Lecture NotesChapter 4-The Structure of the

Atom

Early Ideas of the Atom:

Democritus: atomos: small cells

Aristotle: Rejected the concept of the atom

John Dalton: English teacher.

Dalton’s Atomic Theory: B. Dalton’s Atomic Theory: 1. All matter is made-up of tiny indivisible particles

called atoms.2. The atom is the smallest form of matter.3. Atoms are represented by symbols called

elements.4. Atoms are the foundation of the universe.5. All atoms of the same elements will be the same. 6. All atoms of different elements will be different. 7. Atoms of different element can combine in definite

proportions to form more compounds

Question?

Dalton was right about most of his theory. What part of Dalton’s Theory is not correct with today’s knowledge?

Answer: Atoms are divisible into small subatomic particles.

A Model of the Atom

1. Sir William Crookes: Developed the use of the cathode ray tube in the study of the atom.

a. cathode ray tube: vacuum tube with a (+) end called the anode and a (-) end called the cathode.

b. Crookes study show movement from the cathode (-) to the anode (+). From - +

2. JJ Thomson: Used the cathode ray tube to show the charge to mass ratio of the stream of light in the tube.

a. Compare the charge to mass ratio to other known values

b. Concluded that the charged particles are less than the mass of the lightest atom known.

c. Identified the first subatomic particle called the electron.

3. Robert Millikan: Use Thomson’s charge to mass ratio to define the mass of an electron.a. Mass of electron = 1/1840 the

mass of hydrogen.

4. Ernest Rutherford: Gold Foil Experiment.a. The atom has a dense center

called the nucleus.

b. The nucleus has a positive charge.

c. The positive charge is due to the protons.

d. The atom is mostly empty space.

(diagram: Link)

5. James Chadwick: Discovered the neutrona. Chadwick was a student of Rutherford.

b. Discovered that the nucleus is heavier than could be account for with just protons.

c. The extra mass called the neutron and had NO charge.

1. (no charge=neutral=neutrons)

Structure of the Atom

From the above models, the atom is known to have the following basic points:

**Every atom of the same atom has the same number of protons. THE NUMBER OF PROTONS NEVER CHANGES.

**The number of neutrons can change from atom to atom of the same element. This is called an isotope, (same atom but different number of neutrons).

Conclusion: The Atom

Within the nucleus:

*Protons: (+) charge; half the mass of the atom; number is fixed

*Neutrons: NO charge; the other half of the mass of the atom

Outside the nucleus:

* Electrons: (-) charge; in the electron cloud about the nucleus; NO mass

Conclusion: The Atom• The number of protons NEVER changes for the

same atom. Found in the nucleus• Elements are identified by the number of

protons.• In a neutral atom (net charge = 0), the # of

protons = # of electrons• All the mass of the atom is in the nucleus.• The # of neutrons + the # of protons = the mass

number.• The # of protons = the atomic number• Periodic Table

IonsFormed with an unblanced between p+

and e-

Charged form of an atom brought about by the gain of e- (-) or loss of e- (+)

Ions are formed:

1. response to another atom

2. metals form (+) ions

3. non-metal form (-) ions

4. unlike charges attract; like charges repel.

Isotopes

Same atoms with different number of neutron in the nucleus:

**Accounts for different masses for the same atom.

**amu: atomic mass units, is the average of the mass in one mole.

**Accounts for some radioactivity

Relative AbundanceHow much is present based on the

amount of the sample given.

-It is the decimal form of the percentage

-Used to find the amu.

In a given sample of sulfur, 81.3% has a mass of 32.45 g and 18.7% has a mass of 30.39 g. What is the amu of S?

Textbook: #78 and #104

Radioactivity

The spontaneous emission of radioactive (high energy) particles.

Radioactivity is caused by a proton to neutron ratio. The greater the difference between the p+ and no, the more unstable.

An unstable atom will emit energy until a more stable form is reached. This is called radioactive decay.

Types of Radiation, page 122

• Alpha radiation: (α), contains two protons and two neutrons giving a 2+ charge. Note that the mass and the atomic number changes.

• Beta radiation: (β), contains a single electron and carries a 1- charge. Note the mass does not change.

• Gamma radiation: (γ), Very high energy particle with no mass and no charge.

Radioactive Questions1. Why are some atoms radioactive/

2. Write the symbols used to denote alpha, beta, and gamma radiation. Write the mass and charge of each.

3. What is the primary factor that determines if a nucleus is stable or unstable?

4. Boron-10 emits alpha particles and Cesum-137 emits beta particles. Write a balance equation.