Atoms: The Building Blocks of Matter Ch. 3

Dimensional Analysis *A method of problem-solving using conversion factors Conversion Factor *Is essentially an equality ex/ 12 inches = 1 foot *Is written as a ratio to cancel units 12 inches OR 1 foot 1 foot 12 inches

Dimensional Analysis Problems 1. Convert 15 feet to inches. 2. Convert 176 feet to yards.

1. Convert 2,500 inches to yards. 2. Convert 0.0034 miles to inches.

25 days to seconds 1,000,000 minutes to years

Early atomic theory Democritus (400 B.C.) Aristotle Atoms are nature’s basic particles “atom”  Greek word meaning “indivisible” Aristotle Disagreed, thought matter was continuous Aristotle's theory prevailed for next 2000 yrs.

1700s 1st experimentation Definition of element (pure sub. made of 1 kind of atom) Elements combine  compounds

Late 1700s Quantitative studies of reactions law of conservation of mass

Dalton’s Atomic Theory All matter composed of atoms Atoms… of same element are identical; atoms of diff. elements are diff. cannot be divided, created, destroyed combine in simple whole # ratios to form compounds (Law of Definite Proportions) are combined, separated or rearranged in chem. rxns.

Modern Atomic Theory Not all of Dalton’s ideas were correct Atoms can be divisible Atoms of same element can have diff. masses Most still holds true

Sec. 2 - Structure of the Atom nucleus – protons & neutrons electron cloud

Discovery of the e- (1897) Cathode ray tube – electric current passed through gases, found neg. charged particles JJ Thomson Did more expts. and found particles identical (no matter which gas used) Named them electrons

Choc. chip cookie model Millikan Oil drop exp. Mass of e- extremely small, neg. charged Atom is neutral, therefore must contain + charge also Choc. chip cookie model - - + + - + - - + + + - + - - + +

Discovery of Nucleus Rutherford Chadwick discovered 1900 Gold foil exp. Confirmed dense nucleus with + charge Chadwick discovered neutrons ~30 yrs. Later

Modern atomic model nucleus: p+ and n0 e- cloud All the mass, very little space e- cloud No mass, lots of space

Nuclear forces Like forces repel, normally BUT when protons are really close there is a strong attractive force Same with neutrons Called nuclear forces (energy, bombs)

Size of atom Tiny Radius  from center of nucleus to edge of e- cloud Measured in pm (10-12) Atomic radii range from 40-270 pm

Sec. 3 - Counting Atoms Atomic # Ex: oxygen is 8  8 p+ and 8 e- # of protons in nucleus Determines what element Ex: oxygen is 8  8 p+ and 8 e-

Mass # Total # of p+ and n0 in an isotope Calculate # of neutrons: Mass # - atomic # (p + n) – (p) = n

Isotopes Atoms of same element with diff. # neutrons Diff. masses Ex: hydrogen H protium 99.985% H deuterium 0.015% H tritium trace, synthetic *Don’t differ much in chem. props. 1 1 2 1 3 1

Isotope symbols Hyphen uranium-235 Nuclear symbol U 235 92

Charges Br Ca+2 Atomic # Mass # Protons Neutrons electrons 35 80 45 36 Ca+2 20 40 18 80 35 80 35 40 20

masses so small, came up with scale that is relative amu’s atomic mass units masses so small, came up with scale that is relative amu = mass of 1/12 mass of carbon-12 atom Average atomic masses # on periodic table Decimals  averages

Copper – 63 53% Copper – 64 47% Arsenic – 74 13% Arsenic – 75 80% Arsenic – 76 7%

Relating Mass to Numbers of Atoms The mole is the SI unit for amount of substance A mole (abbreviated mol) is the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12.

Avogadro’s Number Avogadro’s number 6.022 1415  1023 is the number of particles in exactly one mole of a pure substance. Can find: Atoms  moles Moles  atoms

Molar Mass Molar mass - The mass of one mole of a pure substance usually written in units of g/mol numerically equal to the atomic mass of the element in amu’s.

Examples What is the mass in grams of 3.50 mol of the element copper, Cu? A chemist produced 11.9 g of aluminum, Al. How many moles of aluminum were produced?

How many moles of silver, Ag, are in 3.01  1023 atoms of silver? What is the mass in grams of 1.20  108 atoms of copper, Cu?

Chapter Review Pg. 89 # 1-3, 5-7, 11, 14-21, 27