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.

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 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.

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.

•

Assignments

Section 3.2Pg 86 # 1-4, Pg 89 # 1-2

Section Review Pg 89 # 1-8Worksheet #1 Atomic NumberWorksheet #2 Bohr Energy Levels

Worksheet #3 Electron Configurations – Quantum Numbers

QUIZ

Section 3.4Pg 102 # 1-4, Pg 103 # 1-3

Section Review Pg 104 # 1-13QUIZ

TEST – Chapter 3