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# Chapter 3 Atomic Structure

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Chapter 3 Atomic Structure

Objective: You will explain why the model of the atom changed throughout history.

What is a model? Models are used to help us understand things that cannot be seen directly Models are used when something is too large, too small, or too dangerous to be studied directly.

Examples of Scientific Models
This model shows the alignment between the sun, moon, & earth. As it rotates, it shows the phases of the moon and how we measure a year. This model is a mathematical representation of a sound wave. You cannot see sound, but you can see how it affects other objects with its vibrations.

Examples of Scientific Models
What other examples can you think of? Are there other models present in this room?

A good model… Must be based on observations and indirect experimentation. Must explain as many characteristics of the original object as possible. Should be as simple as possible.

When do you change a model?
All models have limitations —No model has ever been totally complete. A model changes when observations of a new situation do not agree with the current model.

Creating a Model The “Think Tube” is also a model for something you cannot see directly. ? THINK TUBE

Creating a Model How many strings are on the inside?
Make your own model showing how the “Think Tube” works.

Atoms The atom has not changed over time, but our idea and model of the atom has.

Definition of Atom: the smallest particle of an element that retains the chemical properties of that element.

Democritus 400 BC By convention there is color, By convention sweetness, By convention bitterness, But in reality there are atoms and space.    -Democritus (c. 400 BCE) Convention means because we said so- doesn’t really exist

Democritus 400 BC Democritus was smashing up sea shells one day and thought that you can break down the shell to tiny pieces, but it can not be completely destroyed.

greek word meaning cannot be cut
Democritus 400 BC Looked at sand on the beach. Cut sand in half and got fewer and fewer grains of sand. What was the smallest piece? He called it atomos = greek word meaning cannot be cut

Democritus 400 BC According to Democritus atoms are:
Invisible Indivisible Solid Eternal Surrounded by an empty space invisible indivisible (no void inside) eternal because they are perfect to explain their movement and changes in density to explain the diversity observed in nature)

Democritus 400 BC continued:
Have an infinite number of possible shapes. Each type of atom had a different size.

Democritus 400 BC Atoms -

Aristotle’s Idea 300 BC All substances are made of 4 elements: Fire, Air, Earth, and Water

Aristotle’s Idea 300 BC There were also four qualities: dryness, hotness, coldness, and moistness. Fire was dry and hot, while water was moist and cold, etc. Each of these elements move naturally in a line to their "proper place," where it will be at rest.

Aristotle 300 BC Water sits on top of the earth, he explained, because it is lighter, yet air floats above the water because it is lighter still—and fire, lightest of all, rises highest. Furthermore, he claimed that the planets beyond Earth were made up of a "fifth element," or quintessence, of which little could be known.

Democritus vs Aristotle
Ancient Greeks accepted Aristotle’s ideas and rejected Democritus. What holds the particles together? Democritus could not answer this question Remained that way until the 17th century

Important Discoveries
Law of Conservation of Mass Law of Definite Proportions Law of Multiple Proportions

Law of Conservation of Mass (Matter)
Lavosier measured the mass of chemicals before and after a chemical reaction and found that the weight did not change. In a chemical reaction, matter is neither created nor destroyed.

Law of Definite Proportions
Also called Law of Constant Composition Proposed by Joseph Proust Elements always react and combine with one another in the same proportions.

Law of Definite Proportions
A chemical compound is always composed of the same combination of atoms - copper carbonate CuCO3

Law of Definite Proportions
Water has the formula H2O. This means that water in the ocean, lakes, or in our sinks always contains 2 atoms of Hydrogen for every 1 atom of Oxygen. What percent of water is Hydrogen and what percent is Oxygen? H: 2g/18g = 11% O: 16g/18g = 89%

Law of Multiple Proportions
If two elements form more than one compound between them, then the ratios of the weights of the two atoms will be ratios that can be reduced to small whole numbers.

Law of Multiple Proportions

John Dalton Quaker Color Blindr English school teacher and public lecturer by the age of 12. As a Quaker, Dalton led a modest existence, although he received many honors later in life. In tribute, more than 40,000 people marched in his funeral procession. Quaker 1 in 300 could read Color blind - of color-blindness came to be known as Daltonism

John Dalton’s Atomic Theory
Dalton’s Theory was a return to the ideas of Democritus Dalton turned the idea into a scientific theory that could be tested Not all of Daltons ideas are still true today. Some ideas were modified.

John Dalton’s Atomic Theory:
All matter is composed of atoms. Atoms of a particular element have identical properties. Elements of a different element have different properties. Atoms cannot be divided or destroyed. Atoms combine to form compounds. During a chemical reaction atoms are rearranged.

John Dalton’s Atomic Theory:
All matter is composed of atoms. Atoms of a particular element have identical properties. Elements of a different element have different properties. Atoms cannot be divided or destroyed. Atoms combine to form compounds. During a chemical reaction atoms are rearranged.

JJ Thomson Excuse me... how can you discover a particle so small that nobody has ever seen one? J.J. was very awkward with his fingers, and I found it very necessary not to encourage him to handle the instruments! But he was very helpful in talking over the ways in which he thought things ought to go." -- H. F. Newall, onetime assistant to the young Professor Thomson

J.J. Thompson’s Model _ _ + + _ + + _ _ +

Discovery of the Electron
JJ Thomson determined that: Atom is a sphere of positive matter that holds electrons in it. Also called the plum pudding model or the raisin dough model.

His experiment

Thomson’s Experiment Voltage source - + Vacuum tube Metal Disks

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

J.J. Thompson’s Cathode Ray Tube
The cathode ray travels from the cathode to the anode when current was passed through the tube. Voltage source - + Cathode Anode

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field he found that the moving pieces were negative

Maltese Cross Tube Using a Maltese cross as the anode, this produced a shadow that glowed at the end of the tube. This showed that the cathode rays traveled in straight lines.

Paddle Wheel Discharge Tube
A paddle wheel placed in the path of the cathode rays turned. This proved that the cathode rays contained mass, and that they might be made of particles.

Thomson's “Plum Pudding Model"

Thomson's “Plum Pudding Model"
Electrons are red. Negative electrons Are embedded in a Blue positive atom.

All atoms contain electrons.
J.J. Thompson’s Model Cathode rays are beams of negatively-charged particles called electrons. All atoms contain electrons. Atoms also contain an equal and opposite positive charge.

Ernest Rutherford "All science is either physics or stamp collecting." Rutherford, Ernest ( ): Born in New Zealand, Rutherford studied under J. J. Thomson His work constituted a notable landmark in the history of atomic research as he developed Bacquerel's discovery of Radioactivity into an exact and documented proof that the atoms of the heavier elements, which had been thought to be immutable, actually disintegrate (decay) into various forms of radiation. Rutherford is best known for devising the names alpha, beta, and gamma rays to classify various forms of "rays" which were poorly understood at his time (alpha and beta rays are particle beams, while gamma rays are a form of high-energy electromagnetic radiation ). Cavendish Laboratory, in Cambridge Studied under JJ Thomson Worked with Gieger, Bacquerel's

Ernest Rutherford’s Model
He became a student of a teacher named Professor J J Thomson at Cambridge University in England

Ernest Rutherford’s Model
Rutherford studied how gold atoms interacted with radioactivity.

Radioactivity is the processes by which unstable atoms emit
subatomic particles (radiation).

Made of 2 protons and 2 neutrons. Charge of +2, and a mass of 4 Relatively slow and heavy.

Charge of minus 1 Mass is very small. They are the same as an electron. They are fast, and light.

Gamma rays are waves, not particles. They have no mass and no charge.

Penetrating Power Alpha particles are easy to stop, gamma rays are hard to stop.

Fluorescent Screen Lead block Uranium Gold Foil Here’s how it looked.

What he expected

Rutherford Expected: The alpha particles to pass through without changing direction. Because…? …the positive charges were thought to be spread out evenly. Alone they were not enough to stop the large alpha particles.

What was expected:

What he got

Rutherford’s Gold Foil Results

Rutherford said that having the alpha particle bounce off of the foil was equivalent to seeing a cannon ball bounce off of a piece of tissue paper.

What Actually Happened…
+ - - - - - -

- - - - - - How he explained it: Atom is mostly empty. It has a small
dense, positive piece at center. Alpha particles are deflected by the nucleus if they get close enough. + - - - - - -

Ernest Rutherford’s Results:
Atom is mostly empty space. Suggests that an atom has a nucleus that holds most of the mass of the atom. + - - - - - -

What Actually Happened

Rutherford’s Atom: A sea of electrons that surrounded a positively charged nucleus.

Rutherford’s Atom His model of the atom was similar to the solar system. Like planets, electrons orbited a central, sun-like nucleus.

Rutherford’s Atom PROTONS +

Rutherford’s Atom 1. Most of the mass of an atom must be located in a small volume at the center of the atom (the nucleus). 2. The nucleus is made of positively charged particles called protons. 3. The electrons move in a large volume which is mostly empty space.

Problems with Rutherford’s Atom
According to "classical" theory the electrons should lose energy by radiating electromagnetic radiation, as they are accelerated electric charges. They should spiral into the nucleus.

Problems with Rutherford’s Atom

2 years after Rutherford…. Neils Bohr 1885 - 1962
Born in Sweden - Went to England to study at Cambridge but he didn’t like Thomson He met Rutherford and liked him so he started working with him Came up with the idea of energy levels Because he was part Jewish when Hitler came to power and invaded Sweden he had to leave. He first went to England then he came to Los Alamos

Bohr Model of the Atom:

Bohr Model of the Atom: Electrons are a particular distance from the nucleus The energy of each electron is not the same Electrons close to center = low Energy Electrons farther away = high Energy

Neils Bohr’s Model There are 2 electrons in the lowest energy level, 8 electrons in the second energy level, and 18 in the third… …this is the model of the atom we will use!

James Chadwick

Chadwick In 1932, Chadwick proved the existence of neutrons - elementary particles devoid of any electrical charge. Located in the nucleus (Rutherford also put out the idea that there could be a particle with mass but no charge) English Worked under Rutherford Later, he found out that a German scientist had discovered the neutron at the same time. But Hans Falkenhagen (Rostock) was afraid of publishing his results. When Chadwick learned of Falkenhagen's discovery, he offered to share the Nobel Prize with him. Falkenhagen was modest and refused this honour.

Chadwicks Atom

Chadwick’s Atom Modern Atom
1. Most of the atom's volume is occupied by electrons. 2. The number and arrangement of electrons in an atom determine its chemical properties.

Chadwick’s Atom 3. The identity of an element is determined by the number of protons in the nucleus. 4. Different isotopes of elements exist and differ only in the number of neutrons and hence the mass of the atom.

Modern Atom Electrons Protons Neutrons

Alpha & Beta particles Born in germany - When he was trying to decide what to study in college His physics professor told him there was nothing else to discover in physics. –

Electrons Electrons are tiny particles which behave like clouds
Electrons carry something called a negative electric charge. Electrons are responsible for the chemistry of the atom.

Electrons http://education.jlab.org/atomtour/listofparticles.html

Atoms - Atomic diameter ~ 1 to 5x10-8 cm
- Are about 10,000,000 in 1 mm - An H atom weighs 1.67x10-24 g Note: (6.02x1023) x (1.67x10-24 g) = 1.00 g 6.02x1023 = Avogadro’s Number

Atoms are made up of three major parts: Part Found Mass Charge
Electrons Outside 9.1x10-28g (small) Protons Nucleus 1.7x10-24g (1.0 AMU) +1 Neutrons Nucleus 1.7x10-24g (1.0 AMU) 0

1) Neutral atoms contain equal number of electrons and protons.
Atomic Structure 1) Neutral atoms contain equal number of electrons and protons.

2) Atoms can loose or gain electrons to become charged = ions
Atomic Structure 2) Atoms can loose or gain electrons to become charged = ions

3) Number protons determines the identity of the atom or ion.
Atomic Structure 3) Number protons determines the identity of the atom or ion. Atomic Number

Atomic Structure 4) Mass Number = # Protons + # Neutrons

APE MAN Atomic Number =Proton # =Electron # Mass # -Atomic Number = Neutron #

Charge (if ion) Mass Number Symbol Atomic Number

An isotopes contribution is determined by its relative abundance.
Atomic Mass- An isotopes contribution is determined by its relative abundance.

Cs 133 55 EXAMPLE Atomic number = protons and electrons
How many protons, neutrons and electrons are found in an atom of 133 55 Cs Atomic number = protons and electrons There are 55 protons and 55 electrons Mass number = sum of protons and neutrons 133 – 55 = 78 There are 78 neutrons

Hydrogen H 1 1 Symbol Atomic Mass Atomic Number

Hydrogen Protons: Electrons: Neutrons: H 1 1

Hydrogen Protons: 1 Electrons: Neutrons: H 1 1

Hydrogen Protons: 1 Electrons:1 Neutrons: H 1 1

Hydrogen Protons: 1 Electrons:1 Neutrons:0 H 1 1

Sodium Protons: Neutrons: Electrons: Na 23 11

Sodium Protons: 11 Neutrons: Electrons: Na 23 11

Sodium Protons: 11 Neutrons: Electrons: 11 Na 23 11

Sodium Protons: 11 Neutrons: Electrons: 11 12 Na 23 11 12

Rhenium 75 Protons: Neutrons: Electrons: Re 111 186 75 75 111

Rhenium Isotope 75 Protons: Neutrons: Electrons: Re 112 187 75 75 112

Re-111

C-12 C 12 6

Isotopes Atoms of a given element with differing numbers of neutrons are called isotopes.

Isotopes An atom is still the same element if it is missing an electron. The same goes for isotopes. They are still the same element. They are just a little different from every other atom of the same element.

An isotopes contribution is determined by its relative abundance.
Atomic Weight- An isotopes contribution is determined by its relative abundance.

Atomic numbers are whole numbers
Mass numbers are whole numbers The atomic mass is not a whole number.

Calculating Atomic Mass
(% abundance of isotope 1)(mass of isotope 1) + (% abundance of isotope 2)(mass of isotope 2) + (% abundance of isotope 3)(mass of isotope 3) + ...

Calculating Atomic Mass
Answer the following questions: "How many naturally occurring isotopes does carbon have?" "What is the abundance of each of the isotopes?"

The sum of all the fractions of abundance
Isotope Atomic Mass Relative Abundance C-12 12.00 98.93 C-13 13.00 1.07 The sum of all the fractions of abundance of each naturally occurring isotopes should equal 1.00 or 100%.

atomic mass of carbon = (0.9893)( amu) +(0.0107)(13.00 amu) = amu amu = amu

What is the atomic mass of Lithium
Isotope Atomic Relative Mass Abundance Li Li

What is the atomic mass of Lithium
0.0759*6.015 = * = 0.4565 6.4828 6.9393

What is the atomic mass of Oxygen?

Isotope Atomic Mass Relative Abundance O-16 16 99.757 O-17 17 0.038 O-18 18 0.205

What is the atomic mass of Oxygen
*16 = *17= *18=

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