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Chapter 3
Atoms and Moles
Section 1: Substances Are Made of Atoms
Section 2: Structure of Atoms
Section 3: Electron Configuration
Section 3.1 – Substances Are Made of
Atoms
OBJECTIVES:
State the three laws that support the existence of atoms
List the five principles of John Dalton’s atomic theory.
Law of Definite Proportions
• Also known as the Law of Constant Composition
• Two samples of a given compound are made of the same
elements in exactly the same proportions by mass,
regardless of the size or sources of the samples
– NaCl
39.3% Na Sodium
60.7% Cl Chlorine
- H2O
11.1% H Hydrogen
88.9% O Oxygen
The Law of Conservation of Mass
• The mass of the reacting system does not change
For example:
Table salt
If I react 23 g of sodium with 35.5 g of chlorine
I get 58.5 g of sodium chloride
Water
If I react 2 g of hydrogen with 16 g of oxygen
I get 18 g of water
Law of Multiple Proportions
When two elements combine to form more than one
compound, the mass ratios of the two compounds are
related by small whole numbers
Examples
• Hydrogen Peroxide (H2O2)
2 atoms of hydrogen react with
two atoms of oxygen
• Water (H2O)
? Atoms of hydrogen react with
? Atoms of oxygen.
Atomic Theory of Matter
John Dalton
1. All matter is composed of extremely small particles
called atoms. (cannot be created/subdivided or
destroyed)
2. All atoms of a given element are identical to one
another in chemical and physical properties
3. The atoms of one element are different from the atoms of all other elements
in physical and chemical properties.
4. Atoms of different elements combine in simple whole number ratios to form
compounds
5. In chemical reactions, atoms are combined, separated, or rearranged but
never created, destroyed, or changed.
The Electron
• Streams of negatively charged particles were found to
emanate from cathode tubes.
• J. J. Thompson is credited with their discovery (1897).
• Thompson measured the charge/mass ratio of the
electron to be 1.76  108 coulombs/g.
The Atom, circa 1900:
• “Plum pudding” model, put
forward by Thompson.
• Positive sphere of matter with
negative electrons imbedded in
it.
Section 3.2 – Structure of Atoms
OBJECTIVES:
Describe the evidence for the existence of electrons, protons, and
neutrons, and describe the properties of these subatomic particles.
Discuss atoms of different elements in terms of their numbers of
electrons, protons, and neutrons, and define the terms atomic number
and mass number.
Define isotope, and determine the number of particles in the nucleus
of an isotope
Discovery of the Nucleus
Ernest Rutherford shot
 particles at a thin
sheet of gold foil and
observed the pattern of
scatter of the particles.
The Nuclear Atom
• Since some particles were deflected at large angles,
Thompson’s model could not be correct.
• Rutherford postulated a very small, dense nucleus with
the electrons around the outside of the atom.
• Most of the volume of the atom is empty space.
Summary of the History of the Atom Model
Democritus
All matter is made up of small particles called atoms.
Dalton
See Dalton’s Atomic Theory of Matter
J. J. Thompson
Discovered the electron
Electrons are embedded in a cloud of positive charge
Plum Pudding Model
Ernest Rutherford
The positive charge is centrally located.
Discovered the nucleus.
Niels Bohr
The electrons orbit the nucleus like the sun orbiting the earth
James Chadwick
Discovered the neutron, located in the nucleus.
Electron
A subatomic particle that has a negative electric charge.
Nucleus
An atom’s central region, which is made up of protons and neutrons.
Protons
A subatomic particle that has a positive charge and that is found in the
nucleus of an atom.
The number of protons of the nucleus is the atomic number, which
determines the identity of an element.
Neutron
A subatomic particle that has no charge and that is found in the nucleus
of an atom.
Atomic Number
The number of protons in the nucleus of an atom.
The number of protons = the number of electrons.
The atomic number is the same for all atoms of an element.
Mass Number
The sum of the number of protons and neutrons of the nucleus of an
atom.
Mass number – atomic number = number of neutrons
Example:
Neon:
Atomic number:
Mass number:
10
20
Number of protons and neutrons (mass number):
- Number of protons (atomic number):
number of neutrons:
20
10
10
Isotopes
Atoms of the same element with different masses.
Isotopes have different numbers of neutrons.
Mass Number
(protons + neutrons)
Atomic Number
(protons)
11
C
6
Symbol
Other Subatomic Particles
Protons were discovered by Rutherford in 1919.
Neutrons were discovered by James Chadwick in 1932.
Subatomic Particles
• Protons and electrons are the only particles that have a
charge.
• Protons and neutrons have essentially the same mass.
• The mass of an electron is so small we ignore it.
Location
Charge
Mass unit
Proton
Nucleus
+1
1
Neutron
Nucleus
0
1
Electron
Orbitals
-1
0
Electron Configurations
• The way electrons are arranged in atoms.
• Aufbau principle- electrons enter the lowest energy first.
• This causes difficulties because of the overlap of orbitals
of different energies.
• Pauli Exclusion Principle- at most 2 electrons per
orbital - different spins
• Hund’s Rule- When electrons occupy orbitals of equal
energy they don’t pair up until they have to.
Atomic Orbital Model for Electrons
S orbitals
• 1 s orbital for every energy level
• Spherical shaped
• Each s orbital can hold 2 electrons
• Called the 1s, 2s, 3s, etc.. orbitals.
Pg 86 # 1-4, Pg 89 # 1-2
Section 3.2
Section Review Pg 89 # 1-8
Assignments
Worksheet #1
Worksheet #2
Atomic Number
Bohr Energy Levels
Electron Configurations –
Quantum Numbers
Worksheet #3
QUIZ
Pg 102 # 1-4, Pg 103 # 1-3
Section 3.4
Section Review Pg 104 # 1-13
QUIZ
TEST – Chapter 3