ATOMS: THE BUILDING
BLOCKS OF MATTER
Chp 3 pg 62
I. The Atom
A. From Idea to Theory
1. Democritus – called nature’s basic
particle an atom
2. Aristotle
a. Didn’t believe in atoms
b. Matter is continuous – accepted for
2000 yrs
3. Foundations of Atomic Theory
a. Law of conservation of mass - Mass is
neither created or destroyed
b. Law of Definite proportions
1) Chemical compound contains the same
elements in exactly the same
proportions
2) Sample size doesn’t matter
3) Salt – always 39.34% Na and 60.66%
Cl
c. Law of multiple proportions
1) If 2 or more different compounds are
made of the same 2 elements, then the
ratio of the masses of the 2nd element
combined with a certain mass of the 1st
element is always a ratio of small whole
numbers.
2) Carbon dioxide C = 1g, O = 2.66g
3) Carbon monoxide C = 1g, O = 1.33g
4) Ratio = 2 to 1
B. Dalton’s Atomic Theory (pg 64)
1. All matter made of small particles (atoms)
2. Atoms of a given element are identical
(size, shape, mass)
3. Atoms can’t be divided, created or
destroyed
4. Atoms of different elements combine in
whole number ratios to form compounds
5. In chem reactions, atoms are combined,
separated, or rearranged
Where did Dalton go wrong?
- Atoms of a given element are identical –
can differ in mass
- Can divide atoms
Section Review – pg 67
C. Structure of the Atom
1. Atom – smallest particle of an element
that keeps chemical properties of that
element
2. Two regions
a. Nucleus – small region at the center
1) Protons – Positively charged
2) Neutrons – no charge
b. Electrons – negative charge
1) Very large compared to nucleus
2) Mostly open space
3) Mass = 9.109 x 10-31 kg
2. Subatomic particles – protons, neutrons,
electrons (Fig. 2.5 pg 72)
4. Discovery of the Electron
a. Cathode-ray – gas
filled tube at low
pressure
b. Pass electricity
through it
c. It glows from
particles
d. Particles go from
cathode to anode
e. Particles reflected
by a negative charge
f. Particles have
same mass
regardless of the gas
g. Particles called
electrons
• Fig 2.2
Thomson’s Plum Pudding Model
5. Discovery of the Atomic Nucleus
a. Shot positively charged alpha particles
into foil
b. About 1 in 8000 were redirected
c. Concluded:
1) Must be a small dense object in the
middle (nucleus)
2) Must have a positive charge
3) Electrons must be around the nucleus
Nucleus = marble then atom = football field
6. Composition of Nucleus
a. Has both protons and neutrons
b. Protons have a charge of +1 to balance
c.
d.
e.
f.
g.
out electrons
Equal number of protons and electrons
Proton mass = 1.673 x 10-27; 1836x
greater than electron
Neutron mass = 1675 x 10-27
Gives atom its weight (electrons give
size)
# of protons determines the element
h. Nuclear force
1) Holds proton-proton together, pro-neu,
and neutron-neu
2) Stable up to 83 protons
#1, 2c, 3-5
D. Counting Atoms
1. Atomic number
a. Number of protons in nucleus
b. Atoms of same element always have
same number of protons
2. Isotopes
a. Atoms of same element with different
masses
b. Change # of neutrons or electrons
Isotopes of Hydrogen
3. Mass number - total number of protons
and neutrons in the nucleus
4.Designating Isotopes
a. Usually written with a hyphen (Uranium235)
b. Uranium-235 = 23592U
1) Top is mass number, bottom is
atomic number
2) Mass # - atomic # = number of
neutrons
• Sample problem A pg 75
• Practice pg 76
5. Relative Atomic Masses
a. Based off Carbon-12
b. amu (atomic mass unit) – 1 amu = 1/12
the mass of carbon-12
6. Relating Mass to Number of Atoms
a. Mole (mol) – amount of substance that
contains as many particles as there are
atoms in exactly 12 g of carbon-12
b. It is a counting unit
Moles of different substances
c. Avogadro’s number
1) # of particles in 1 mol of substance
2) 6.02 x 1023 particles (or atoms)
d. Molar Mass
1) Mass of one mole of a pure
substance
2) Unit = g/mol
3) Mol mass of an element = atomic
mass
a) M.M. Oxygen = 15.999 g/mol
b) M.M. Hydrogen = 1.009 g/mol
4) Molar mass of compounds
a) Pb
b) H2O
c) C6H12O6
d) Na2(SO4)
e) (CH4)3
e. Gram/mole/atom conversions (pg 80)
1) How many grams of helium are there in
two moles of helium?
2) If you have 5 moles of oxygen how
many grams do you have?
3) If you have 125g of sodium how many
moles do you have?
4) If you have 15g of water how many
moles do you have?
Mole assignment 1
5) If you have 7 moles of N how many
atoms do you have?
6) How many atoms are in 23 moles of F?
7) If are given a bottle of 10 grams of CO2
how many moles are in the bottle? How
many particles?
mole assignment 2