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Chapter 2: Atoms, Molecules, and Ions

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1 Chapter 2: Atoms, Molecules, and Ions
Chemistry 1061: Principles of Chemistry I Andy Aspaas, Instructor

2 Atomic theory of matter
Dalton’s atomic theory All matter composed of indivisible atoms Atom: small particle, retains identity in reactions Element: type of matter composed of only one kind of atom Compound: type of matter composed of fixed proportion of 2 or more elements Chemical reaction: rearrangement of atoms to give new chemical combinations

3 Deductions from Dalton’s Atomic Theory
Law of conservation of mass Law of definite proportions Example: 1.00 g C reacted with oxygen 2 compounds formed g O : 1.00 g C g O : 1.00 g C

4 Structure of the Atom Dalton said atoms were indivisible, but experiments starting around 1900 showed that atoms themselves consist of particles Nucleus: atom’s central core Positively charged Contains most of atom’s mass Electron Very light, negatively charged particles

5 Discovery of the electron
1897, British physicist J.J. Thomson Experiments showed atoms are not indivisible Cathode ray tube Calculated ratio of electron’s mass to its charge 1909, American Robert Millikan Charge on electron: x coulomb Mass: x kg

6 Nuclear model of the atom
1911, British physicist Ernest Rutherford Gold foil experiment Most alpha particles pass straight through gold foil Most of atom = empty space If a golf ball was the nucleus, the atom would be about 3 miles in diameter

7 Gold foil experiment

8 Nuclear isotopes Nucleus comprised of 2 kinds or particles Protons Have positive charge equal in magnitude to electron’s negative charge More than 1800 times more massive than electron Neutrons Mass nearly identical to proton, no charge

9 Atomic number and mass number
Atomic number (Z): total number of protons in the nucleus of an atom New element definition: substance whose atoms all have the same atomic number Mass number (A): total number of protons and neutrons in a nucleus Nuclide: atom characterized by certain mass number and certain atomic number Nuclide symbol: Z subscript, A superscript, element

10 Isotopes Isotopes: atoms whose nuclei have the same atomic number but different mass numbers Some elements have only one naturally occurring isotope (sodium-23) Some have several naturally occurring isotopes: Oxygen contains the following isotopes 99.769% oxygen-16 0.037% oxygen-17 0.204% oxygen-18

11 Atomic weights Dalton: an atom of a certain element has a characteristic mass But, naturally occurring elements may be a mixture of isotopes Isotope percentages are generally constant Dalton actually calculated average atomic masses They were relative to the mass of hydrogen, the smallest element

12 Atomic mass units and atomic weight
Carbon-12 isotope chosen arbitrarily as standard 12 atomic mass units (amu) Atomic weight: average atomic mass for a naturally occuring element, expressed in amu Appears on periodic table along with atomic number

13 Calculating atomic weight
Fractional abundance: fraction of a total number of atoms, which consists of a particular isotope Isotopic mass is not exactly equal to mass number Neon-20, mass = amu, abund = Neon-21, mass = amu, abund = Neon-22, mass = amu, abund = Multiply isotopic mass by fractional abundance, and sum total to get atomic weight Gives a “weighted average”

14 Periodic table of the elements
1869, Dimitri Mendeleev (and J. Lothar Meyer, independently) Arranged elements in order of atomic number Placed elements in horizontal rows so that vertical columns of elements with similar properties formed

15 Periods Period: elements in any one horizontal row of the periodic table First has only 2 2nd and 3rd have 8 4th and 5th have 18 6th has 32 7th is incomplete

16 Groups Group: elements in any one vertical column of the periodic table Are numbered with Roman numeral and letter (or just numbered with a number) Main-group elements (labeled with A) Transition elements (labeled with B) Inner transition elements (shown below rest of table)

17 Groups Elements in a single group have similar properties For example: IA: alkali metals - all react vigorously with water (except hydrogen) VIIA: halogens - all react vigorously with sodium

18 Metals, Nonmetals, Metalloids
Metal: substances or mixtures that… Have a characteristic shine Generally are good conductors of heat and electricity Are usually malleable (can be hammered into sheets) Are usually ductile (can be drawn into wire) Metallic elements are solid at room temp., except for mercury

19 Metals, nonmetals, metalloids
Nonmetal: element that does not exhibit characteristics of a metal Most are gases Solid nonmetals are usually brittle and hard Bromine is only liquid nonmetal Metalloid (or semimetal) - both metallic and nonmetallic properties Many are semiconductors (silicon, germanium) Don’t conduct when pure, do when doped or at very high temperature

20 Chemical formulas Chemical formula: atomic symbols with numeric subscripts that show relative proportions of atoms of different elements in a substance Al2O3: ratio of Al atoms to O atoms is 2 : 3 No subscript: ratio of 1 (NaCl, 1:1)

21 Molecular substances Molecule: group of atoms that are chemically bonded together - tightly connected by attractive forces Molecular substance: composed of all the same molecules Molecular formula: shows number of different atoms of an element in a molecule Structural formula: shows how atoms are bonded together, a line indicates a chemical bond

22 Molecular substances Some elements are molecular substances
Cl2, O2, N2, S8 Some exist as individual atoms He, Ne Some exist as a very large but indefinite number of atoms bonded together C Polymer: extremely large molecules made of small molecules repeatedly bonded together (monomers) Natural (wool, cotton), synthetic (plastics, Nylon, polyester, Kevlar)

23 Ionic substances Unlike molecular substances, ionic substances are formed from charged atoms or groups of atoms called ions Metal atoms tend to lose electrons: result is a positive charge (cation) Nonmetals tend to gain electrons: result is a negative charge (anion) Sodium loses one electron to become Na+ Calcium loses two electrons to become Ca2+ May consist of a group of bonded atoms that have a deficiency or surplus of electrons (SO42+)

24 Ionic compounds Ionic compounds are composed of cations and anions Ionic formula: given by smallest possible integer number of different ions in substance Charges are omitted Ionic compounds are uncharged as a whole Ex. Na+ and Cl- = NaCl Fe3+ and SO42- = Fe2(SO4)3

25 Naming simple compounds
Chemical nomenclature: systematic naming of chemical compounds Organic compounds: carbon-containing molecular substances Inorganic compounds: composed of elements other than carbon (except CO, CO2, cyanides, and carbonates)

26 Ionic compounds Named given by name of cation followed by name of anion Potassium sulfate Monatomic ion: formed from a single atom

27 Predicting a monatomic ion’s charge
Most main-group metallic ions have one monatomic cation with charge equal to group number Aluminum, Group IIIA, Al3+ Many high-atomic-number metals have a cation equal to group number AND one equal to the group number minus 2 Thallium, Group IIIA, Tl3+, Tl+ Most transition elements can have several monatomic cations. +2 is very common A nonmetal main-group ion forms an anion Charge = 8 - group # Oxygen, Group VIA, O2-

28 Naming monatomic ions Monatomic cations are named after element if there’s only one possible cation for that element If more than one possible cation, use charge as roman numeral after name Fe2+ = iron(II), Fe3+ = iron(III) Or use suffixes with latin name (-ous for lower charge, -ic for higher) Cu2+ = cuprous, Cu3+ = cupric Monatomic anions use -ide suffix (bromide, sulfide)

29 Polyatomic ions Polyatomic ion: ion consisting of 2 or more atoms chemically bonded together, which carry a net electric charge Oxoanions: central element + some oxygens Suffixes indicate relative amount of oxygens Fewer oxygens: -ite More oxygens: -ate Ex. SO32- is sulfite, SO42- is sulfate

30 Polyatomic anions If more than 2 oxoanions, use prefixes hypo- and per- ClO- hypochlorite ion ClO2- chlorite ion ClO3- chlorate ion ClO4- perchlorate ion

31 Binary molecular compounds
Name first element with its exact name Suffix second element with -ide Use Greek prefixes to indicate number Don’t use mono unless there are more than one possible amount Ex. CO vs CO2

32 Acids and corresponding anions
Acids have H+ as the cation Oxoacids contain an oxoanion -ate becomes -ic -ite becomes -ous End name with “acid” Binary compounds of hydrogen and nonmetal Prefix hydro- Suffix -ic acid

33 Hydrates Simply indicate how many water molecules are present by using the Greek numerical prefix and -hydrate at the end MgSO4 • 7H2O = magnesium sulfate heptahydrate

34 Writing chemical reactions
Reactant ----> Product Arrow means “reacts to form” or “yields” Useful to indicate state or phase (g) = gas, (l) = liquid, (s) = solid, (aq) = aqueous solution Use a coefficient to indicate relative number of particles involved ∆ over arrow means heat is applied A compound written over the arrow is usually a catalyst

35 Balancing chemical equations
Mass must be conserved so use coefficients to make sure the same number of each atom occurs on each side of the equation Start by balancing atoms for elements that occur in only one substance on each side Ex: H3PO3 ---> H3PO4 + PH3 Start by balancing oxygen Practice!

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