Atomic Theory

The Movers and Shakers of the

Subatomic World.

Dalton•In the early 1800’s, the

English chemist John Dalton did a number of experiments that eventually led to the acceptance of the idea of atoms.

Dalton’s Atomic Theory

•All elements are composed of atoms. Atoms are invisible and indestructible particles.

continued...

•Atoms of the same element are exactly alike.

H H

continued...

•Atoms of different elements are different.

O H

continued...

•Compounds are formed by the joining of atoms of two or more elements.

H H O

Questions...•What were the four parts of the Dalton’s atomic theory?

•Say it again...

Was he right?•Dalton’s atomic theory of

matter became one of the foundations of chemistry.

•BUT it needed work…other scientists had to modify the theory.

Thomson•In 1897, the work of an English

scientist J. J. Thompson provided the first hint that atoms were made up of even smaller particles

•Atoms are divisible…Dalton was wrong!

Thompson’s Model•He stated that atoms are

made up of positively (+) and negativelynegatively (-) charged particles

And...•He thought the positive (+) and

negative (-)negative (-) particles were s p r e a d o u t in atoms like p p u l d u d m i s n

•(plums in puddin’)

Questions...•What kind of particles did Thompson find in atoms?

•What did his idea of an atom look like?

Was Thomson Right?•Not really…there are positively (+) and negativelynegatively (-) charged particles inside atoms.

•BUT...

Rutherford’s Model•In 1908, Rutherford discovered

the nucleus.•He stated that atoms have a

small, dense, positively (+) charged center called a nucleus.

Questions...•What did Rutherford find inside the atom?

•What did Rutherford’s atom look like?

So...•Particles are not spread out like

plums in pudding…•Positive particles (protons) are

in the center (nucleus).•Where are the negativenegative particles (electrons)?

Bohr’s Model•In 1913, Niels Bohr

proposed an improvement to Rutherford’s Model:

•Electron’s move in definite orbits around the nucleus, like planets around the sun.

Questions...•What did Bohr propose?•Where were the particles in Bohr’s model of an atom?

•What did it look like?•What was special about the electrons in Bohr’s model?

Are we there yet?•Bohr’s model was used for a

long time…

•BUT...

Modern Atomic Model

•An atom has a small, positively charged nucleus surrounded by a large region in which there are enough electrons (-) to make the atom neutral (0).

P

E

E

E

E

E

E

E

E

E

E

E

PPPPPPPP P

PP

E

Questions...•Do electrons have specific orbits in the modern model?

•What does the atom look like in the new model?

Make Your Timeline•After the sheet is passed out:

–write your name on it!–Fill out the timeline according to date, scientist, notes, and picture (if there is one) for each scientist we went over.

–Neatness counts!!!!!–HAVE FUN