Who was Democritus?

How did Dalton describe atoms?

Do Now: Take out HW and Stamp sheet Answer the following in your notes

Do Now: Take out HW and Stamp sheet Answer the following in your notes

Dalton’s model (1803)

Thomson’s plum-pudding model (1897)

Rutherford’s model (1909)

Bohr’s model (1913)

Charge-cloud model (present)

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

Greek model(400 B.C.)

"In science, a wrong theory can be valuable and better than no theory at all."- Sir William L. Bragg

His atomic theory of matter contains four hypotheses:

Elements are made of tiny particles called atoms. All atoms of a given element are identical. The atoms

of a given element are different from those of any other element

Atoms of one element can combine chemically with atoms of another element to form compounds. A given compound always has the same relative number and types of atoms.

Atoms are indivisible in chemical processes. Atoms are not created or destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together.

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

Thompson’s 1897 Experiment Using a cathode ray tube,

Thomson was able to deflect cathode rays with an electrical field.

The rays bent towards the positive pole, indicating that they are negatively charged.

Thomson proposed that atoms consist of small, negative electrons embedded in a massive, positive sphere.

The electrons were like raisins in a plum pudding, called the ‘plum pudding’ model of the atom.

J.J. Thomson discovered the electron and knew that electrons

could be emitted from matter (1897).

beam of alpha particles

radioactive substance

gold foil

circular ZnS - coatedfluorescent screen

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

Rutherford ModelRutherford Model

In 1911, Ernest Rutherford In 1911, Ernest Rutherford conducted his gold foil experiment conducted his gold foil experiment which helped improve our which helped improve our understanding of atomic understanding of atomic structure.structure.

He directed a He directed a narrow beam of narrow beam of alpha particlesalpha particles at a very thin sheet at a very thin sheet of gold foil.of gold foil.

Alpha particles (Alpha particles () are ) are He atomsHe atoms that have been stripped of that have been stripped of

their their electrons electrons

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57

n +

What he expected…

Because he thought the mass was evenly distributed in the atom.

- -

-

-

-

What he got…richochetingalpha particles

Rutherford ModelRutherford Model

According to ThomsonAccording to Thomson’’s model, s model, the heavy, positive alpha the heavy, positive alpha particles should pass easily particles should pass easily through the gold, with only a through the gold, with only a slight deflection slight deflection And mostly thatAnd mostly that’’s how it s how it

happened.happened. However, they found However, they found 1 in every 1 in every

80008000 particles had actually been particles had actually been deflected back toward the source.deflected back toward the source.

Rutherford ModelRutherford Model

Rutherford suggested a new Rutherford suggested a new structural modelstructural model of the atom. of the atom. He stated that all the He stated that all the positive positive

chargecharge and the mass is and the mass is concentrated in a small core in the concentrated in a small core in the center of the atom, AKA center of the atom, AKA nucleusnucleus

And that the atom is mostly And that the atom is mostly empty empty spacespace with electrons surrounding with electrons surrounding the positively charged nucleus like the positively charged nucleus like planets around the sunplanets around the sun..

Take a minute and draw out what Rutherford thought that the atom looked like. (Don’t forget about the charge)

Proton +++++++

Electrons

Two atoms are walking down the street.One atom says to the other, “Hey! I think I lost an electron!”The other says, “Are you sure?”“Yes, I’m positive!”

A neutron walks into a restaurant and orders a couple of drinks. As she is about to leave, she asks the waiter how much she owes. The waiter replies, “For you, No Charge!!!”

Atoms are incredibly tiny. Measured in picometers (10-12 meters)

Hydrogen atom, 32 pm radius Nucleus tiny compared to atom

Radius of the nucleus near 10-15 m. Density near 1014 g/cm3

IF the atom was the size of a stadium, the nucleus would be the size of a marble.

California WEB

7Km and 27 km

electronelectron ee-- -1-1 00Outside of Outside of

nucleusnucleus

protonproton pp++ +1+1 11Inside Inside

nucleusnucleus

neutronneutron nn00 00 11Inside Inside

nucleusnucleus

Structure of the AtomStructure of the Atom

There are two regions1. Nucleus

Contains protons and neutrons

Has a positive charge Almost all the mass

2. Electron Cloud Where electrons are

found Has a negative charge Almost all the volume

Single Atom

Water Molecule