Download Atoms: The Building Blocks of Matter

Atoms: The Building
Blocks of Matter
Chapter 3
Democritus vs. Aristotle:
Philosophical Ideas
• Democrituscalled nature’s basic
particle an atom, based on the Greek
word meaning “indivisible.”
• Aristotlecame after Democritus; did
not believe in atoms, thought all
matter was continuous.
• Neither view supported by
experimental evidence until the 18th
Century.
Foundations of Atomic Theory
• Law of conservation of mass
• Law of definite proportions
• Law of multiple proportions
Dalton’s Atomic Theory (1808)
1.
2.
3.
4.
5.
All matter is composed of extremely small
particles called atoms.
Atoms of a given element are identical in
size, mass, and other properties;
Atoms cannot be subdivided, created, or
destroyed.
Atoms of different elements combine in
simple whole-number ratios to form
chemical compounds.
In chemical reactions, atoms are combined,
separated, or rearranged.
How did Dalton do?
Modern Atomic Theory
Two important concepts,
(1) All matter is composed of atoms and
(2) Atoms of any one element differ in
properties from atoms of another
element,
remain unchanged.
Atoms
• Composed of:
– Nucleus protons (+) & neutrons (no
charge)
– Electrons (-)
– Collectively referred to as subatomic
particles.
Electrons
• Joseph John Thompson and Robert A.
Millikan
• Discovered through experiments with
cathode-ray tubes.
• Negative charge
• Very small mass 9.109 x 10-31 kg
• Found in electron cloud around
nucleus.
• Discovery of the electron led to 2
other inferences about atomic
structure:
– Because atoms are electrically
neutral, they must contain a
positive charge to balance the
negative electrons.
– Because electrons have so much
less mass than atoms, atoms must
contain other particles that
account for most of their mass.
Atomic Nucleus
• Ernest Rutherford, Hans Geiger, and
Ernest Marsden
• Discovered through experiments with
alpha particles and gold foil.
• The neucleus contains protons and
neutrons.
– Protons have a positive charge equal to
the negative charge of the electron.
• Atoms are electrically neutral because they
contain equal #s of protons and electrons.
– Neutrons are electrically neutral.
• Proton mass = 1.673 x 10-27 kg
– 1,836 times greater than the mass of
the electron
• Neutron mass = 1.675 x 10-27 kg
Nuclear Forces
• Nuclear forces are short-range
proton-neutron, proton-proton, and
neutron-neutron forces that hold the
nuclear particles together.
Atomic Number
• The atomic number of an element is
the number of protons in the nucleus
of each atom of that element.
• Determines the identity of the atom.
• Look at the periodic table, what is
the atomic number for hydrogen?
Nitrogen? Sodium?
Info on the Periodic Table
Atomic #
Symbol
Mass #
Isotopes
• Isotopes are atoms of the same
element that have different masses.
– The isotopes of a particular element all
have the same number of protons and
electrons but different numbers of
neutrons.
– Most of the elements consist of
mixtures of isotopes.
– Although isotopes have different
masses, they do not differ significantly
in their chemical behavior.
– Nomenclature
Isotopes of Hydrogen
• The number of protons defines the
element.
• The number of neutrons varies.
1
1
H
Protium
Hydrogen-1
2
1
H
3
1
H
Deuterium
Tritium
Hydrogen-2
Hydrogen-3
Mass Number
• The total number of protons and
neutrons in the nucleus of an atom is
equal to the mass number.
mass number
atomic number
4
2
He
Practice
How many protons
are in the nucleus
of this helium atom?
How many neutrons?
4
2
He
2 protons and 2 neutrons
# of protons + # of neutrons = mass number
Practice Problems 
• How many protons, electrons, and
neutrons are there in an atom of
chlorine-37?
17 protons
17 electrons
20 neutrons
More Practice
• How many protons, electrons, and
neutrons are in each of the following
atoms:
– Bromine-90
– Carbon-13
• What element contains 15 electrons
and 15 neutrons?
Relative Atomic Masses
• Expressing the mass of atoms in
grams results in very small numbers
that are not fun to work with.
Therefore, scientists use relative
atomic masses, based on carbon-12.
• One atomic mass unit (amu) is exactly
1/12 the mass of a carbon-12 atom.
Average Atomic Masses
of Elements
• Weighted average of the isotopes of an element
• The sum of the masses of each isotope times
their relative abundances
Isotope
U-238
U-235
U-234
Percent Abundance
99.23%
0.71%
0.0058%
Relative Abundance
Practice Problems 
• Calculate the average atomic mass of
argon to 2 decimal places, given the
following relative atomic masses and
abundances of each of the isotopes:
argon-3635.97 amu; 0.337%
argon-3837.96 amu; 0.063%
argon-4039.96 amu; 99.600%
Answer: 39.95 amu
Stop
Relating Mass to #s of Atoms
• The relative atomic mass scale makes
it possible to know how many atoms of
an element are present in a sample of
the element with a measurable mass.
– Mole
– Avogadro’s number
– Molar mass
The Mole
• A mole (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 is the number of
particles in exactly one mole of a pure
substance.
• 6.0221367 x 1023
• Typically rounded to 6.022 x 1023
Molar Mass
• The mass of one mole of a pure
substance is called the molar mass of
that substance.
• Usually written in units of g/mol.
• Molar mass of an element is
numerically equal to the atomic mass
of the element in atomic mass units.
• Can be used as a conversion factor in
chemical calculations.
How do they all relate?
Practice Problems 
• What is the mass in grams of
3.50 mol of the element copper,
Cu?
Answer: 222 g Cu
• A chemist produced 11.9 g of
aluminum, Al. How many moles of
aluminum were produced?
Answer: 0.441 mol
• How many moles of silver, Ag, are
in 3.01 x 1023 atoms of silver?
Answer: 0.500 mol
• What is the mass in grams of
1.20 x 108 atoms of copper, Cu?
Answer: 1.27 x 10-14 g/mol
Now, you try some!!!
• What is the mass in grams of 2.25
mol of the element iron, Fe?
• What is the mass in grams of 0.0135
mol of the element sodium, Na?
• How many moles of calcium, Ca, are in
5.00 g of calcium?
• How many moles of gold, Au, are
there in 3.60 x 10-10 g of gold?
Yes, keep working!!!
• How many moles of tin, Sn, are in 2500
atoms of tin?
• How many moles of lead, Pb, are in 1.50 x
1012 atoms of lead?
• What is the mass in grams of 7.5 x 1015
atoms of nickel, Ni?
• What mass of gold, Au, contains the same
number of atoms as 9.0 g of aluminum, Al?
Just a few more!!!
• Just kidding!!!
• Any questions?