Chapter 4: Structure of the Atom. 4.1 Early theories and 4.2 Defining the atom Historical Background: Models of the Atom: -see reference chart (On

Chapter 4:

Structure of the Atom

4.1 Early theories and 4.2 Defining the atom

Historical Background:

Models of the Atom: -see reference chart (On Pg. 10 of your packet)


Models of the Atom Date scientist discovery________________ 100 BC Democritus/Greeks concept of the atom 1770 Antoine Lavosier Law of conservation of mass 1800 Joseph Proust Law of definite proportions 1803 John Dalton Law of multiple proportions Atomic Model I 1880 William Crookes Cathode Rays (electrons) 1885 Goldstein Canal Rays (protons) 1900 J.J. Thomson Plum Pudding Model

Electron Atomic Model II

1909 Ernest Rutherford nucleus of atom Atomic Model III 1913 Niels Bohr Planetary Model Atomic Model IV 1920- Schroedinger/Planck/ Modern or Wave Model Present DeBroglie/Einstein/etc. Atomic Model V

4.1 Early theories and 4.2 Defining the atom 1. Atomists and Democritus

Greeks approx 2,500 years ago Matter was made up of atoms “atomos” or

“Indivisible” particles Seashell experiment—broken into smaller &

smaller pieces

4.1 Early theories and 4.2 Defining the atom 2. John Dalton

1766-1844; returned to theory of atoms Atoms are like billiard balls (solid spheres)

which cannot be broken down further 4 major postulates

1) All elements are composed of atoms 2) Atoms of the same element are identical 3) Atoms can physically mix or chemically combine

in simple whole number ratios 4) Reactions occur when atoms separate, join, or

rearrange

4.1 Early theories and 4.2 Defining the atom 3. William Crookes

developed Crookes tube (CRT) in 1870’s first evidence for existence of electrons

because you could “see” electrons flow and confirm their existence.

tube is precursor to today’s TV picture tubes

4.1 Early theories and 4.2 Defining the atom 4. J.J. Thomson

discovered electron in 1897 discovered positively charged particles

surrounded by electrons found the ratio of the charge of an electron to

its mass to be 1/1837



cathode ray tube experiments – advancement of Crookes tube

“plum-pudding model”

4.1 Early theories and 4.2 Defining the atom 5. Ernest Rutherford

Discovered nucleus (dense core of atom) in 1911

Gold foil experiments

Quote from E.R.’s Lab Notebook “It is about as incredible as if you had fired a

15-inch shell at a piece of tissue paper and it came back and hit you.” -ER


Video Clip: Rutherford Gold Foil Experiment

4.1 Early theories and 4.2 Defining the atom 6. Robert Milliken

Oil drop experiment determined the charge and mass of an

electron

Video Clip: Milliken Oil Drop Experiment

4.1 Early theories and 4.2 Defining the atom 7. James Chadwick

discovered the neutron (no charge, but same mass as proton)

Neutrons help disperse the strong repulsion of positive charges

Relative Sizes Nucleus diameter = 10-5 nm Atom diameter = 10-1 nm Nucleus = basketball --> Atom = 6

miles wide!

4.1 Early theories and 4.2 Defining the atom 8. Niels Bohr

improved on Rutherford’s work “planetary model”- positive center is

surrounded by electrons in defined orbits circling the center

4.1 Early theories and 4.2 Defining the atom defined the following:

energy level – the location where an electron is found at a set distance from the nucleus dependent on the amount of energy it has

ground state – the typical energy level where an electron is found; lowest energy

excited state – an energy level higher than the ground state for an electron; temporary condition

4.1 Early theories and 4.2 Defining the atom 9. Quantum Mechanical Model

Erwin Schroedinger; Mathematical model Electron locations are based on probability Electrons are not particles, but waves!

http://phet.colorado.edu/en/simulation/hydrogen-atom

Defined: Orbital – region where an electron is likely to

be found 90% of the time

4.3 How atoms differ

Atoms – vocabulary and classifications

Atom – the smallest particle of matter that retains its properties.

can “see” individual atoms with a scanning tunneling microscope.


Subatomic particles – the component parts of an atom: proton, neutron, and electron


Ion- atom with the same number of protons but a different number of electrons. If the atom has a (+) charge it has fewer

electrons than protons, If the atom has a (-) charge it has more electrons than protons.

4.3 How atoms differSubatomic

ParticleMass and Abbreviation Charge Location Discoverer

Proton 1 amu, p+ +1 Nucleus None

Neutron 1 amu, n 0 Nucleus Chadwick in 1932

Electron Almost zero, e- -1 Electron cloud

Thomson

4.3 How atoms differ Calculations involving Subatomic

Particles:

atomic number = # of protons mass number = # of protons + # of neutrons (neutral atom): # of protons = # of electrons (charged ion): charge = #p+ - #e-

4.3 How atoms differ Isotopes and Calculations:

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

Atomic mass – weighted average of the masses of all the isotopes of an element

4.3 How atoms differ Isotope (Isotopic Notation)

Mass #

Atomic #

Atomic Symbol

Z

AX

Example: Uranium-238

4.3 How atoms differ Isotope Problems:

Multiply the mass number of the isotope by the decimal value of the percent for that isotope

Add the relative masses of all of the isotopes to get the atomic mass of the element

4.3 How atoms differ Example:

If 90% of the Beryllium in the world has a mass number of 9 and only 10% has a mass number of 10, what is the atomic mass of Beryllium?

4.4 Unstable Nuclei and Radioactive Decay

Vocabulary

Radioactivity-the spontaneous emission of radiation from substances

Nuclear reactions- changes in an atom’s nucleus

Radiation-rays and/or particles emitted from radioactive material


Types of Radiation

Alpha radiation -stream of high energy alpha particles

alpha particles consist of 2 protons and 2 neutrons and are identical to helium-4 nucleus.

symbol 4He 2+

2

not much penetrating power, travel a few centimeters, stopped by paper, no health hazard


mass number decreases by 4 atomic number by 2

alpha decay: 226 Ra 222Ra + 4 He 88 86 2

Example: Uranium-238


Beta radiation high speed electrons

To form beta radiation a neutron splits into a proton and an electron

The proton stays in nucleus and the electron propels out at high speed.

Symbol 0e- 0e- 0B

-1 -1 -1

100 times more penetrating then alpha, pass through clothing to damage skin


Beta decay: 131I 131 Xe + 0B 53 54 -1

Example: Astatine-220


Gamma radiation similar to X rays doesn’t consist of particles

symbol: 0 0

penetrates deeply into solid material, body tissue, stopped by Pb or concrete, dangerous

usually emitted with alpha and beta radiation no mass or electrical charge emission of gamma rays by themselves cannot

result in the formation of a new atom


Practice:

What is the alpha decay of plutonium-250?


2. What is the beta decay of Carbon-14?


Models of the Atom Date scientist discovery________________ 100 BC Democritus/Greeks concept of the atom 1770 Antoine Lavosier Law of conservation of mass 1800 Joseph Proust Law of definite proportions 1803 John Dalton Law of multiple proportions Atomic Model I 1880 William Crookes Cathode Rays (electrons) 1885 Goldstein Canal Rays (protons) 1900 J.J. Thomson Plum Pudding Model

Electron Atomic Model II

1909 Ernest Rutherford nucleus of atom Atomic Model III 1913 Niels Bohr Planetary Model Atomic Model IV 1920- Schroedinger/Planck/ Modern or Wave Model Present DeBroglie/Einstein/etc. Atomic Model V

End of Chapter 4!

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Chapter 4: Structure of the Atom. 4.1 Early theories and 4.2 Defining the atom Historical Background: Models of the Atom: -see reference chart (On