Chapter 4: The Structure of the Atom



The Structure of the Atom

Summary

1. Early Theories

• Dalton’s Atomic Theory

• Discoveries of electron, proton and neutron

• Rutherford’s experiments

2. Properties of e, p, n

• Location

• Charge

• Mass

• Nucleons: protons and neutrons

• Atomic symbols and # of p, e and n calculations

• Neutral atom versus ions

• Relative mass, amu vs. actual mass of electron

• Amu of electron = 0.000549, proton = neutron = 1

• Amu = [pic]of 12C, 12C = 12 amu

3. Distinguish between

• Atomic number = number of protons = number of electrons (for a neutral atom)

• Mass number = # protons + # neutrons

• Atomic mass = weighted average mass of the isotopes of that element

4. Average atomic mass calculations

• Mass

• % abundance

• See sample calculations

5. Radioactivity

• Spontaneous emission of radiation by certain atoms

• The structure of atomic nuclei and the changes they undergo

• Radiation: Rays and particles produced during radioactive decay

• An atom undergoes radioactive decay in order to form a more stable atom by releasing energy.

6. Types of Radiation

a. Alpha Radiation: Deflected towards negatively charged plate, hence alpha particle is positively charged.

• Alpha particle [pic]

• Has +2 charge

• Has a mass of 4

• Has no electrons

• Represented as [pic]or [pic]

Example

[pic]

[pic]

b. Beta Radiation = fast moving electrons attracted to a positively charged plate.

• Represented as [pic]

• Has mass of 0

• Has a charge of –1

Example

[pic]

c. Gamma rays

• Represented as [pic]

• Has mass of 0

• Has a charge of 0

• It is a high energy radiation

• It is usually produced along with [pic]

Example

[pic]

7. Positron Emission: Same mass as electron but opposite charge

[pic] Positron

8. Electron Capture: The nucleus pulls in surrounding electron from the lowest energy level

[pic]

The electron combines with a proton to form a neutron.

9. Nuclear Stability

• Depends on neutrons to protons ratio (n/p)

• The closer the n/p ratio is to unity (1), the more stable the atom

• The further below 1 or above 1, the less stable the atom.

10. Other Terms

• Radioactive Series: A series of reactions that begins with an unstable nucleus and results in the formation of a stable nucleus.

• Transmutation: The conversion of an atom of one element to an atom of another element.

• Induced transmutation: A type of transmutation that involves striking nuclei with high-velocity charged particles.

[pic]

Half-life: The time required for one half the original amount of a sample to decompose is referred to as the half-life of

the reaction.

In general,

Let n = number of half lives

Let t1/2 = half life

Then:

1. [pic]

n = t/T where t is the time elasped and T is the duration of the half-life

2. Let original amount = Xo

Let final amount = Xf

Then, Xf = Xo(0.5)n

3. [pic]

4. log10 Xf = log10 Xo + n log10 0.5

5. log10 Xf - log10 Xo = n log10 0.5

• Radiochemical Dating: The process of determining the age of an object by measuring the amount of certain radioisotope remaining in the object. This is possible because the half-life of radioisotope materials remain constant.

• Nuclear Fission: The splitting of a nucleus into fragments is known as nuclear fission. This process is usually accompanied by large release of energy. Therefore, this is a process used at Nuclear Power Plants to generate energy

• Nuclear Fusion: The combining of atomic nuclei is called nuclear fusion. This process also produces large amount of energy. However, energy must the supplied in order to initiate and sustain nuclear fusion reactions.

• Geiger counter: This is a radiation detection device for ionizing radiation.

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