History of Atomic Theory

History of Atomic Theory

Figuring Out Atoms

Dalton’s Atomic Theory1.Matter is composed of extremely

small particles called atoms. 2.Atoms are indivisible and

indestructible. 3.Atoms of a given element are

identical in size, mass, and chemical properties.

Dalton’s Atomic Theory4. Atoms of a specific element are

different from those of another element.

5.Different atoms combine in simple whole-number ratios to form compounds.

6.In a chemical reaction, atoms are separated, combined or rearranged.

Dalton’s Model

Atoms are the

smallest division of matter.

They cannot be divided.

Modern Advancements

It wasn’t until almost 80

years after Dalton’s theory

that we saw further

discoveries.http://www.animatedgif.net/clockscounters/clockscounters4.shtml

J.J. Thomson and the Cathode Ray Tube

• Some sort of particles were hitting the Zinc-Sulfide coating• If you bring a magnet towards

the beam, the beam moves (towards the positive and away from the negative)• These particles were in all

sorts of elements

J.J. Thomson and the Cathode Ray Tube

• These small particles are now called electrons.

• Thomson designed a new model of an atom by changing Dalton’s model.

• This model is called the plum pudding model.

J.J. Thomson and the Cathode Ray Tube

http://reich-chemistry.wikispaces.com/Fall.2008MMA.Riley.Timeline

Plum Pudding Model

A uniformly positive atom

with small negative particles mixed in

Rutherford’s Gold Foil Experiment

To test the plum pudding model, Ernest Rutherford

designed a simple experiment.

http://www.daviddarling.info/encyclopedia/R/Rutherfords_experiment_and_atomic_model.html

Rutherford’s Gold Foil Experiment

• Each atom has a small, dense nucleus• Most of the atom is empty space• Within the nucleus are particles that

have a positive 1 charge. He called these particles protons.• He was able to improve the plum

pudding model

Rutherford’s Conclusions

Rutherford’s Model

Small, dense, positively charged

nucleus with smaller

negative charges

surrounding it.

Last Piece of the Puzzle• One of Rutherford’s colleagues,

James Chadwick discovered that there must be another particle in

the nucleus. • This particle is the same mass as

a proton but does not have a charge.

• He called these particles neutrons.

http://nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-bio.html

Atomic ParticlesSo now we know….

•Electrons are small, negatively charged particles•Protons are positively charged particles in the nucleus•Neutrons are neutral particles in the nucleus

Atomic Structure

Atomic Structure• In the nucleus

there are two types of particles: Protons and Neutrons

• Around the nucleus there are electrons

http://www.theo-phys.uni-essen.de/tp/ags/guhr_dir/research.php

Practice• The number of protons determines the

element• How many protons are in each of the

following elements?• He: • F:• Ga:• Be:

• The number of electrons = the number of protons

Periodic Table Atomic Number = Number of Protons or

Number of Electrons

Elemental Notation• Rather than writing out the element’s

information each time, we use a special notation to organize information. –Mass Number– Atomic Number– Elemental Symbol

The Numbers• The mass number of an element is the number of protons +

neutrons. • The atomic number of an element is the number of protons the

element has.• To find the number of neutrons, subtract the atomic number from

the mass number.

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Mass Number

Atomic Number

Element

Symbol

136 Number of Neutrons

Isotopes• Isotope- Atoms with the same

number of protons but different number of neutrons.

• Atoms of the same element have the same number of protons

• Atoms of the same element can have different numbers of neutrons11

6C 126C 13

6C 146C

Ions• In the nucleus there are protons and

neutrons– Protons are positive– Neutrons are neutral

• If the nucleus has 3 protons it has a charge of 3+

Neutral AtomIn a neutral atom, there are

equal numbers of protons and electrons

Each positive is cancelled

by a negative.

Ions• In an ion, there are unequal numbers

of protons and electrons

3 + charge4 – charge1 – charge

Ions• In an ion, there are unequal numbers

of protons and electrons

3 + charge2 – charge1 + charge

What is the charge of an ion that has 29 protons and 32 electrons?

What is the charge of an ion that has 12 protons and 13 electrons?

What is the charge of an ion that has 54 protons and 52 electrons?

Modern Model

Finding out more about atoms

Do NowGet one of each paperKeep your homework

Complete the do now on your notes page!

What we know…• We know that there are protons, neutrons

and electrons. • Protons and Neutrons are in the nucleus. • The nucleus has a positive charge.• Electrons are in the space around the

nucleus.• But scientists were learning more about the

behavior of atoms while they were studying light.

Photoelectric Effect• Scientists continued to explore the

properties of matter and light.

High Energy Low Energy

Properties of Light• Studies showed that light behaved as

packages of energy. • We call these packages photons.

Low Energy

High Energy

Photoelectric Effect

e- e- e- e-e-e-e-e-

Experiment: Shining light on a piece of

metalWith low energy photons, no

electrons are removed.

Photoelectric Effect

e- e- e- e-e-e-e-e-

Experiment: Shining light on a piece of

metalWith many low energy

photons, no electrons are removed.

Photoelectric Effect

e- e- e- e-e-e-e-e-

Experiment: Shining light on a piece of

metalWith high energy photons,

electrons are removed.

Niels Bohr• Bohr reviewed these results and

came up with a new model of the atom.

• If only certain types of energy could remove electrons, he thought maybe there were energy levels for electrons.

Bohr’s Model

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Bohr’s Model

e-

Bohr’s Model• This idea of energy levels in an atom

were represented by rings.

Bohr’s Model• In the first energy level there are two

electrons.

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Bohr’s Model• In the second energy level there are

8 electrons.

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Bohr’s Model• In the third energy level there are 18

electrons.

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Energy With Rings• Electrons in the outer rings have

more energy than those in the inner rings

Low Energy

High Energy

Electron Configuration• On our periodic table, the electron

levels are represented by the electron configuration.

Fluorine: 2-7Boron: 2-3

Electron Configuration• What is the electron configuration for

the following elements? •Oxygen (O): • Beryllium (Be):

Drawing Bohr’s Model

Fluorine

Drawing Bohr’s Model

Boron

Drawing Bohr’s Model

Oxygen

Drawing Bohr’s Model

Beryllium

Wave-Mechanical Model• The modern structure of the atom

has maintained many of the same features we discussed with one small difference…

The Electron Cloud

Electron Cloud• Rather than defined rings, the

electrons have areas they are most likely found (highest probability) that are similar to Bohr’s Rings.

Do now• Get one of each paper• Complete the “What we know”

section of 2-9.

Valence Electrons and

Lewis Dot Diagrams

What we know• To find mass number, add protons

and neutrons• The number of protons always equals

the atomic number• To find neutrons, subtract protons

from mass number• For an atom, the number of electrons

equals the number of protons

Valence Electrons• The outermost

electrons are responsible for all of the chemical properties of the element.

• These outermost electrons are called valence electrons.

2-5Valence Electrons

Periodic Table

The last number in

the electron configuratio

n are the valence

electrons.

Lewis Electron Dot Diagrams

• G. N. Lewis was a college professor in 1902 teaching chemistry.

• He wanted a way to represent the valence electrons.

• He decided to develop his own method.

Lewis Electron Dot Diagrams

• Each of the valence electrons are represented by a dot.

• Start by putting one dot on each side of the symbol then pairing them up.

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Lewis Electron Dot Diagram

C Cl N Br

Excited vs. Ground State

Ground State• When an atom is at its lowest

energy, the electrons are in the ground state.

FluorineP=9N=9

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e-

e- e-

e-e-

e-

e-2

7

Excited State• When an atom absorbs energy, an

electron is moved up a level. This is called the excited state.

FluorineP=9N=9

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e- e-

e-e-

e-

e- 27

18

1-8

Energy of Electrons• Specific amounts of energy is gained

when electrons go to an excited state and move up an energy level

• Specific amounts of energy is lost when electrons return to ground state.

Absorption Spectrum

BoronP=5N=6

e-

e-

e-

e-

e-Only certain photons of light can be

absorbed or released by an atom based on

its electron configuration.

Emission Spectrum

Emission spectrum show which colors of light are released.

Emission Spectrum

BoronP=5N=6

e-

e-

e-

e-

e-Light is produced when electrons down an energy level and energy is released.

This is when an electron goes from excited to ground

state.

To determine what elements are in a mixture, match up the lines in the mixture to

the lines in the known spectra.

Vocabulary1. Proton2. Neutron3. Electron4. Valence Electron5. Mass Number6. Ion7. Isotope8. Emission/Bright

line Spectrum

9. Excited State10.Ground State11.Wave-mechanical

model12.Electron Cloud13.Orbital14.Lewis Dot Diagram15.Bohr’s Model