Unit 2 – Atomic Theory

Chapter 4 & 11.2

Unit Test:

Democritus

Lived around 400 B.C.

Came up with the concept of the atom

John Dalton - 1807

Known for Billiard Ball Model

Called the father of Atomic Theory

Dalton’s Atomic Theory - 1807

All matter is composed of atoms which are indivisible

All atoms of the same element are identical

Atoms of different elements are different

Atoms bond in different proportions to form compounds (Law of Definite Proportions)

J. J. Thomson - 1903

Discovered the electron (1st subatomic particle) through experiments with cathode ray tube

Plum Pudding model (or Chocolate Chip Cookie model)

Thomson’s Model

“Pudding” or “Cookie” is the positive charge and most of the mass of the atom

“Plums” or “Chocolate Chips” are the scattered electrons

POSITIVE CHARGE ELECTRON

S

Ernest Rutherford - 1911

Nuclear Model (atom contains a nucleus)

Gold foil Experiment

Rutherford’s Experiment

Rutherford’s Model

Atoms have: A nucleus Protons (positive

charge) in nucleus Mostly open space Electrons found

somewhere around the nucleus

Niels Bohr - 1913

Planetary Model Electrons (e-) have

definite path around the nucleus (orbit)

e- arranged around the nucleus according to energy level

e- with lowest energy level are closest to nucleus

Bohr’s Model

Quantum Mechanical Model - 1923

Electron Cloud (modern theory) Calculates the probability of finding the

electron within a given space Electrons, instead of traveling in

defined orbits, travel in diffuse clouds around the nucleus

Quantum Mechanical Model

Present Model of the Hydrogen Atom

Stepwise Timeline of Atomic Theory

Dalton 1803

Thomson1897

Rutherford1909

Bohr1913

Modern Theory

Subatomic Particles

amu – atomic mass unit; based on carbon-12

1 amu = 1/12 mass of C-12 = mass H Impractical to use actual mass of subatomic

particles

Name Symbol Relative Mass

Charge Position

Proton 1H or p+ 1 amu 1 Nucleus

Electron e- 0 amu -1 Outside

Neutron 1n 1 amu 0 Nucleus

Elements of the Periodic Table

Atomic Number Identifies element # protons # electrons in neutral

atom Atomic Mass

a.k.a. mass number # protons + # neutrons

Atomic Number

Atomic Symbol

Atomic Mass

Isotope Notation

Element Symbol with mass number and atomic number

Can also be the element name dash mass number

XMass

Number

Atomic Number

or Element- Mass

Practice

Symbol # Protons

# neutrons

# electrons

Atomic Number

Mass Number

9 10 9 9 19

28 59

150 94

30 65

F199

C146

Isotopes

Atoms of the same element with different numbers of neutrons

Think of it as different sized shirts!6 neutrons Carbon-12

7 neutrons Carbon-13

8 neutrons Carbon-14C

C

C

146

136

126

Isotopes

Average Atomic Mass

Mass listed on the periodic table Determined by averaging the masses

of all naturally occurring isotopes of that element

Explains why the atomic mass is usually not a whole number

Finding Average Atomic Mass

You are told there are 3 isotopes of Cycreekium and their percent abundance: Cycreekium-20; 10% Cycreekium-22; 20% Cycreekium-23; 70%

Find their average atomic mass!

Finding Average Atomic Mass

Formula: % abundance as decimal × atomic mass for each isotope; sum results for avg. atomic mass.

10% × 20amu = 0.10 × 20amu = 2.0amu

20% × 22amu = 0.20 × 22amu = 4.4amu

70% × 23amu = 0.70 × 23amu = 16.1amu

Total: 22.5amu

Percent Abundance

If you are given information about an elements isotope you can estimate the most abundant isotope!

Example: Carbon-12, Carbon-13, Carbon-14. Look at the atomic mass on the periodic table. Which isotope is the mass closest to?

End of Unit 2 Notes!

Study for Unit 2 Test on: