1. Chapter 3: Atoms – The Building Blocks of Matter
Section 2: The Structure of the Atom

2. Lesson Starter
Even though the two shapes look different, the characteristics of the various parts that compose them are the same.
The same is true with the atom.
Though atoms of different elements display different properties, isolated subatomic particles have the same properties.

3. Objectives
Summarize the observed properties of cathode rays that led to the discovery of the electron.
Summarize the experiment carried out by Rutherford and his co-workers the led to the discovery of the nucleus.
List the properties of protons, neutrons, and electrons.
Define atom.

4. The Structure of the Atom
ATOM – the smallest particle of an element that retains the chemical properties of that element.
NUCLEUS – small region located near the center of the atom
PROTON – positively charged particle found in the nucleus

5. The Structure of the Atom
NEUTRON – electrically neutral particle found in the nucleus
ELECTRON – negatively charged particle surrounding the nucleus
SUBATOMIC PARTICLES – protons, neutrons, and electrons

6. Cathode Rays and Electrons
JJ THOMSON -- Experiments in the late 1800s showed that cathode rays were composed of negatively charged particles.
When a CURRENT was passed through a cathode ray tube, the cathode glowed
caused by a stream of particles called a CATHODE RAY
traveled from the CATHODE to the ANODE

7. Cathode Rays and Electrons

8. Cathode Rays and Electrons
An object placed between the cathode and the opposite end of the tube cast a SHADOW on the glass.

9. Cathode Rays and Electrons
A PADDLE WHEEL placed on rails between the electrodes, rolled along the rails from the CATHODE toward the ANODE.
CRT Paddle Wheel Video

10. Cathode Rays and Electrons
Cathode rays were deflected by a MAGNETIC FIELD in the same manner as a wire carrying ELECTRIC CURRENT (which has a NEGATIVE CHARGE)

11. Cathode Rays and Electrons
The rays were deflected AWAY FROM a NEGATIVELY charged object.
These particles were named ELECTRONS.

12. Charge and Mass of the Electron
Millikan’s experiment measured the CHARGE-TO-MASS RATIO of an electron.
Robert A. Millikan’s oil drop experiment measured the CHARGE of an ELECTRON.
the mass of an electron is ABOUT 1/2000 the mass of the simplest hydrogen atom
electrons carry a NEGATIVE charge
electrons are present in ATOMS OF ALL ELEMENTS

13. Charge and Mass of the Electron
inferences were made about atomic structure
Because atoms are electrically neutral, they must contain a POSITIVE CHARGE to BALANCE the negative electrons
Because electrons have so much less mass than atoms, atoms must contain other particles that ACCOUNT FOR MOST OF THEIR MASS.

14. Charge and Mass of the Electron
Oil Drop Experiment Video

15. Discovery of the Atomic Nucleus
Gold Foil Experiment Video
GOLD FOIL EXPERIMENT – 1911 – ERNEST RUTHERFORD
a thin piece of gold foil was bombarded with ALPHA PARTICLES
most of the particles were expected to GO STRAIGHT THROUGH the foil and they did

16. Discovery of the Atomic Nucleus
About 1 in 8000 particles that struck the foil were deflected back towards the source
conclusions were drawn about the structure of an atom
particles must have been deflected by a DENSELY PACKED BUNDLE of POSITIVELY charged matter

17. Discovery of the Atomic Nucleus
the VOLUME of a nucleus is SMALL compared to that of the atom itself
like a MARBLE in a FOOTBALL FIELD
Rutherford suggested that the ELECTRONS SURROUND THE NUCLEUS like planets around the sun

18. Composition of the Nucleus
Except for the nucleus of the simplest type of hydrogen atom, ALL ATOMIC NUCLEI are made of PROTONS and NEUTRONS.
A ­PROTON­­­ has a POSITIVE CHARGE EQUAL in magnitude to the NEGATIVE CHARGE of an ELECTRON.

19. Composition of the Nucleus
Atoms are ELECTRICALLY NEUTRAL because they contain EQUAL NUMBERS of protons and electrons.
A neutron is electrically NEUTRAL.

20. Composition of the Nucleus
The nuclei of atoms of DIFFERENT ELEMENTS differ in their number of PROTONS and therefore in the amount of POSITIVE CHARGE they possess.
Thus, the NUMBER OF PROTONS determines that atom’s IDENTITY.

21. Forces in the Nucleus
When TWO PROTONS are EXTREMELY CLOSE to each other, there is a STRONG ATTRACTION between them.
A similar attraction exists when NEUTRONS are very close to each other or when PROTONS AND NEUTRONS are very close together.
The SHORT-RANGE proton-neutron, proton-proton, and neutron-neutron FORCES that hold the nuclear particles together are referred to as NUCLEAR FORCES.

22. The Sizes of Atoms
The RADIUS of an atom is the distance from the CENTER OF THE NUCLEUS to the outer portion of its ELECTRON CLOUD.
Because atomic radii are so small, they are expressed using a unit that is more convenient for the sizes of atoms.
The unit is the PICOMETER, pm.