Lesson 15 History of Atomic Theory Objectives: * The student will list and explain the five points in Dalton’s atomic theory. * The student will describe.

Lesson 15 History of Atomic Theory

Objectives:Objectives:* The student will list and explain the five points * The student will list and explain the five points

in in Dalton’s atomic theory.Dalton’s atomic theory.

* The student will describe and reenact * The student will describe and reenact Rutherford’s Gold Foil Experiment, and explain his Rutherford’s Gold Foil Experiment, and explain his theory theory of an atom.of an atom.

* The student will describe Neils Bohr’s atomic * The student will describe Neils Bohr’s atomic theory.theory.

History of Atomic Theory Virtual Lab 1 video

I. Dalton’s Atomic Theorya.a. John Dalton looked over knowledge that had been accumulated before his time, and John Dalton looked over knowledge that had been accumulated before his time, and

write five basic principles dealing with matter.write five basic principles dealing with matter.b.b. Democritius (460-370 BC) was the first to propose the idea of “atoms” . This theory Democritius (460-370 BC) was the first to propose the idea of “atoms” . This theory

was rejected by Aristotle (384-322) and lay dormant for 2000 yearswas rejected by Aristotle (384-322) and lay dormant for 2000 years           i.      All matter is made of indivisible and i.      All matter is made of indivisible and indestructible atomsindestructible atoms                           ii. Atoms of a given element are identical in their ii. Atoms of a given element are identical in their physical physical and chemical and chemical

propertiesproperties                         iii.    Atoms of different elements have different iii.    Atoms of different elements have different physical and chemical physical and chemical

propertiesproperties                     iv.    Atoms of different elements combine in iv.    Atoms of different elements combine in simple, whole-number simple, whole-number

ratios to form ratios to form compounds – also known as compounds – also known as the Law of Multiple Proportionsthe Law of Multiple Proportions

                   v.    Atoms cannot be subdivided, created, or destroyed when they are v.    Atoms cannot be subdivided, created, or destroyed when they are combined, separated, or rearranged in chemical reactions – combined, separated, or rearranged in chemical reactions – also known as the Law also known as the Law of Conservation of Massof Conservation of Mass

c.c. Dalton’s Atomic Theory Stated:Dalton’s Atomic Theory Stated:i.      All matter is made of indivisible and i.      All matter is made of indivisible and

indestructible atomsindestructible atoms                           ii. Atoms of a given element are identical in their ii. Atoms of a given element are identical in their

physical physical and chemical and chemical propertiesproperties

                         iii.    Atoms of different elements have different iii.    Atoms of different elements have different physical and chemical propertiesphysical and chemical properties

                     iv.    Atoms of different elements combine in iv.    Atoms of different elements combine in simple, whole-number ratios to simple, whole-number ratios to

form form compounds – also known as compounds – also known as the Law of Multiple the Law of Multiple

ProportionsProportions                   v.    Atoms cannot be subdivided, created, or v.    Atoms cannot be subdivided, created, or

destroyed when they are combined, destroyed when they are combined, separated, or rearranged in separated, or rearranged in

chemical reactions chemical reactions – also known as the Law of – also known as the Law of Conservation of Conservation of MassMass

d. Dalton’s theory of the atom makes it outd. Dalton’s theory of the atom makes it out to be something similar to a marble.to be something similar to a marble.

II. Finding the Structure of the Atom                  a.       Electrons are negatively charged particles that have a a.       Electrons are negatively charged particles that have a

small masssmall mass                                                            i.    Cathode ray tubes (CRT) are used in i.    Cathode ray tubes (CRT) are used in

televisions and other devices.televisions and other devices.                                                            ii.   Small particles travel from the cathode to ii.   Small particles travel from the cathode to

the anode.the anode.           

1. 1. CathodeCathode – a negative electrode through which – a negative electrode through which current flowscurrent flows

            2. 2. AnodeAnode – a positive electrode through which – a positive electrode through which

current flowscurrent flows                                                                                                    

iii.iii. J.J. Thompson discovered that electrically J.J. Thompson discovered that electrically charged plates could deflect these beams of charged plates could deflect these beams of particles. He determined they were particles. He determined they were negatively charged.negatively charged.

iv.iv. He was able to determine the mass of what He was able to determine the mass of what is now called the electron, identifying the is now called the electron, identifying the first subatomic particle, disproving first subatomic particle, disproving Dalton’s TheoryDalton’s Theory

v.v. Robert Millikan was then able to determine Robert Millikan was then able to determine the exact charge of this electronthe exact charge of this electron..

Thompson's Experiment Virtual Lab 1 video

Cathode Ray Tube Drawing:

                                            V.   His model of the atom was called the Plum V.   His model of the atom was called the Plum Pudding model. Pudding model.

1. He is given credit for the discovery of these 1. He is given credit for the discovery of these negatively charged negatively charged

electronselectrons embedded in a embedded in a ball of positive ball of positive charge.charge.

            2. Resembles “plum pudding”, a bread with fruit 2. Resembles “plum pudding”, a bread with fruit pieces embedded in it.pieces embedded in it.

3. Draw the “Plum Pudding Model” in this 3. Draw the “Plum Pudding Model” in this space:space:

b.       Each atom has a positively charged inner core           i.      Ernest Rutherford did an experiment known as the

gold foil experiment.

1.      He fired tiny positively charged particles (alpha particles) at a very thin piece of gold foil, and

determined where the electrons ended up going.

2.      He found that most passed straight through, but some were deflected anywhere from a little to almost straight back

3.      He determined from this that the atoms were mostly empty space and had a positively charged

“core”, and it was named the nucleus.

4.      Draw the setup of the Gold Foil Experiment in this space:

The majority of the particles passed through with an unaltered path.

But a few alpha particles had their pathway drastically changed

Rutherford’s Experiment Virtual Lab 1 video

                        i.      His model of the atom is similar to that of the solar system, with electrons traveling around the nucleus in well-defined paths.

                        ii.      Draw a Rutherford “solar system model” in this space:

LAB ACTIVITY• SUBATOMIC BOWLING!SUBATOMIC BOWLING!

Group Total Trials Number of HITS

TOTALS

Conclusions from Rutherford’s Experiment

• Most of the atom is empty spaceMost of the atom is empty space

• Atoms have a solid core called the nucleusAtoms have a solid core called the nucleus

• Nucleus is positively chargedNucleus is positively charged

• They measured the approximate size of the They measured the approximate size of the nucleusnucleus

• ADD THIS!ADD THIS!

c.       Electrons occupy energy levels within an atom

                   i.      Neils Bohr came up with the idea that electrons would be found only in specific energy levels, similar to the rungs on a ladder

        ii.      The energy levels closer to the nucleus have lower energy than those

farther away.                                              iii.      The difference in energy between any one level and the next is

called a quantum.                                            iv.      Electrons can only be found in those energy levels, never in between. They are “quantized”

v. Bohr’s model of the atom can be compared to a stepladder

d.       Neutrons add mass to the nucleus                             i.      It was found that the entire mass of the nucleus couldn’t be

accounted for with just protons                                                  

ii.      Neutrons were discovered by James Chadwick in 1932, and some of his research was based on discoveries by Rutherford and Irène and Frédéric Joliot-Curie.

                                                  iii.      This particle was very difficult to discover, because of the fact that it does not possess a charge.

                                                         iv.      Originally, the neutron was thought to be a combination of a

proton and an electron, but later it was determined that it was a unique particle.