Atomic Theory and Structure

Chapters 4-5

Atomic Theories

Democritus ~ 400 BC believed that atoms were indivisible and

indestructible

Dalton ~ 1800’s Developed through experiments First Atomic Model

Dalton’s Atomic Model

All elements are composed of tiny indivisible particles called atoms

Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

Dalton’s Atomic Model (cont)

Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.

Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

Discovery of Electron

1897 – JJ Thomson, using cathode ray tube, discovered negatively charged particles called electrons

1909 – Robert Millikan - Oil Drop Experiment Determined charge on an electron.

“Plum Pudding” Model

Uniform positive sphere with negatively charged electrons embedded within.

Radiation

Late 1800’s – discovery of radiation

Three Types Alpha Beta Gamma

Rutherford Gold Foil Experiment - 1909

Shot alpha particles at gold foil Most went through foil with little or no

deflection. Some were deflected at large angle and

some straight back.

A.K.A. Geiger Marsden Experiment

Rutherford Gold Foil Experiment - 1909

Rutherford Model

Conclusions from Gold Foil Experiment Atom is Mostly Empty Space Dense positive nucleus Electrons moving randomly around nucleus

Subatomic Particles

Electron Discovered in 1897 by JJ Thomson Negative charge (-1) Mass = 9.109389*10-28g Approx mass ~ 0 Found outside of nucleus

Subatomic Particles

Proton Discovered in 1919 by Rutherford Positive charge (+1) Mass = 1.672623*10-24g Approx mass ~ 1 atomic mass unit (u) Found inside nucleus

Subatomic Particles

Neutron Discovered in 1932 by James Chadwick No charge (0) Mass = 1.6749286*10-24g Approx mass ~ 1 atomic mass unit (u) Just slightly larger than a proton Found inside nucleus

Atomic Structure

Atoms have no net charge

# of electrons = # of protons

# of electrons around nucleus = # of protons in nucleus

Atomic Structure

Atomic Number Number of protons in an element All atoms of the same element have the same

number of protons

Mass Number Number of protons and neutrons in an atom

Atomic Structure

# of Neutrons = Mass Number – Atomic Number

Atoms of the same elements can have different numbers of neutrons

Isotope – atoms of the same element with different number of neutrons

Chemical Symbols

Cl-35 Chlorine-35

Cl3517

Mass Number

Atomic Number

Ion

Atom or group of atoms that have gained or lost one or more electrons Have a charge

Example: H+, Ca2+, Cl-, OH-

Ions

H+ 1 proton 0 electrons

Ca2+ 20 protons 18 electrons

Cl- 17 protons 18 electrons

OH- 9 protons 10 electrons

Atomic Theories

Rutherford’s model could not explain the chemical properties of elements

Niels Bohr believed Rutherford’s model needed to be improved

Bohr proposed that electrons are found only in circular paths around the nucleus

Bohr Model

Dense positive nucleus

Electrons in specified circular paths, called energy levels

These energy levels gave results in agreement with experiments for the hydrogen atom.

Bohr Model

Bohr Model

Each energy level can only hold up to a certain number of electrons

Level 1 2 electrons Level 2 8 electrons Level 3 18 electrons Level 4 32 electrons

Electron Configuration

The way in which electrons are arranged in the atom

Example: Na 2-8-1

Valence Electrons Electrons in the outermost energy level

Energy Level Transitions

Electron energy is quantized

Electrons can move between energy levels with gains or losses of specific amounts of energy.

Energy Level Transitions

Gaining energy will move an electron outward to a higher energy level (Absorption)

When an electron falls inward to a lower energy level, it releases a certain amount of energy as light (Emission)

Energy Level Transitions

Ground State vs. Excited State

Ground State When the electrons are in the lowest available

energy level Ex: Na 2-8-1

Excited State When one or more electrons are not in the lowest

available energy level Ex: Na 2-7-2 or 2-8-0-1 or 2-6-1-1-1

Line Spectra

Emission Spectra Shows only the light that is emitted from an

electron transition

Absorption Spectra Shows a continuous color with certain

wavelengths of light missing (absorbed)

Energy Level Transitions

Energy Level Transitions

Wave Mechanical Model

More detailed view of the Bohr Model

Schrödinger Wave Equation and Heisenberg Uncertainty provides region of high probability where electron COULD be. Orbital

Modern Model AKA Quantum Mechanical Model, Electron Cloud Model

Wave Mechanical Model

Orbital Regions of space where there is a high probability

of finding an electron

Wave Mechanical Model

Each energy level is divided into sublevels 1st Energy level has 1 sublevel, s 2nd Energy level has 2 sublevels, s and p 3rd Energy level has 3 sublevels, s, p, and d 4th Energy level has 4 sublevels, s, p, d, and f

These sublevels start to overlap as you move away from the nucleus

Wave Mechanical Model

Sublevels are divided into orbitals s sublevel has 1 orbital p sublevel has 3 orbitals d sublevel has 5 orbitals f sublevel has 7 orbitals

Each orbital can hold up to 2 electrons

Atomic Orbitals

Electron Orbital Configuration

Sublevel order

1s

2s 2p

3s 3p 3d

4s 4p 4d 4f

5s 5p 5d 5f 5g

6s 6p 6d 6f 6g 6h

7s 7p 7d 7f 7g 7h

Electron Orbital Configuration

One sublevel must be full before you can move to the next sublevel

For sublevels with multiple orbitals Each orbital must have one electron before you

can double up

Electron Orbital Configuration

H ____ 1s1

He ____ 1s2

Li ____ ____ 1s2 2s1

1s

1s

1s

2s

Electron Orbital Configuration

C ____ ____ ____ ____ ____

C 1s2 2s22p2

N ____ ____ ____ ____ ____

N 1s2 2s22p3

1s

1s

2s 2p

2s 2p

Electron Orbital Configuration

O ____ ____ ____ ____ ____

O 1s2 2s22p4

F ____ ____ ____ ____ ____

F 1s2 2s22p5

1s

1s

2s 2p

2s 2p

M&M’s Demo

What colors are found in a regular M&M’s bag? Green Yellow Orange Blue Red Brown

M&M’s Demo

Do you get an equal amount of each color in each bag?

If we opened up all the regular M&M bags in the world would we get an equal number of each color?

Are you supposed to?

M&M’s Demo

Color 1 bag World

Blue % 24%

Green % 16%

Yellow % 14%

Orange % 20%

Red % 13%

Brown % 13%

M&M’s Demo

M&M’s come in certain abundances (percentages)

So do isotopes of each element

Relative Abundance Percent of each naturally occurring isotope found

in nature

Average Atomic Mass

Atomic Mass Weighted average based on the relative

abundance and mass number for all naturally occurring isotopes

Example C-12 98.9% 12.011u C-13 1.1%

Atomic Mass

C-12 98.9% C-13 1.1%

Carbon = 0.989*12 + 0.011*13 = 12.011u