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Chapter 3Atoms 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.
• 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 ModelDemocritus
All matter is made up of small particles called atoms.
DaltonSee Dalton’s Atomic Theory of Matter
J. J. ThompsonDiscovered the electronElectrons are embedded in a cloud of positive chargePlum Pudding Model
Ernest RutherfordThe positive charge is centrally located.Discovered the nucleus.
Niels BohrThe electrons orbit the nucleus like the sun orbiting the earth
James ChadwickDiscovered 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: 10Mass number: 20
Number of protons and neutrons (mass number): 20- Number of protons (atomic number): 10
number of neutrons: 10
IsotopesAtoms of the same element with different masses.
Isotopes have different numbers of neutrons.
Mass Number (protons + neutrons)
SymbolAtomic Number
(protons)
C116
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 1Neutron Nucleus 0 1Electron 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.
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.
•