1 unit 2: atomic theory chapter 4 chemistry 1k cypress creek high school

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Unit 2: Atomic Theory

Chapter 4Chemistry 1KCypress Creek High School

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Table of Contents

Chapter 4: The Structure of the Atom– 4.1: Early Theories of Matter– 4.2: Subatomic Particles & the Nuclear Atom– 4.3: How Atoms Differ– 4.4: Unstable Nuclei & Radioactive Decay

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Democritus

Lived around 400 B.C. Came up with the concept of the atom

4.14.1 Early Theories of MatterEarly Theories of Matter

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Development of the Modern Atomic Theory

In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container.– He observed that the mass of reactants in the

container before a chemical reaction was equal to the mass of the products after the reaction.

– Lavoisier concluded that when a chemical reaction occurs, mass is neither created nor destroyed but only changed.

Lavoisier’s conclusion became known as the law of conservation of mass.


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Development of the Modern Atomic Theory

In 1799, another French chemist, Joseph Proust, observed that the composition of water is always 11% hydrogen and 89% oxygen by mass.

– Regardless of the source of the water, it always contains these same percentages of hydrogen and oxygen.

Proust studied many other compounds and observed that the elements that composed the compounds were always in a certain proportion by mass. This principle is now referred to as the law of definite proportions.

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Dalton’s Atomic Theory

John Dalton (1766-1844), an English schoolteacher and chemist, studied the results of experiments by Lavoisier, Proust, and many other scientists.

Dalton proposed his atomic theory of matter in 1803.

– Although his theory has been modified slightly to accommodate new discoveries, Dalton’s theory was so insightful that it has remained essentially intact up to the present time.

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Dalton’s Billiard Ball Model

Called the father of Atomic Theory The following statements are the main

points of Dalton’s atomic theory.1) All matter is made up of atoms.

2) Atoms are indestructible and cannot be divided into smaller particles. (Atoms are indivisible.)

3) All atoms of one element are exactly alike, but are different from atoms of other elements.

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J. J. Thomson - 1903

Discovered the electron (1st subatomic particle) through experiments with cathode ray tube

Plum Pudding model (or Chocolate Chip Cookie 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 CHARGEELECTRONS


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Ernest Rutherford - 1911

Nuclear Model (atom contains a nucleus)

Gold foil Experiment Atoms have:

– A nucleus– Protons (positive charge)

in nucleus– Mostly open space– Electrons found

somewhere around the nucleus


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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


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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


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Stepwise Timeline of Atomic Theory

Dalton 1803

Thomson1897

Rutherford1909

Bohr1913

Modern Theory


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TAKS Review

The graph shows the results of a study testing chemical pesticides on a pest species common to cotton plants. Different chemical pesticides were used in five different areas. According to these results, which of the following is the most effective chemical for controlling this pest species?

– a. R– b. S– c. T– d. V

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4.24.2 Subatomic Particles & the Nuclear AtomSubatomic Particles & the Nuclear Atom

The Electron

Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts.

– In 1897, a British physicist, J.J. Thomson, discovered that this solid-ball model was not accurate.

Thomson’s experiments used a vacuum tube.

At each end of the tube is a metal piece called an electrode.

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Cathode-Ray Tube

When the electrodes are charged, rays travel in the tube from the negative electrode, which is the cathode, to the positive electrode, the anode.

Thomson found that the rays bent toward a positively charged plate and away from a negatively charged plate.

– He knew that objects with like charges repel each other, and objects with unlike charges attract each other.

Click box to view movie clip.

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Cathode-Ray Tube

Thomson concluded that cathode rays are made up of invisible, negatively charged particles referred to as electrons.

– These electrons had to come from the matter (atoms) of the negative electrode.

Matter is not negatively charged, so atoms can’t be negatively charged either.

– If atoms contained extremely light, negatively charged particles, then they must also contain positively charged particles—probably with a much greater mass than electrons.

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Protons

In 1886, scientists discovered that a cathode-ray tube emitted rays not only from the cathode but also from the positively charged anode.

– These rays travel in a direction opposite to that of cathode rays.

– Thomson was able to show that these rays had a positive electrical charge.

Years later, scientists determined that the rays were composed of positively charged subatomic particles called protons.

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Protons

At this point, it seemed that atoms were made up of equal numbers of electrons and protons.

However, in 1910, Thomson discovered that neon consisted of atoms of two different masses.

– Atoms of an element that are chemically alike but differ in mass are called isotopes of the element.

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Neutrons

Because of the discovery of isotopes, scientists hypothesized that atoms contained still a third type of particle that explained these differences in mass. – Calculations showed that such a particle should

have a mass equal to that of a proton but no electrical charge.

– The existence of this neutral particle, called a neutron, was confirmed in the early 1930s.

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Subatomic Particles

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

Name Symbol Relative Mass

Charge Position

Proton 1H or p+ 1 amu 1 Nucleus

Electron e- 0 amu -1 Outside

Neutron 1n 1 amu 0 Nucleus


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Rutherford’s Gold Foil Experiment

In 1909, a team of scientists led by Ernest Rutherford in England carried out the first of several important experiments that revealed an arrangement far different from the cookie-dough model of the atom.

– The experimenters set up a lead-shielded box containing radioactive polonium, which emitted a beam of positively charged subatomic particles through a small hole.

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Rutherford’s Gold Foil Experiment

Today, we know that the particles of the beam consisted of clusters containing two protons and two neutrons and are called alpha particles.

– The sheet of gold foil was surrounded by a screen coated with zinc sulfide, which glows when struck by the positively charged particles of the beam.

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The Nuclear Model of the Atom

To explain the results of the experiment, Rutherford’s team proposed a new model of the atom.

Because most of the particles passed through the foil, they concluded that the atom is nearly all empty space.

– Because so few particles were deflected, they proposed that the atom has a small, dense, positively charged central core, called a nucleus.

– The new model of the atom as pictured by Rutherford’s group in 1911 is shown below.

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TAKS Review

Trade winds blow from east to west across the Pacific Ocean. The winds move surface waters westward across the ocean. This causes deeper, colder water to rise to the surface along the coast. This upwelling of deep ocean waters brings with it nutrients that would otherwise lie near the bottom of the ocean. Which of the following conclusions is supported by the information above?

– a. Trade winds help maintain some food chains.– b. Trade winds can reverse parts of the water cycle.– c. Trade winds produce useful minerals in some oceans.– d. Trade winds may be able to reduce greenhouse gases

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4.34.3 How Atoms DifferHow Atoms Differ

Atomic Numbers

The atomic number of an element is the number of protons in the nucleus of an atom of that element.

– It is the number of protons that determines the identity of an element, as well as many of its chemical and physical properties.

– Because atoms have no overall electrical charge, an atom must have as many electrons as there are protons in its nucleus.

Therefore, the atomic number of an element also tells the number of electrons in a neutral atom of that element.

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Masses

The mass of a neutron is almost the same as the mass of a proton. The sum of the protons and neutrons in the nucleus is the mass

number of that particular atom. – Isotopes of an element have different mass numbers because they

have different numbers of neutrons, but they all have the same atomic number.

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Atomic Mass

In order to have a simpler way of comparing the masses of individual atoms, chemists have devised a different unit of mass called an atomic mass unit, which is given the symbol .

– An atom of the carbon-12 isotope contains six protons and six neutrons and has a mass number of 12.

– Chemists have defined the carbon-12 atom as having a mass of 12 atomic mass units.

Therefore, 1 = 1/12 the mass of a carbon-12 atom. 1 is approximately the mass of a single proton or neutron.

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Information in the Periodic Table

The number at the bottom of each box is the average atomic mass of that element.

This number is the weighted average mass of all the naturally occurring isotopes of that element.

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Isotope Notation

Element Symbol with mass number and atomic number

Can also be the element name dash mass number

XMass

Number

Atomic Number

or Element- Mass


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Practice

Symbol # Protons # neutrons # electrons Atomic Number

Mass Number

9 10 9 9 19

28 59

150 94

30 65

F199

C146


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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


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Calculating Atomic Mass

Copper exists as a mixture of two isotopes. – The lighter isotope (Cu-63), with 29 protons and 34 neutrons,

makes up 69.17% of copper atoms. – The heavier isotope (Cu-65), with 29 protons and 36

neutrons, constitutes the remaining 30.83% of copper atoms.

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Calculating Atomic Mass The atomic mass of Cu-63 is 62.930 amu, and the atomic mass

of Cu-65 is 64.928 amu. Use the data above to compute the atomic mass of copper.

First, calculate the contribution of each isotope to the average atomic mass, being sure to convert each percent to a fractional abundance.

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Calculating Atomic Mass

The average atomic mass of the element is the sum of the mass contributions of each isotope.

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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?


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Question 1

Calculate the atomic mass of germanium. Answer

– 72.59 amu

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Radioactivity

Reactions which involve a change in an atom’s nucleus are called nuclear reactions.– In the late 1890’s scientists noticed that some

substances spontaneously emitted radiation in a process they called radioactivity.

– The rays and particles emitted by the radioactive material were called radiation.

Unstable nuclei lose energy by emitting radiation in a spontaneous process called radioactive decay.

4.44.4 Unstable Nuclei & Radioactive DecayUnstable Nuclei & Radioactive Decay

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Types of Radiation

Scientists named the radiation that was deflected toward the negatively charged plates alpha radiation.

– This radiation is made up of alpha particles contains 2 protons & 2 neutrons.

Scientists named the radiation that was deflected toward the positively charged plate beta radiation.

– This radiation consists of fast moving electrons called beta particles containing an electron.

The third type of radiation is called gamma radiation or gamma rays, high energy radiation that has no mass.

4.44.4 Unstable Nuclei & Radioactive DecayUnstable Nuclei & Radioactive Decay

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TAKS Review

A medical researcher hypothesizes that a newly developed medication can reduce high blood pressure. Which of these would most likely be the dependent variable in a study involving this medication?

– a. The number of participants in the study– b. The ages of people treated for high blood pressure with

other medications– c. The blood pressure of the participants in the study– d. The number of people treated for high blood pressure with

other medications

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End of Unit 2

Be Prepared for Unit 2 Test.

1 unit 2: atomic theory chapter 4 chemistry 1k cypress creek high school

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