The AtomFrom Philosophical Idea to Scientific Theory

Dalton’s Model of the Atom Solid sphere:

Solid indivisible sphere

Dalton’s Atomic Theory 1. All matter is composed of extremely small

particles called atoms. 2. Atoms of a given element are identical in

size, mass, and other properties… different elements have different properties

3. Atoms cannot be subdivided, created, or destroyed

Dalton’s Atomic Theory

4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds

5. In chemical reactions, atoms are combined, separated, or rearranged.

Modern Atomic Theory (what we believe about the atom today)

Keeps some of Dalton’s postulates but others have been disproved. Now we know:

1. Atoms are divisible even into smaller particles

2. A given element can have atoms with different masses

Cathode-ray experiments

Result of experiments between electricity and matter

Carried out in cathode ray tubes: Many of these experiments involved passing electric current through gases at low pressures

CATHODE RAY TUBE

Picture – Label Parts

CATHODE RAYS cont.

Experiments to Investigate Relationship between Energy and Matter

– Procedure: Pass current from the cathode to the anode

1st Subatomic Particle Discovery

RESULTS & HYPOTHESIS:

Current passed through tube surface of the tube directly opposite the cathode glowed

Hypothesized glow caused by a stream of particles, which they called a cathode ray

EXPERIMENTS TO TEST INITIAL HYPOTHESIS

MAIN RESULTS 1. Object placed between cathode and

opposite end of tube cast a shadow on the glass

2. A paddle wheel placed on rails between the electrodes rolled along the rails from cathode toward anode.

RESULTS CONTINUED

3. Cathode rays were deflected by a magnetic field in the same manner as a wire carrying electric current, which was known to have a negative charge

4. The rays were deflected away from a negatively charged object.

HYPOTHESIS FROM THE OTHER EXPERIMENTS

The particles that compose cathode rays are negatively charged

JJ Thomson

Supported Hypothesis (that cathode rays are negatively charged particles)

Measured the ratio of the charge of cathode-ray particles to their mass: found it was always the same Therefore concluded that all cathode rays are composed of identical negatively charged particles (called electrons)

CHARGE & MASS OF ELECTRON

Thomson’s Experiment

Showed that the electron has a very large charge for its tiny mass

Thomson’s Model of the Atom

Plum Pudding

Millikan

Oil-drop Experiment

ROBERT A. MILLIKAN’S EXPERIMENTS (1909)

He showed that the mass of the electron is about one two-thousandth (1/2000) the mass of the simplest type of hydrogen atom

More accurate experiments show the mass is actually 1/1837 the mass of H

DISCOVERY OF THE ATOMIC NUCLEUS

Rutherford’s Experiment (Draw Picture)

STARTLING RESULTS

Expected the alpha particleds to pass through with only a slight deflection (assumed mass and charge were uniformly distributed throughout the atoms of the gold foil)

Mostly true but about 1 in 8000 was redirected back toward source

RUTHERFORD’S CONCLUSIONS

1. Rebounded alpha particles must have experienced some powerful force within the atom

2. The force must occupy a very small amount of space in the atom. The atom must be mostly empty space

RUTHERFORD’S CONCLUSIONS

The force must be caused by a very densely packed bundle of matter with a positive electric charge.

He called this bundle of matter the nucleus.

QUESTIONS LEFT TO PONDER

WHERE WERE THE ELECTRONS?

Rutherford suggested that the electrons surrounded the nucleus like planets around the sun but did not know what kept the electrons in motion around the nucleus

INSIDE THE ATOM – PART 2

SUBATOMIC PARTICLESISOTOPESAVERAGE ATOMIC WEIGHT

Subatomic particles

Protons (p+) Nucleus Determine the identity of the atom Moseley’s organization of periodic table

Neutrons (n0) Nucleus

Electrons (e-) Most important in determining element’s

properties

Using Periodic Table

Periodic Table – Element information

Atomic ## protons

ElementSymbol

Average Atomic Weight (amu)

Using Periodic Table

Periodic Table – Element information

Atomic ## protons

ElementSymbol

Average Atomic Weight (amu)

Using Periodic Table

Subatomic particle values: 1. Atomic #

2. Atomic Weight

3. Mass #

Using Periodic Table

Isotopes

1. Definition Isotope: atoms of the same element with

different numbers of neutrons

2. Notation Symbol

Hyphen

Average atomic weight

1. Definition Average atomic weight: weighted

average of all naturally occurring isotopes of a given element

(Found on the periodic table)

Average atomic mass

2. Formula:

100

(%)(mass(mass)(%) )

Avg.AtomicMass

Average atomic weight

Example (Round to 2 decimal places)

Silver exists as 51.84% 107Ag and 48.16% 109Ag. The actual mass of 107Ag is 106.90509 amu and the actual mass of 109Ag is 108.90476 amu. What is the average atomic mass?