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Chemistry 100. Chapter 2 Atoms, Molecules and Ions. Law of Conservation of Matter. By 1800, chemists had noted that the mass of reactants equals the mass of products - provided you capture any escaping gas Matter is not created or destroyed in a chemical reaction. Law of Constant Composition.
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Chemistry 100 Chapter 2 Atoms, Molecules and Ions
Law of Conservation of Matter • By 1800, chemists had noted that the mass of reactants equals the mass of products - provided you capture any escaping gas • Matter is not created or destroyed in a chemical reaction
Law of Constant Composition • Chemists (notably Proust) • The relative amount (percentage) of each element in a compound was the same no matter how the compound was made • These two laws lead Dalton to revive the Atomic Theory • Matter is made up of small, indivisible particles
Dalton’s Atomic Theory • An element is composed of atoms. • All atoms of a given element are the same. • Atoms of different elements are different and have different properties.
Dalton’s Theory-II • Atoms are not changed, created or destroyed in a chemical reaction. • Compounds are the combination of more than one element. • A given compound has the same relative number and kind of atoms.
Law of Multiple Proportions • Accorrding to Dalton’s Theory • Two elements (A and B) form two distinct compounds • The amounts of B combining with a fixed amount of A would be a small whole number ratio. • Water: 1 g hydrogen + 8 g oxygen • Hydrogen peroxide: 1 g hydrogen +16 g oxygen
Atomic Structure • Roentgen discovered X-rays (1895) • Becquerel discovered radioactivity (1896) • J. J. Thompson discovered the electron (1897) • Rutherford classified radioactivity emissions: alpha (), beta () and gamma ()
Alpha, beta, gamma • Rutherford’s findings: • Alpha are positive particles (+2) ; heavier than electrons • Beta are high speed electrons; negatively charged particles (-1) • Gamma are neutral rays • Alpha particles are nucleus of He atom
Thompson’s Model “Plum pudding” model A cloud of positive charge holding the negatively charge electrons in place
Rutherford’s alpha experiment • Scattering of alpha particles by gold foil • Most particles were undeflected • Some were deflected by large angles
Rutherford’s Model • Centre - the nucleus - is small but positively charged • Most of the atom is empty spaces • Electrons rotate about the nucleus - like the solar system
Modern Model • Additional experiments showed • Nucleus consists of protons (positive) and neutrons (neutral) • Electrons (negative) exist around the nucleus • Number of protons = number of electrons
Mass of Elementary Particles • Protons and neutrons have a mass of about 1 amu • Electrons have very small mass • Most of the mass of an atoms comes from nucleus (1 amu is 1.66054 10-24 grams)
Isotopes • All atoms of a given element have the same number of protons. All Carbon atoms have 6 protons (and 6 electrons). • The number of protons is different for each element. • Atoms of a given element that differ in the number of neutrons are called isotopes. • Examples : carbon-12 and carbon-14
Atomic Masses • By international agreement, the carbon-12 atom is defined as having a mass of exactly 12 atomic mass units (amu’s). • All atomic masses are referenced to this standard.
Ca 20 40.078 The Periodic Table • A typical entry in the periodic table Atomic number Atomic mass
Periodic Table (II) • Elements in the periodic table are arranged in • Groups or families – they have similar chemical and physical properties • Metals – towards the left • Nonmetals – towards the right • Metalloids – in the middle region
Atomic Masses in the Periodic Table • Question: why is the mass of C in the periodic table reported as 12.01 amu and not as 12.000 … amu, exactly? • Another example: the atomic mass of Cl is = 35.453 amu’s. We would expect Cl to be 35 amu?
Ionic Compounds • Tables of common ions in textbook (pages 60 and 63). • Ionic compounds • Cation name followed by anion name, e.g., sodium bromide (NaBr) • Multiple ion types • FeCl2 – iron (II) chloride • FeCl3– iron (III) chloride
Binary Molecular Compounds • Binary compounds containing two nonmetals • name of the first element in the formula followed by the stem of the name of the second element with the suffix -ide. • The number of atoms of each element in the compound is indicated by a prefix. • Some common names must be committed to memory. • Examples – nitrous oxide, N2O, and nitric oxide, NO. mono di tri tetra 1 2 3 4
Formulas and Names of Acids • An acid usually is a compound of hydrogen and a nonmetal or a polyatomic anion. • Treat the hydrogen atoms of the acid as H+ ions. • For acids containing monatomic anions, • When these acids are found in water solution, add the prefix hydro- and the suffix -ic to the stem of the name of the anion • Hydrofluoric acid (HF), hydrochloric acid (HCl)
Acids From Polyatomic Anions • If the anion name ends in ‘ate’, the ‘ate’ in the name of the anion is replaced by ‘ic acid ’ • The acid of the sulfate ion is sulfuric acid (H2SO4) • The acid of the nitrate ion is nitric acid (HNO3) • If the anion names end in ‘ite’, we change the suffix to –ous and add the word acid. • The acid of the nitrite is called nitrous acid, HNO2 • The acid of the hypochlorite ion is called hypochlorous acid, HClO
Average Atomic Masses • Most elements in nature exist as mixtures of isotopes. • Atomic masses reported in the periodic table - weighted averages of the different isotopes. • The amount of each isotope in a sample of as an element may vary considerably with the source of the sample. • This is the reason why some elements in the periodic table have few significant figures for their mass.
Organic Compounds • Many organic compounds have complex three dimensional structures • chains, and/or rings • branches. • The highlighted groups are called functional groups. • They are primarily responsible for the chemical and some physical properties of the molecules.
