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 ELECTRONS

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 Rutherford 1909 Modern Theory Thomson 1897 Bohr 1913

Subatomic Particles Name Symbol Relative Mass Charge Position Proton 1H or p+ 1 amu 1 Nucleus Electron e- 0 amu -1 Outside Neutron 1n 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

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

X Isotope Notation Element- Mass Element Symbol with mass number and atomic number Can also be the element name dash mass number Mass Number X Element- Mass or Atomic Number

Practice 9 10 19 28 59 150 94 30 65 Symbol # Protons # neutrons # electrons Atomic Number Mass Number 9 10 19 28 59 150 94 30 65

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-14

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: