Chapter 4 The Bohr Model

The Bohr Model Electrons move in ORBITS around the positively charged nucleus in an atom. The nucleus contains +1 protons and neutral neutrons

The Bohr Model Electrons move in orbits of fixed size. These orbits limit how close or far away an electron can get from the nucleus.

The Bohr Model Orbits are “quantized” – only orbits of specific energy exist in an atom An electron must be in an orbit & can have only the corresponding energy e’s further out have higher energy e’s closer to nucleus have lower Principal quantum number n = 1, 2, 3, 4……….. (We’ll study in CH 5)

Ground State Ground state is the most stable state for an atom The electrons are travelling in the lowest available energy orbit or “shell”

Excited State Light is emitted by the electron as it falls from the excited state back to ground state.

Bohr Model Bohr’s Model explains why hydrogen emits very, very specific kinds of light. Excited Hydrogen Gas

Bohr Model Electrons falling from specific levels cause the different wavelengths of light Excited Hydrogen Gas

Bohr Model – The Rules Every Bohr model atom is drawn the same way. The nucleus is in the center 2 p+ 2 n0

Bohr Model – The Rules e- e- There are electron orbits surrounding the nucleus In those shells are electrons e- 2 p+ 2 n0 e-

Bohr Model – The Rules e- e- Each shell can hold a specific number of electrons The outer shell can be partially empty, but only if the inner shells are already full e- 2 p+ 2 n0 e-

2 e- 8 e- 18 e- 32 e- 50 e- Bohr Model – The Rules First Shell: ____________________ Second Shell: _______________________ Third Shell: _______________________ Fourth Shell: _____________________ Fifth Shell: ______________________ 8 e- 18 e- 32 e- 50 e-

Bohr Model – Example Diagram the bohr model of carbon-14 This C has: + - - + + - - + + + -

Bohr Model – Example Diagram the bohr model of magnesium-22 This Mg has: _______ p+ _______ n0 _______e - - - - - + + - + - + + - + - - + + + + + - + - -

Bohr Model – Your Turn! Diagram the bohr model of the following isotopes Oxygen – 17 Argon – 35 Titanium – 40 Iron – 57

Bohr Model Poster On a piece of construction or computer paper: Create a Bohr Model for an isotope An isotope of elements: 3-38 Must get instructor to approve isotope before starting Use your notes to draw it correctly Must use 4 different colors in your model Bonus for artistic ability

Chapter 4 Average Atomic Mass

Atomic Mass In case you forgot: Weighted Average Based on ALL isotopes of an element This is why it is not a whole number NOT the same thing as mass number

Atomic Mass Unit of Atomic Mass = AMU 1 AMU = 1/12th the mass of Carbon-12

Atomic Mass Hydrogen has two stable isotopes: The average atomic mass of hydrogen: 1.00794 amu

Atomic Mass Which isotope is more abundant? Is there more H-1? More H-2? How can you tell? Because the average mass (1.00794 amu) is closer to the mass number of H-1: There is more hydrogen-1 in the universe!

Steps for Average Atomic Mass Convert the percent abundance to decimal This is known as relative abundance Do this by dividing by 100 Multiply the abundance of each by their mass number. Add the new numbers together. This is your average atomic mass!

Example: The element gallium has two stable isotopes: Ga-69 and Ga-71. Ga-69 has a percent abundance of 60.11% and Ga-71 has a percent abundance of 39.89%. What is the average atomic mass of Ga? 1) Convert % to decimal. 60.11/100 39.89/100 2) Multiply decimal by mass # * 69 * 71 = 0.6011 = 0.3989 = 41.48 = 28.32 + 3) Add the products together 69.80 amu

Example: Thallium has two stable isotopes, Th-203 and Th-205. Th-203 has an abundance of 29.52%. Th-205 has an abundance of 70.48%. What is the average atomic mass of Thallium? 1) Convert % to decimal. 29.52/100 70.48/100 2) Multiply decimal by mass # * 203 * 205 = 0.2952 = 0.7048 = 59.95 = 144.48 + 3) Add the products together 204.43 amu

Your Turn! Do the problems on your practice worksheet Your instructor will circulate to make sure you are doing it correctly.

Calculate % from atomic mass. Steps for Calculating Percent Abundance (When given mass, but not % abundance) Because percent abundances will always add up to 100% assign one isotope to have a percentage of X and the other isotope to have a percentage of 1-X

Calculate % from atomic mass. Look up the average atomic mass of the atom on the periodic table. Set up your problem so it looks like the setup below: X*(Mass:Isotope-A) + (1-X)*(Mass:Isotope-B) = Average Atomic Mass

Calculate % from atomic mass. Solve for “X. Multiply by 100 to turn it into a percentage. This is the percent abundance of isotope A. The percent abundance of isotope B is 100% - % Abundance Isotope-A

Example Hydrogen comes in two stable isotopes, H-1 and H-2. The mass of H-1 is 1.00 AMU. The mass of H-2 is 2.00 AMU. Determine the percent abundance of each isotope. 1) Set abundance = X % H-1 = X % H-2 = 1-X 2) Look up AAM 1.0079 amu 3) Plug into equation X (1.00) + (1-X)(2.00) = 1.0079 4) Solve for X 1X +2-2x=1.00794 5) Multiply X by 100 to get percent. 0.99206 * 100 =99.21% H-1 6) Subtract % of A to get % of B 100 – 99.21 = 0.79% H-2 -1X =0.99206 X = 0.99206

Now you Try! IN YOUR NOTEBOOK ANSWER THIS: Oxygen comes in two stable isotopes, O-16 and O-18. The mass of O-16 is 15.99 AMU. The mass of O-18 is 17.99 AMU. Determine the percent abundance of each. Set abundance = X % O-16 = X % O-18 = 1-X 2) Look up AAM 15.9994 amu 3) Plug into equation X (15.99)+(1-X)(17.99) = 15.9994 4) Solve for X 15.99X+17.99-17.99x=15.9994 5) Multiply X by 100 to get percent. 0.9953 * 100 =99.53% O-16 6) Subtract from 100 100 – 99.53 = 0.47% O-18 -2X =-1.9906 X = 0.9953