structure (with ‘full’ outer shells). An electronic structure such as this is known as a noble gas structure. IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 8 Elements, mixtures and compounds A substance which contains just one type of atom is known as an element. There are well over 100 types of atom. For example, the element carbon contains only carbon atoms. An atom is the smallest part of an element you can get elements consist of just one type of atom (but there are countless numbers of atoms, not just one)! Each element has its own symbol. For example the symbol for carbon is C and the symbol for iron is Fe. The atoms of different elements can join together in chemical reactions, forming compounds. A COMPOUND is two or more different elements which are chemically bound together. The properties of compounds are usually very different from the properties of the elements they contain. For example sodium is a very reactive shiny metal and chlorine is a very poisonous green gas. Sodium chloride, which is made when these two elements react, is commonly known as salt and is safe to eat. A MIXTURE is when two more elements (or compounds) are mixed together but not chemically joined. Element Element IGCSE TOPIC 10.3: ATOMS AND ELEMENTS Compound 9 Mixture Mixture Metals and non-metals About one-quarter of the elements are non-metals. They are found on the righthand side of the periodic table. Nonmetals Non-metals have different properties to metals and are summarised in the table below: Properties of metals Properties of non-metals Shiny (when polished) They are solids at room temperature (except mercury) They conduct electricity Dull They can be solids, liquids or gases at room temperature They don’t conduct electricity (apart from carbon (graphite) They don’t conduct heat Brittle They conduct heat Strong (apart from group 1 metals which can be cut with a knife) Malleable Ductile Non-malleable Not ductile Alloys such as steel or brass are not elements and so are not found on the periodic table They are mixtures of metals often with several other metals and non-metals too. The properties of the component metals are modified by alloying, allowing the alloy to be used in new applications. IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 10 Topic 3:Atoms & Elements Summary questions 1 2 3 IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 11 4 5 6 IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 12 7 8 IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 13 IGCSE TOPIC 10.3: ATOMS AND ELEMENTS 14