Download The Atom

The Atom
Chapter 3
From Idea to Theory

Democritus, Greek philosopher, 400 B.C.,
introduced the concept of an atom, an
‘indivisible’ particle.
Basic Laws of Matter

Law of Conservation of Mass- mass can’t be
created or destroyed during ordinary chemical
rxns and physical changes.

Carbon + oxygen  carbon dioxide
12 g
32 g
44 g

Law of Definite Proportions- a certain cmpd
will always have the same proportion of each
element, by mass and by atoms.

Ex. Carbon dioxide always has one carbon atom to 2
oxygen atoms OR 12 g carbon and 32g oxygen
Basic Laws of Matter

Law of Multiple Proportions- more than one
cmpd can be formed from the elements by
having a different whole number ratio of the
atoms

Ex. CO, CO2
Dalton’s Atomic Theory
1.
2.
All matter is composed of small particles
called atoms.
Atoms of the same element are the same
physically and chemically.
3. Atoms can’t be divided, created, or destroyed.
4. Atoms of different elements combine in
whole-number ratios to form cmpds.
5. In chemical rxns, atoms are combined,
separated, or rearranged.
Section Review p69
Structure of Atom

Atom- smallest particle of an element that
retains the chemical properties of that element.

Subatomic particles:
Nucleus- center of atom
 Proton- Positively charged particle in nucleus
 Neutron- neutral particle in nucleus, a tiny bit larger
than proton
 Electron- negatively charged particle, about 2000X
smaller in mass that proton

Cathode Ray Tube

Cathode Ray tube experiment by JJ Thomson
discovered the electron (See pg. 70)

Conclusions from Experiment:
1. Rays cast shadow on other end of tube- light
2. Rays push paddle along- some form of matter
3. Rays deflected by magnet- not light
4. Rays deflected by negative charge- therefore
cathode rays are made up of negatively charged
particles called electrons.
Discovery of Nucleus

Robert Millikan determine the mass of an
electron to be 9.109 X 10-31 kg. Since atoms are
neutral, atoms must have positive charge.

Ernest Rutherford’s Gold Foil Experiment
discovered the nucleus. See pg. 73
Gold Foil Experiment

Conclusion of Gold Foil Experiment
1. Most of the alpha particles passed through –
atom mostly empty space.
2. Few deflected and reflected alpha particles small, dense, positive, nucleus in atom
Note: nuclear forces hold neutrons together in the
nucleus.
Section Review p74
Counting Atoms

Atomic number- number of protons in nucleus
of an atom. Number of protons in the nucleus
determines which element it is.

Ex. All C atoms have 6 protons. No other element
has 6 protons. Locate the atomic number of carbon
on the periodic table
Isotopes

Isotopes- atoms of the same element with
different number of neutrons, so their masses
are different. Isotopes of the same element are
chemically different

Ex. 3 forms of hydrogen
1. Protium- one proton, no neutrons – 99.985%
in nature
2. Deuterium- one proton, one neutron –
0.015% in nature
3. Tritium- one proton, two neutron –
radioactive- very little in nature, can be made
Counting Atoms


Mass number- number of protons and neutrons
in the nucleus of an isotope
Ex.
Protium’s mass number is 1 (1p + 0n),
hydrogen-1 (hyphen notation), nuclear
symbol =
Deuterium’s mass number is 2 (1p + 1n),
hydrogen-2, nuclear symbol =
Tritium’s mass number is 3 (1p +2n),
hydrogen-3 , nuclear symbol =
Practice
1. How many protons, electrons, and neutrons are
in chlorine-37?
Practice p78
Atomic Mass

Mass of one atom of carbon-12 is 12 amu
(atomic mass unit). So all atomic masses use C12 as standard.

Average atomic mass of an element is the
weighted average of all the naturally occurring
isotopes of that element

The average atomic mass depends on the mass and
abundance of each element’s isotope.
Atomic Mass Calculations
1.
Calculate the average atomic mass of copper
given the following abundances and atomic
masses for each isotope.
Isotope 1: 62.929598 amu and 69.17%
Isotope 2: 64.927793 amu and 30.83%

What is the mass of one aluminum atom? One
gold atom? One neon atom?
Mass to Number of Atoms



1 mole = 6.02 X 1023 particles = Avogadro’s
number
Molar Mass = number of grams of one mole of
a substance
Practice: What is the molar mass of each
element? He, Li, Hg
Mass and Moles
1.
What is the mass in grams of 3.50 moles of
copper?
2. How many moles of aluminum are in 11.9 g?
Practice p83
Moles to Atoms
3. How many moles of silver are 3.01 X 1023
atoms of silver?
4. How many atoms of tungsten make up 1.34 X
10-2 moles?
Atoms to Mass
5.
What is the mass in grams of 7.5 X 1015 atoms
of nickel?
6.
How many atoms of sulfur, S, are in 4.00 g of
sulfur?
7. What is the mass of 3.10 X 1023 atoms of Cl?