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Chapter 2 Atoms, Molecules, and Ions LAW OF CONSERVATION OF MASS • Antoine Lavoisier (1743-1794) • During a chemical change, the total mass remains constant LAW OF DEFINITE PROPORTIONS • Joseph Proust (1754-1826) • Law of Constant Composition • Different samples of a pure compound always contain the same proportion of elements by mass. LAW OF MULTIPLE PROPORTIONS • John Dalton (1766-1844) • When two or more elements form more than one compound, the ratio of the masses of one element in these compounds for a fixed mass of the other element is a small whole number. ATOMIC THEORY OF MATTER (1808) • John Dalton (1766-1844) • Elements (matter) are composed of atoms • Each element is characterized by the mass of its atoms. • A compound is a chemical combination of atoms in definite proportions. • A chemical reaction is the rearrangement of atoms leading to new compounds ATOMIC STRUCTURE Subatomic Particles (Table 2.1) • ELECTRON (cathode ray) 1899 JJ Thomson – Negative charge, -1 – Mass about 1/1836 of proton or neutron mass Milliken • PROTON 1911 Rutherford – Positive charge, +1 • NEUTRON – No charge, 0 Relative mass = 1 amu 1932 Chadwick Relative mass = 1 amu •MODELS OF THE ATOM • Thomson (Plum Pudding) Model: positive mass with electrons embedded in it • Rutherford Model (1911): positive charge in small volume with (diameter = 1E-15 m) electrons occupying mostly empty space (d = 1E-10 m) around the nucleus • Bohr Atom - Chapter 5 • Quantum Mechanical Atom - Chapter 5 ATOMIC STRUCTURE • Atomic Symbol – Shorthand notation for element – One or two letters on Periodic Table • Atomic Structure – Atomic Number (Z) = #protons, uniquely defines an atom – Mass Number (A) = #protons + #neutrons – If atom is neutral, Z = #electrons NUCLIDE SYMBOL • Atomic symbol, E; symbol in the middle of each element box on the Periodic Table. • Z (left subscript); number on the top of each element box on the Periodic Table. • A (left superscript) • If species is an ion (has a charge), add + or charge (right superscript) • A ch E Z IONS • A charged species with unequal numbers of protons and electrons. • If # protons > # electrons, the ion has a net positive charge and is called a cation • If # protons < # electrons, the ion has a net negative charge and is called a anion • An ion may consist of an atom or a group of atoms ISOTOPE • Atoms which have the same Z but a different A • This means that the #protons is the same but the #neutrons is different. • Most elements have isotopes that occur in nature in precise proportions (fractional abundances, %). • A few elements have no naturally occurring isotopes. ATOMIC MASS • One C-12 atom weighs exactly 12 amu • amu = atomic mass unit – 1.661E-24 g • Atomic mass of an element is defined as a weighted average over all naturally occurring isotopes of the element. • Number on bottom on each element box on the Periodic Table. PURE SUBTANCE (Fig. 2.7) • Pure substance, not separable by physical means, has a constant composition – Element: smallest entity which retains all properties of element, made up of atoms of one type (@116 with 90 occurring naturally) • May be an atom (Na, Si) or a molecule (S6, N2) – Compound: consists of more than one element combined in definite proportions; separable into constituent atoms by chemical means • There are over 20 million compounds CHEMICAL BONDS • Compounds contain atoms connected by chemical bonds which involve electron interactions. The electrons act as the “glue” between atoms. – If electrons are shared between two atoms, the bond is a covalent bond. I.e., the bond between two non-metal atoms. – If electrons are transferred to produce ions, the bond is ionic. • Ions are charged particles which form via the gain (anion, negative ion, formed from nonmetal elements) or loss (cation, positive ion, formed from metal elements) of electrons. • Oppositely charged ions attract and form an ionic bond. • Type of bond between a metal and a non-metal atom. • Polyatomic ions are charged groups of atoms; they can form ionic bonds. MIXTURE • Mixture: two or more pure substances present in any proportions; separable by physical means – Homogeneous: one uniform phase of the same composition and properties, a solution – Heterogeneous: > 1 phase with varying composition and properties CHEMICAL REACTION • A chemical reaction involves rearrangements of atoms; breaking initial chemical bonds (in the reactants) and making new chemical bonds (in the products). • R1 + R2 P1 + P2 + P3 CHEMICAL FORMULA • Qualitative description of the constituent elements : NH3, C12H22O11 • Quantitative description of the relative numbers of atoms of each element • Empirical - includes all atoms in molecule in correct smallest integral ratios • Molecular - includes all atoms in molecule in actual numbers and correct ratios Types of Chemical Formulas • Chemical - shows type and number of atoms (p. 52) • Structural - shows chemical bonds (p. 52) • Ball and Stick - shows spatial arrangement, 3D (Fig 2.9a) • Space filling - shows space atoms fill, 3D (Fig 2.9b) ACIDS, BASES, SALTS • Acid: Produces H+ ions and anion in water – HCl(aq) H+ (aq) + Cl-(aq) – Polyprotic acids produce more than one H+ ion. • Base: Produces OH- ions and cation in water – KOH(aq) K+ (aq) + OH-(aq) • Acid and Base react to form Salt (ionic compound) and Water. (neutralization) PERIODIC TABLE (Front End Page) • An arrangement of elements according to increasing atomic number which shows the periodic or regularly repeating nature of elemental properties. – Rows = periods – Columns = groups or families; note similarity of properties – Metals Nonmetals Semimetals – Main group (A) Transition (B) NOMENCLATURE or NAMING COMPOUNDS • Binary Ionic Compounds (Fig. 2.11, 2.12) – Metal atoms tend to lose electrons and form cations. – Nonmetal atoms tend to gain electrons and form anions. – Use Periodic Table to determine charges and number of each ion in the compound. Note that the ionic compound must be neutral overall. – Name cation first as element and anion second with “ide” ending. – Some transition metal elements form more than one common ion. Designate charge with Roman numeral NOMENCLATURE (con’t) • Polyatomic Ions (Table 2.3) – Memorize – Oxoanion = nonmetal + oxygen • Acids (Table 2.4) – Memorize – Oxoacids (named from oxoanions) NOMENCLATURE (con’t) • Binary Molecular Compounds (Table 2.2) – Name more “cation-like” first, then the more “anion-like) second with “ide” ending. Hydrogen is almost always named first. – Indicate number of each using prefix as needed. – Note historic names