Chapter 4 Atoms and Elements Atoms - indestructible building blocks from which are substances are constructed. 2006, Prentice Hall CHAPTER OUTLINE Atoms, Elements and Symbols Periodic Table of the Elements Properties of Metals and Non-Metals The Atomic Theory The Modern Atom Atomic Structure Ions Isotopes and Atomic Mass 2 Experiencing Atoms • atoms are incredibly small, yet they compose everything • atoms are the pieces of elements • properties of the atoms determine the properties of the elements If every atom within a pebble were the size of the pebble itself, then the pebble would be larger than Mt. Everest (~29,000 ft) Experiencing Atoms • there are about 117 elements found in nature – • and 23 have been made in laboratories each has its own, unique kind of atom – – they have different structures therefore they have different properties If you keep dividing matter into smaller and smaller pieces you end up with an atom. EARLY CONCEPTS OF THE ATOM The smallest particle of matter that still retains its properties is called an atom. In the fifth century B.C., the Greek philosopher Democritus proposed that matter is composed of a finite number of discrete particles, named atomos (meaning un-cuttable or indivisible) 5 DALTON’S ATOMIC THEORY In Dalton’s 1808, John modelDalton, represented built on the ideas of atom as Democritus, a featureless and ball of uniform formulated a precise definition density. themodel building blocks of of This is referred to matter. as the “soccer ball” model. 6 DALTON’S ATOMIC THEORY Dalton’s atomic theory, explains the difference between an element and a compound. explains two scientific laws, and predicts a new scientific law. 7 DALTON’S ATOMIC THEORY Postulate 1 2 Deduction Each element consists of indivisible, small particles called atoms. All the atoms of an element are identical to one another, but different from others. Gives a more precise definition for an element. 8 DALTON’S ATOMIC THEORY Atoms of oxygen are different from All atoms atomsof ofoxygen hydrogen are identical to one another All atoms of hydrogen are identical to one another Atoms of consists each element are identical to one Atoms of indivisible, small particles. another, but different from others. 9 5.1 DALTON’S ATOMIC THEORY Postulate 3 4 Atoms combine chemically in definite whole-number ratios to form compounds. Atoms can neither be created nor destroyed in chemical reactions. Deduction Supports Law of Definite Composition; predicts Law of Multiple Proportions. Supports Law of Conservation of Mass. 10 LAW OF DEFINITE COMPOSITION As a result Atoms combine compounds in definite always whole-number contain elements ratios in to form the same compounds. proportions by mass. H 2 = O 1 H 1 = O 1 11 LAW OF MULTIPLE PROPORTIONS Two or more elements may combine in different ratios to form more than one compound. H 2 = O 1 H 1 = O 1 12 Sizes of Atoms • using compositions of compounds and assumed formulas, Dalton was able to determine the relative masses of the atoms – Dalton based his scale on H = 1 amu • we now base it on C-12 = 12 amu exactly – unit = atomic mass unit = amu • another term for amu is ? dalton • absolute sizes of atoms – mass of H atom= 1.67 x 10-24g – volume of H atom = 2.1 x 10-25cm3 Some Notes on Charges • Two Kinds of Charge called + and – • Opposite Charges Attract • Like Charges Repel • To be Neutral, something must have no charge or equal amounts of opposite charges The Atom is Divisible! • Work done by J.J. Thomson and others proved that the atom had pieces called electrons • Thomson found that electrons are much smaller than atoms and carry a negative charge – the mass of the electron is 1/1836th the mass of a hydrogen atom – the charge on the electron is the fundamental unit of charge which we will call –1 charge units DISCOVERY OF THE ELECTRON Smaller Negatively particles charged than particles the atom from also cathode exist and were are called pulled towards subatomic positively particles. charged plate, anode, allowed pass through and beexperiments detected on and In 1897, J.J.to Thomson performed a fluorescent screen. with a cathode ray tube. 16 DISCOVERY OF THE ELECTRON In These absence presence observations ofofaamagnetic magnetic indicated field, andthat electric the the cathode cathode fields, rays the were were cathode rays notrays deflected. negatively were deflected charged. towards the positive plate. These rays were later named electrons. 17 ATOMIC MODEL Based on these findings, Thomson proposed an atomic model, composed of negatively charged electrons embedded in a uniform positively charged sphere. This model is called the “plum pudding” model. 18 DISCOVERY OF THE NUCLEUS In these 1910, experiments Ernest Rutherford he bombarded carried out a thin a sheet number of experiments to further the of gold foil with -particles (large,probe positively nature of emitted charged) the atom. from a radioactive source. 19 DISCOVERY OF THE NUCLEUS Some Few majority The ofofthe theparticles particles of the particles were wereobserved observed were observed totobebeturned to pass through deflected at large un-deflected slightly back towards the angles. directionor they camedeflected. from. 20 NUCLEAR MODEL OF THE ATOM The Based deflections scatterings on these were observations, caused by glancing head-on Rutherford proposed collision of a model of particles with thethe atom consisting of a small, nucleus. massive positive Deflection center (nucleus), surrounded by electrons in mostly empty space. Scattering 21 5.5 Structure of the Atom • Rutherford proposed that the nucleus had a particle that had the same amount of charge as an electron but opposite sign – based on measurements of the nuclear charge of the elements • these particles are called protons – protons have a charge of +1 and a mass of 1 amu • since protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons Some Problems • How could beryllium have 4 protons stuck together in the nucleus? – shouldn’t they repel each other? • If a beryllium atom has 4 protons, then it should weigh 4 amu; but it actually weighs 9.01 amu! Where is the extra mass coming from? – each proton weighs 1 amu – remember, the electron’s mass is only about 0.00055 amu and Be has only 4 electrons – it can’t account for the extra 5 amu of mass The Must Be Something Else There! • to answer these questions, Rutherford proposed that there was another particle in the nucleus – it is called a neutron • neutrons have no charge and a mass of 1 amu – the masses of the proton and neutron are both approximately 1 amu THE MODERN ATOM The electrons current model (e-) move of the atom describes rapidly throughitthe as aatomic neutral spherical volume, held by the entity, composed forces attractive of a positively to the charged nucleus nucleus. surrounded by negatively The nucleus consists of charged electrons. positively charged protons (p+) and neutrally charged neutrons (n0). 25 ATOMIC STRUCTURE The number modern atom of protons consists in an of atom 3 subatomic determines its identity, particles:and is called atomic number (Z). In a neutral atom, the number of protons (+) are Relative equal to the number of electrons (–). Particle Charge Mass Almost all the mass of the atom rests in the Proton +1 ~1800 nucleus. Therefore the number 0of protons and neutrons in Neutron ~1800 an atom is called the mass number (A). Electron –1 1 26 Elements • each element has a unique number of protons in its nucleus • the number of protons in the nucleus of an atom is called the atomic number – the elements are arranged on the Periodic Table in order of their atomic numbers • each element has a unique name and symbol – symbol either one or two letters • one capital letter or one capital letter + one lower case The Periodic Table of Elements Elements: Origins of the Names of the Elements • Most chemical symbols are based on the English name of the element. • Some symbols are based on Latin names. • The symbol for potassium is K, from the Latin kalium, and the symbol for sodium is Na, from the Latin natrium. • Additional elements with symbols based on their Greek or Latin names include the following: lead Pb mercury Hg iron Fe silver Ag tin Sn copper Cu plumbum hydrargyrum ferrum argentum stannum cuprum Elements: Origins of the Names of the Elements • Early scientists gave newly discovered elements names that reflected their properties: • Argon, from the Greek argos, means “inactive,” referring to argon’s chemical inertness. • Other elements were named after countries: • Polonium after Poland • Francium after France • Americium after the United States of America. • Other elements were named after scientists. • Every element’s name, symbol, and atomic number are included in the periodic table (inside the front cover) and in an alphabetical listing (inside the back cover) in this book. Review • • • • What is the atomic number of boron, B? What is the atomic mass of silicon, Si? How many protons does a chlorine atom have? How many electrons does a neutral neon atom have? • Will an atom with 6 protons, 6 neutrons and 6 electrons be electrically neutral? • Will an atom with 27 protons, 32 neutrons and 27 electrons be electrically neutral? • Will a Na atom with 10 electrons be electrically neutral?