Atomic Theory · Theory ” Used all ... Atoms combine with each other in specific ratios to make...

Atomic Theory

Democritus

400 B.C.

Earth

Air

Fire

Water

Greek “Thinkers”

Atoms – “indivisible” – smallest part of

an element that retains the property of

that element

1. All atoms have a definite size

(“incompressible”)

2. All atoms are neutral

3. All matter is composed of atoms too

small to be seen which cannot be divided.

Greek “Thinkers”

4. There is empty space between atoms

5. Atoms are completely solid

Later proved incorrect

6. Atoms are different sizes, shapes, and

weights

Alchemists

Middle Ages

Spent most of their time trying to

change metals into silver and gold

Transmutation – changing one

element into another

Added knowledge about the properties

of elements

Antoine Lavoisier 1789

Able to list 33 known

elements

Law of Conservation of

Mass

“Recycling” Law

Matter can be changed;

BUT it cannot be created

or destroyed.

John Dalton

1803

“The Father of

Modern Atomic

Theory”

Used all former ideas

to come up with his

atomic theory

John Dalton

1. Each element consists of individual

atoms

2. All atoms of the same element are

identical in mass and properties

3. All atoms of different elements are

different in mass and properties

John Dalton

4. Atoms combine with each other in

specific ratios to make compounds

5. Chemical reactions occur when

atoms are separated, joined, or

rearranged, but the atoms of one

element are not changed into the

atoms of another by chemical

reaction

Dmitri Mendeleev 1869

Russian Scientist

Devised the Periodic Table

Arranged the known

elements by atomic weight

and noticed that properties

re-occurred periodically

Left blanks for undiscovered

elements

400-1890’s

Atom was viewed as solid and indivisible

Marie & Pierre Curie

Henri Becquerel

Discovered Radioactivity

Polonium (Po), Radium (Ra), Uranium (U)

Smaller particles are emitted from the atom

Gamma Rays – neutral

Beta rays – electrons

Alpha rays - protons

J.J. Thomson

Discovered the electron using the cathode ray tube

Electrons are NEGATIVE particles

e- are very small in weight

Plum pudding model

Later found positive particles

(protons)

Robert Millikan

Determined the mass of the e-

(9.11 x 10-31 kg)

This mass is MUCH smaller than the

mass of the proton

Ernest Rutherford

Gold foil experiment

Found an atom has a very small positive nucleus

surrounded by tiny moving e and lots of empty

space

Protons must be in center

Modern Gold Foil

Experiment

James Chadwick

Discovered the neutron

No charge

Same weight as proton

Atomic Structure

Subatomic Particles

protons (p+) = positive

electrons (e-) = negative

neutrons (n0) = neutral

Atomic Structure

Atomic number = # p+

Bromine

A.N. = 35

35 protons

Protons determine the element

Atomic Structure

Atoms are neutral

# protons (p+) = # electrons (e-)

Bromine

a.n. = 35

35 protons (+)

Therefore, 35 electrons (-)

Atomic Structure

Atomic Mass

The weight of the atom is in the nucleus

Protons and Neutrons in the nucleus

Atomic mass = #p+ + #n0

Bromine a.m. = 80

80 = 35 + #n0

#n0 = 45

Atomic Structure

Atoms are neutral

# protons (p+) = # electrons (e-)

Atomic number = # p+

Atomic mass = #p+ + #n0

= neutrons = protons

C-14

Nucleus

C-12

Nucleus

= neutrons = protons

C-14

Nucleus

C-12

Nucleus

-Both are Carbon (both have 6 protons)

-Have different number of neutrons!

Isotopes

Atoms of the same element (same

atomic number) with different atomic

masses (different # of neutrons).

C-14 and C-12

Mr. D Periodic Table of Elements