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Atoms, Molecules and Ions Chapter 2 In This Chapter • History of atoms. • Subatomic particles. • Atomic numbers, mass numbers. • Isotopes and Atomic weights. • Compounds, Molecules and Ions. • Nomenclature. 2 1 Elements • An element is a pure substance that can’t be changed into a simpler form of matter by a chemical reaction. • Elements consist of atoms; the smallest structural unit of an element that retains the chemical properties of that element. • Atoms are considered as the building blocks of every chemical around us. 3 The Origin of Elements • After a very short time (matter of seconds) that the universe was produced by the Big Bang the only elements present were hydrogen and helium. • After millions of years the cooling of the universe caused the hydrogen and helium to collect together and form large clouds, due to gravity. • These clouds eventually contracted, became hotter and hotter and burst to become stars. • Intense heat within the stars made the hydrogen and helium atoms to smash together and produce other heavier elements. • This merging releases even more heat which is responsible for starlight. 4 2 Names of Elements • Names come from various sources including early Greek, Latin or German names describing some property of the element, names of places, names of people. – Iodine from Greek word iodes – violet like. – Bismuth from German weisse masse – white mass, miners called it wismat. – Germanium and Francium – discovered in – Element number 110 was recently named Darmstadtium to honor the German city in which it was discovered. – Einsteinium and Curium – to honor 5 Symbols of Elements • An universally accepted abbreviation for the name of the element. • Elemental symbols are unique. • 14 elements have single letter symbols, while the rest have 2. • The following rules are typically followed: – Symbols have either one or two letters. – If only one letter is used it is capitalized, C, H, O. – If 2 letters are used only the first letter is capitalized, Na, Ca, Hg. 6 3 Early Atomic Theories • Democritus, a Greek philosopher (460 –370 BC) proposed the idea of an “atom” as an indivisible component of matter, using the word atomos which means indivisible. • John Dalton, in 1808, published A New System of Chemical Philosophy in which he presented his theory of atoms. • Dalton’s theory was the first one to provide a systematic description of the relationship between atoms and matter. 7 Dalton’s Atomic Theory • Dalton’s basic postulates (ideas) were: – All matter is composed of atoms which are indivisible and indestructible. Atoms are considered as the ultimate chemical particles. – An element is composed of identical atoms with fixed, identical properties and masses. – Compounds are formed by the combination of atoms of 2 or more different elements in a fixed whole number ratio. – A chemical reaction involves a combination, separation or rearrangement of atoms. Atoms are neither created nor destroyed. 8 4 Problems with Dalton’s Theory • Atoms can be divided further. • Elements can have more than one mass. • Here is what we know about the atom now: 9 Regions of an Atom • All atoms consist of 2 distinct regions. • The nucleus: A small dense region at the center which contains positively charged protons and neutral (with no charge) neutrons. • Surrounding the nucleus is a diffuse region which contains the negatively charged electrons. • Atoms of different elements have different # of protons, electrons and neutrons. • The # of protons decides the identity of the element. 10 5 Properties of Subatomic Particles Proton (p) Charge = 1 Mass = 1.6725 x 10-24 g Neutron (n) Charge = 0 Mass = 1.6750 x 10-24 g Electron(e-) Charge = -1 Mass = 9.1095 x 10-28 g • Protons and neutrons have masses that are similar to each other. • The electron is about 1800 times lighter than the other 2. 11 Historical Discoveries • J. J. Thomson studied electrical beams in partially evacuated cathode-ray tubes and proposed that the cathode-ray consisted of negatively charged particles, which we now know of as the electron. • Thomson determined the charge to mass ratio for the electron and proposed a model for the atom called the plum pudding atom. • Thomson won the Nobel Prize for Physics in 1906 for his work. 12 6