Chapter 2 Atoms, Molecules, and Ions

LAW OF CONSERVATION OF MASS Antoine Lavoisier ( ) During a chemical change, the total mass remains constant

LAW OF DEFINITE PROPORTIONS Joseph Proust ( ) 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 ( ) 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 ( ) 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 massMilliken PROTON1911Rutherford –Positive charge, +1Relative mass = 1 amu NEUTRON1932Chadwick –No charge, 0Relative 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 Z E ch

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 with 90 occurring naturally) May be an atom (Na, Si) or a molecule (S 6, N 2 ) –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 : NH 3, C 12 H 22 O 11 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 NonmetalsSemimetals –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