chapter 5: atomic structure & the periodic table
TRANSCRIPT
Atoms: the smallest particle of an element
that retains the properties of that element.
(Greek: atomos = indivisible)
Democritus (Greek teacher in the 4th
century BC)
First suggested the idea that atoms
existed
Section 3.1: The Atom: From
Philosophical Idea to Scientific Theory
1700’s – chemists were able to relate
changes to individual atoms
Average atom size:
Mass = 1 x 10 –23 g
Diameter = 1 x 10-8 cm
How small is that?100,000,000 copper
atoms in a row would = 1 cm in length!
The Atom
Law of Definite Proportions
Definition: a chemical compound contains the
same elements in exactly the same proportions
by mass regardless of the size of the sample or
source
Example:
▪ Sodium chloride: NaCl always consists of exactly
39.34% sodium & 60.66%chlorine by mass
▪ Water: H2O always consists of exactly 11.18%
hydrogen & 88.82% oxygen by mass
Law of Multiple Proportions
Definition: if two or more different compounds
are composed of the same two elements, then
the ratio of the masses of the second element
combined with a certain mass of the first
element is always a ratio of small whole
numbers
Examples:
CO & CO2 : 1:1 ratio & a 1:2 ratio
H2O & H2O2: 2:1 ratio & a 2:2 ratio
1. All elements are composed of tiny
indivisible particles called atoms.
2. Atoms of the same element are
identical. The atoms of any one
element are different from those of
any other element.
Dalton’s Atomic Theory
3. Atoms of different elements can combine
with one another in simple whole number
ratios to form compounds.
H2O C12H22O11 NOT H2.5O¾
4. Chemical reactions occur when atoms are
separated, joined or rearranged. Atoms of
one element are not changed into atoms of
another by chemical means!
Most of Dalton’s Atomic Theory is accepted
One major revision includes that idea that
atoms are indivisible….
There are 3 parts to an atom….
1. electrons
2. protons
3. neutrons
Section 3.2 – The Structure of the Atom
Negatively charged subatomic particles J.J. Thomson discovered in 1897
Passed an electric current through gases at low pressures called a “Cathode Ray Tube”
Noticed the surface of the tube directly opposite the cathode glowed.
Why? Opposites attract and the electrons were attracted to the positive ends and lights up!
Cathode Ray Tube
Discovery of the Electron
Cathode rays are identical regardless of the element
Therefore all elements must have electrons!
Other important findings:
Atoms are electrically neutral, so they must contain a positive charge to cancel it out
Since electrons are so small, atoms must contain other particles that account for their mass
J.J. Thomson – plum-pudding model
e- are spread evenly throughout the positive
charge of the rest of the atom
Ms. Agostine’s “mint chocolate chip ice
cream model”
Robert Millikan (1868-1953)
Found quantity of charge in 1
electron (e-)
Also determined the ratio of the
charge to the mass of 1 e-
Calculated the mass of 1 e-
Electrons weigh 9.109 x10-31 kg
Most particles go straight through Positively charged particles deflect off of the
positively charged nucleus(~1/8,000)
Gold Foil Experiment
“…it was as if you fired a 15-inch [artillery]
shell at a piece of tissue paper and it came
back and hit you.”
Nucleus was very small
If a nucleus were a marble
the atom would be a football field
Protons (p+)
Positively charged
particles
Mass = 1.673 x 10-27
kg
1,836 times heavier
then an electron
Neutrons (no)
Subatomic particles
with no charge
Discovered by Sir
James Chadwick
Mass is nearly the
same as a proton
Mass = 1.675x10-27 kg
Particle
Symbol
Relative
Charge
Relative
Mass
(amu)
Actual
Mass (kg)
Electron e- 1- 1/1836 9.11x10-31
Proton p+ 1+ 1 1.67x10-27
Neutron no 0 1 1.68x10-27
Atomic Number : the number of protons in the
nucleus of an atom of an element
Atoms are electrically neutral
Tells how many electrons there are also!
Periodic Table
#1 – Hydrogen: has 1 p+ and 1 e-
#6 – Carbon: has 6 p+ and 6 e-
Mass Number – total number of protons and neutrons in a nucleus
# of neutrons = mass # - atomic # = (# p+ + # no) - (# p+) Ex) Beryllium – 9 Hyphen notation: The number “9” is the
mass number # of p+? # of no? # of e-?
Definition – atoms that have the same number
of protons but different numbers of neutrons
Different types of the same element
Ex) Carbon – has 3 isotopes
Carbon – 12
Carbon – 13
Carbon – 14
Differ by # of no
All have the same # of p+
If not, it would be a different element
All have 6 protons
Carbon – 12
Has 6 neutrons
Carbon – 13
Has 7 neutrons
Carbon – 14
Has 8 neutrons
Hydrogen-1: 1 p+ and 0 no
Relative abundance = 99.985 %
Commonly called normal “hydrogen”
Hydrogen-2: 1 p+ and 1 no
Relative abundance = 0.013%
Commonly called heavy hydrogen or “deuterium”
Hydrogen-3: 1 p+ and 2 no
Relative abundance = 0.002%
Commonly called “tritium”
Definition – weighted average mass of the
atoms in a naturally occurring sample of the
element
Carbon-12 = 98.89 % abundant
Carbon-13 = 1.11% abundant
Carbon-14 = ~0.0000001% abundant
Formula:
Atomic = relative • mass # + relative • mass # +
mass abund. abund.
Repeats for as many isotopes as exist for that element….
Units: atomic mass unit (amu): defined as
exactly 1/12 the mass of a carbon-12 atom
1 amu = approximately the mass of 1 proton
amu’s are used so you don’t have to use scientific notation
when talking about such small masses
Remember that the mass of 1 proton ≈ mass of 1 neutron
Sample Problem:
Chlorine has 2 isotopes:
chlorine-35 which is 75.77%
abundant and chlorine-37
which is 24.33% abundant.
What is the atomic mass of
chlorine?
35 Cl = 75.77% abundant = 0.7577 rel. abund. 37 Cl = 24.33% abundant = 0.2433 rel. abund.
Atomic mass =
= (35 amu x 0.7577) + (37 amu x 0.2433)
= (26.5195 amu) + (9.0021 amu)
= 35.5216 amu
Compare to value on Periodic Table = 35.453 amu
which rounds to 35.45 amu
The amount of a substance that
contains as many atoms, molecules,
ions, or other elementary units as the
number of atoms in 12.01 g C.
That number is equal to 6.02 × 1023
It is called Avogadro's number.
Proposed:
Equal volumes of different gases at
the same temperature and pressure,
contain the same number of
molecules.
How do you buy donuts?
How do you buy computer paper?
How do you buy pencils?
How do you buy soda?
You use a counting/measuring
amount!
1 dozen donuts = 12 donuts
1 ream of paper = 500 sheets
1 gross of pencils = 144 pieces.
1 case of soda = 24 cans
602,000,000,000,000,000,000,000
1 mole of paperclips: goes around earth 4
trillion times
1 mole of large marshmallows: covers the
continental USA 650 miles deep
1 mole of marbles: 116 times the size of Mt.
Everest
1 mole of pennies stacked up: distance to
the moon 1.55x1012 times
Mole of iron = 55.85 g Mole of sulfur = 32.07 g It’s not individual grains of iron or
sulfur! Where are these numbers coming
from? …Periodic table!!!
Definition: the mass of one mole of a substance (Units: g/mol)
AKA the atomic mass on the periodic table
Round all elements masses to two decimal places
Exception: Hydrogen gets 3 decimal places
12.01 grams of Carbon and 1.008 gram of Hydrogen contain the same number of atoms
6.022 x 1023 atoms
The gram atomic mass of any two elements must contain the same number of atoms
Carbon = 12.0107 g/mol
Rounds to 12.01 g/mol
Oxygen = 15.9994 g/mol
Rounds to 16.00 g/mol
Hydrogen = 1.00794 g/mol
Rounds to 1.008 g/mol
What if you have a compound?
1 mole NaCl = 58.44 g/mol
NaCl
1 Na = 22.99 g/mol
+ 1 Cl = 35.45 g/mol
58.44 g/mol
C12H22O11
12-C x 12.01g/mol = 144.12 g/mol
22-H x 1.008 g/mol = 22.176 g/mol
+ 11-O x 16.00 g/mol = 176.0 g/mol
342.30 g/mol
Al2(SO4)3
2-Al x 26.98 g/mol = 53.96 g/mol
3-S x 32.07 g/mol = 96.21 g/mol
+ 12-O x 16.00 g/mol = 192.0 g/mol
342.17 g/mol
GAM: molar mass of an atom or element
Ex) Fe, Cu, P, S… GMM: molar mass of a molecule (nonmetal compound)
Ex) H2O, Cl2, O2, CO2, P2O5
GFM: molar mass of a formula
unit (ionic compounds)
Metal-nonmetal compounds
Ex) NaCl, FeBr3, Zn3(PO4)2
If one mole of water is 18.02 g, how much
would two moles weigh?
36.04 g
How much would 3 moles weigh?
54.06 g
If one mole of water is 18.02 g, how many
moles are in 90.1 g?
5 moles
Three types of Mole Conversions
Mole – mass (g)
Mole – representative particles
(molecules, atoms, formula units)
Mole – volume (L)
If one mole of Helium weighs 4.00
g/mol, how much does 2 moles weigh?
Can you weigh out something less dense
then air?
The volume of a mole of a gas is much more
predictable than that of a liquid or solid
That is, under the same physical conditions
(STP)
Standard temperature and pressure
▪ Standard Temperature is 00C
▪ Standard Pressure is 1 atm
At STP, 1 mole of any gas will occupy a
volume of 22.4 L
22.4 L is known as the molar volume of a gas
What does it mean?
It means that 22.4 L of any gas at STP contains
6.02 X 1023 representative particles of that gas.
Standard Temperature and Pressure (STP): specific conditions that can be reached in the lab
Standard Pressure: 1 atmosphere (atm)
Standard Temperature: 0oC or 273 K
How many liters of gas are 3.612x1024
atoms of Neon gas at STP?
Given:
Unknown:
Conversion Factor:
Solve:
Temperature = a measurement of the average kinetic energy of the particles of an object
Scales
Fahrenheit (oF)
Celsius (oC)
Kelvin (K)
Absolute Zero – coldest possible temperature where all movement stops, theoretical value
0 Kelvin = -273.15 oC = -459 oF