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1 Atoms and theories of matter 2 © Zanichelli editore 2015 What are atoms? 3 © Zanichelli editore 2015 The atomic theory The atom is the smallest particle of an element that preserves its chemical properties. Molecules are made up of combined atoms. Ions are atoms or molecules that have a positive or negative electrical charge. 4 © Zanichelli editore 2015 Dalton’s law of multiple proportions The law of multiple proportions states that when two elements combine to give more than one compound, the masses of one element that combine with the fixed mass of the other are in a ratio of small whole numbers. Compound Oxygen mass Carbon mass Ratio Carbon monoxide 4g 3g 1.33 : 1 Carbon dioxide 8g 3g 2.66 : 1 5 © Zanichelli editore 2015 The atomic theory /1 Dalton developed the first atomic theory in order to explain the law of conservation of mass, the law of definite and constant proportions and the law of multiple proportions. According to the first atomic theory, matter is made up of small atoms that cannot be created, divided or destroyed. 6 © Zanichelli editore 2015 The atomic theory /2 All atoms of an element are identical and have the same mass. Atoms of different elements combine in simple number to form compounds. Carbon monoxide 1 g carbon + 1.33 g oxygen Carbon dioxide 1 g carbon + 2.66 g oxygen 7 © Zanichelli editore 2015 Electrical properties of matter + + + - Bodies can have a positive (+) or negative (-) charge or they can be neutral, which means they have the same number of positive and negative charges. Objects with opposite charges attract each other; objects with the same charge repel each other. 8 © Zanichelli editore 2015 Subatomic particles Atoms are made up of subatomic particles: • protons (p), with charge +1 and mass equal to 1.673 10-27 kg; • electrons (e–), with charge -1 and negligible mass; • neutrons (n), with neutral charge and mass equal to 1.675 10-27 kg. Particle Mass Charge Electron 9.109 10-31 – 1.602 10-19 Proton 1.673 10-27 1.602 10-19 Neutron 1.675 10-27 0 9 © Zanichelli editore 2015 Atomic number and mass number Each atom is identified by an atomic number (Z) and a mass number (A). Mass number = number of p + n Atomic number = number of p A Z E Z is the number of protons and it also indicates the number of electrons in a neutral atom. A is the sum of protons and neutrons. 10 © Zanichelli editore 2015 The structure of the atom 11 © Zanichelli editore 2015 The atomic nucleus Atoms have a tiny, dense nucleus with a positive charge. The nucleus is made up of protons and neutrons and is surrounded by an empty space in which electrons move. Nuclear force keeps protons and neutrons together inside the nucleus, overcoming the mutual repulsion caused by the positive charges. 12 © Zanichelli editore 2015 Isotopes Isotopes are atoms that have the same atomic number but different atomic mass. They have the same nuclear charge and chemical properties, but they differ in their mass and in some physical properties. 13 © Zanichelli editore 2015 From atoms to ions E n+ E n- Number of positive charges = number of lost electrons Number of negative charges = number of acquired electrons Na – e– Na+ Cl + e– Cl– Atoms can lose or acquire electrons, becoming positive ions (or cations, when they lose one or more electrons) or negative ions (or anions, when they acquire one or more electrons). 14 © Zanichelli editore 2015 Electronic structure The number and disposition of an atom’s electrons influence its tendency to join other atoms. + In an atom, only certain energy levels are allowed. The first level can have two electrons, the second and third levels can have eight electrons each. An atom is stable when its outermost energy level is complete. 15 © Zanichelli editore 2015 Chemical bonds 16 © Zanichelli editore 2015 Chemical bonds With a chemical bond, atoms try to reach the most stable electronic configuration; by sharing, losing or gaining one or more electrons they can complete an energy level. 17 © Zanichelli editore 2015 Covalent bond When two atoms share a couple of electrons, they form a covalent bond. Each atom provides one electron in the couple. 18 © Zanichelli editore 2015 Ionic bond – + Na + Cl Na + Cl When two atoms have very different properties, with one atom attracting electrons more strongly than the other, the former gains one or more electrons, while the latter loses them. The atoms become ions, connected by electrostatic attraction, called ionic bond. 19 © Zanichelli editore 2015 Chemical formulae, equations and reactions 20 © Zanichelli editore 2015 Chemical formulae CO2 It indicates the element carbon It indicates the number of atoms of oxygen It indicates the element oxygen Elements and compounds are represented by chemical formulae: the formula indicates the type and number of atoms or ions in the smallest particle that has all the chemical properties of the substance. 21 © Zanichelli editore 2015 Formulae of the elements S S S S Al Chemical formula: Al Cl Cl Chemical formula: Cl2 S S S S Chemical formula: S8 In nature, seven elements are made of diatomic molecules, four elements contain more than two atoms, while the others are formed by single atoms. 22 © Zanichelli editore 2015 Formulae of compounds H H N H Chemical formula of ammonia : NH3 H H H C H Chemical formula of methane: CH4 The formula of a molecular compound describes the composition of a single molecule, while the formula of an ionic compound indicates the numerical ratio between ions, without showing their charges. 23 © Zanichelli editore 2015 Atomic mass units The relative mass of atoms and molecules is expressed in atomic mass units (u). It is a relative mass: 1u = 1/12 of the mass of a 12C. The atomic mass unit, also called dalton, is very small: 1 u = 1.6605 10-24 g 24 © Zanichelli editore 2015 Atomic mass and molecular mass The atomic mass of an element is the average mass of the atoms present in a natural sample. It is expressed in u. The molecular mass is the relative mass of a molecule and it is calculated by summing up the mass in u of the atoms of the molecule’s formula. 25 © Zanichelli editore 2015 The mole The mole (mol) is the quantity of substance that contains 6.02 1023 particles. The molar mass is the mass of a mole in grams; numerically it is equal to the mass of one particle of the substance in u. Mass (g) Quantity of substance (mol) m __ n= M Molar mass (g/mol) 26 © Zanichelli editore 2015 Chemical equations Chemical equations describe reactions, indicating changes in composition and the ratio between particles of the reagents and products. 2H2O The coefficient 2 indicates 2 molecules of water. O2 + 2H2 The coefficient 2 indicates 2 molecules of hydrogen. There is no coefficient before the oxygen: this means that there is only one molecule. 27 © Zanichelli editore 2015 Balancing chemical equations A chemical equation is balanced if the total number of atoms of each element in the reagents and in the products is equal. 2H2O O2 + 2H2 Reagents: 4 atoms of H, 2 atoms of O Products: 4 atoms of H, 2 atoms of O 28 © Zanichelli editore 2015 Balancing chemical equations: three practical rules In order to balance a chemical equation it is a good practice to follow these procedures: 1.balance the atoms of metals and non metals first; 2.even when they are present in compounds, oxygen and hydrogen should be balanced after the others, because they often appear in many formulae; 3.balance water and pure elements last. 29 © Zanichelli editore 2015