A Brief History of Atomic Theory

This Section will focus on Scientists who have had an impact on the

study of the atom.

Ready To Meet Them?Ready To Meet Them?

DemocritusDemocritusDemocritus proposed that matter cannot be broken down indefinitely. At some point you end up with a piece that can’t be divided. That smallest piece he called an atom, from the Greek word atomos, which means “indivisible”.

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470-380 B.C.

Democritus’ Model

“Atomos”

ATOMOS was the word Democritus used the point, or stage where matter cannot be broken

down any further. ATOMOS literally means “indivisible”

John DaltonJohn DaltonJohn Dalton was a British chemist.He was the first modern scientist to propose the existence of atoms.He described an atom as an invisible indestructible, solid sphere, like a billiard ball.

1766 - 1844

Dalton’s Model

The “Indivisible Sphere”

Sir Joseph “J.J.” Sir Joseph “J.J.” ThomsonThomson

J.J. Thomson was a British physicist who was the first scientist to propose the plum pudding model. He was also the first to propose the theory of the negatively charged electron. He was credited for the discovery. He also proposed the isotope.

1856 - 1940

Thomson “plum pudding” model

+

-

Ernest Ernest RutherfordRutherford

Ernest Rutherford experiments proved that atoms are mostly empty space Discovered the nuclear, which contains positively charged particles .Was the first to suggest that electrons circle the dense nucleus. 1871- 1937

Rutherford’s Model

Electron Negative charge

Nucleus

It has a

+ charge

Niels BohrNiels BohrNiels Bohr stated that electrons move in different orbits, or energy levels, around the nucleus like planets orbit the sun. Each energy level is located a specific distance from the nucleus and contains a certain number of electrons.

1885 - 1962

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Bohr Model

Electron-

negative charge

Energy

levels

Nucleus

Current ModelThis model is based upon Bohr’s model, except that electrons orbit the nucleus in random patterns. The region where these particles are found is referred to as the electron cloud.

Electron Clouds

NucleusNext SlideNext Slide

Demitri Mendeleev

Demitri Mendeleev was a Russian scientist. He was the first person to make the periodic table. He also wrote a chemistry book.The periodic table is a tool to help read elements.

Mendeleev’s Table

Modern Periodic Table

How to Read the Table

6 2

C 4

carbon

12.01115

Atomic Mass

Number of Electrons in each energy field

Atomic Number

Chemical Symbol

Element Name

Calculating Atomic Mass

6

CCarbon

12.01115

To find the Atomic Mass, you look at the bottom number in the Atom tile and round it to the nearest whole number.

Atomic Mass

How to find number of protons

6

C CARBON

12.01115

The number of Protons in atom is also the Atomic number, so therefore the Atomic number also represents the amount of Protons in the nucleus of that Atom.

Atomic Number/ number of Protons

How to Find the Amount of Neutrons in an Atom

To find the amount of neutrons in an atom you have to subtract the atomic mass from the atomic number .

16

S

sulfur

32.064

2

8

6

Atomic Mass 32

Atomic Number

16

16

Number of protons

To determine amount of electrons

6

C

CARBON

12.01115

2

4

To find the number of

electrons in an atom, look at the numbers in the right side. To determine the

number of energy levels count the number of side numbers and

there you go!!!

33

AS

ARSENIC

74.9160

How to tell what period the element is listed

2

8

18

5

4 ENERGY LEVELSAPPEARS IN THE 4TH PERIOD

What's the BIG difference

The big difference between an element, compound, and a molecule is……

Element: a substance made up of only 11 kind of atom

Compound: a substance made up of 22 or more DIFFERENTDIFFERENT elements

Molecule: a grouping of 2 2 or more more atoms joined together

For instance . . .

This is an atom of an element. As you can see, it is made up of only one kind of atom

. . In this case HydrogenH

Now, with a compound

H

O

O

When two or more atoms are put together it makes a compound.

How elements are formedHow elements are formed ElementsElements are formed with a nucleus, and are formed with a nucleus, and

inside the nucleus is protons and neutrons. inside the nucleus is protons and neutrons. Around the nucleus are energy fields, and Around the nucleus are energy fields, and inside the energy fields are are electrons.inside the energy fields are are electrons.

Sulfur

Vocabulary

Electron

Neutron

Proton

Nucleus

Compound

Atom

Isotope

element

Electron fields

Mass

Atomic structure

Periodic table

Chemical symbol

Suggested Vocabulary

Element name

chemist

properties

matter

electron cloud

synthetic

metal

metalloid

Solid

liquid

gas

noble gasses

subatomic

Nonmetals

particles

Check out some of these helpful websites to do some research of your own . . .

•A periodic table of Elements

•Periodic Table Adventure

•Science Education at Jefferson Lab

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