The Development of the Atomic Theory



The Development of the Atomic Theory

The Indivisible Atom

GREEK – DEMOCRITUS (450 BC)

DALTON (1803)

The Divisible Atom

FRANKLIN ( 1760’s)

DEMO: Static Electricity – laser disc (Electric Charges)

THOMSON (1890's)

MILLIKAN (1909)

DEMO: Rutherford’s Experiment – laser disc

RUTHERFORD (1908)

MOSELEY

ATOMIC NUMBER

ISOTOPES / MASS NUMBER

ATOMIC MASS

SC3. Students will use the modern atomic theory to explain the characteristics of atoms.

a. Discriminate between the relative size, charge, and position of protons, neutrons, and electrons in the atom.

b. Explain the relationship of the proton number to the element’s identity.

c. Explain the relationship of isotopes to the relative abundance of atoms of a particular element.

The Development of the Atomic Theory

THE INDIVISIBLE ATOM

Greek - Democritus (450 BC)

- Atom was indivisible

- Theorized the existence of the atom

- Also, theorized that there were just four 'elements' - fire, water, air, earth

Benjamin Franklin ( 1760’s)

- discovers that an object can have an electrical charge.

- Charge can be positive or negative

VIDEODISC

1. Can you explain what is happening?

John Dalton (1803)

- Atom was indivisible

- All elements are composed of atoms

- The same atoms for one element are exactly alike

- Atoms are neither created or destroyed in a chemical reaction

- In a chemical reaction, atoms are separated, combined, or rearranged

- Different atoms combine in simple whole number ratios to form compounds (Law of Definite Proportions)

THE DIVISIBLE ATOM

Sir William Crookes (1880)

- determined that rays were traveling from one end to another in the cathode ray tube

- the cathode ray tube led to the invention of the television

- others experimented with the cathode ray tube and discovered that the type of gas had no effect so the ray must be a part of all matter. A magnet deflected the ray, so it must be composed of charged particles, and it deflected toward the positive, so the charged particles must be negative.

J. J. Thomson (late 1890's)

- discovered the electron using the cathode ray tube

- determined that the electron was smaller than a hydrogen atom. This was a shocking discovery. Many thought Dalton was wrong.

- Knew the atom was neutral and the electron was negative, so there must be positive material with a lot more mass. Said the atom was a positive pudding-like material throughout which negatively charged electrons were scattered - Plum Pudding or Chocolate Chip Cookie Model ( Page 94, figure 4-9).

Robert Millikan (1909)

- Knew the electron was negative, but actually determined the charge (as a measurement). Was able to use this measurement to determine the mass of an electron. It was 1/1840 of a hydrogen atom

VIDEODISC

1. What is an alpha particle?

2. What did Rutherford expect to see? What did he actually see?

3. What is Rutherford’s model of the atom?

Ernest Rutherford (1909) (1919)

- Did a famous gold foil experiment (the alpha scattering experiment)

- Calculated that the atom was mostly empty space through which electrons move.

- Concluded that the atom has a small, dense, positively charged, centrally-located nucleus surrounded by negatively charged electrons

- By 1919 he had refined the concept of the nucleus. Called the positive particles protons.

Rutherford and James Chadwick (1932)

- showed the nucleus also had a neutron.

- The neutron was basically equal in mass to the proton but had no electrical charge.

SUBATOMIC PARTICLES:

Fill in the blanks with the appropriate information.

Relative How to determine

Particle Location Charge Mass (amu) the number in a neutral atom

PROTON

____________________________________________________________________________________________________

NEUTRON

____________________________________________________________________________________________________

ELECTRON

The atom at this point: is spherically shaped with a dense, centrally located, positive nucleus surrounded by one or more negatively charged electrons in an electron cloud. Most of the atom consists of fast moving electrons traveling through empty space surrounding the nucleus. Electrons are held within the atom by an attraction to the positive nucleus. The nucleus has neutral neutrons and positive protons and 99.9% of the mass. Since atoms are electrically neutral, the number of protons must equal the number of electrons.

Henry Moseley (1912)

- Discovered that atoms of each element contain a unique positive charge.

- The number of protons in an atom identifies it.

- ATOMIC NUMBER = number of protons (and number of electrons). It is unique for each element

The MASS NUMBER is the number of neutrons and protons (called nucleons) in the nucleus. This means that you can calculate the number of neutrons in an atom by taking the mass number and subtracting the number of protons.

ISOTOPES

Isotopes are atoms of the same element that have a different number of neutrons. They differ in mass, but the atom’s chemical behavior are the same. To identify an isotope, you write the element’s name and follow it with its mass number. Examples: Carbon-12, Uranium-238. It can also be identified by writing

C U

The mass of individual atoms is too small to measure so the mass of an atom is compared to a standard, Carbon-12 – Carbon-12 = 12 amu. AMU stands for atomic mass unit. The ATOMIC MASS is the weighted average of the isotopes of an element and can be calculated if you know the isotope’s mass numbers and the percentage abundance of each. For more practice, page 103.

PRACTICE

The atomic mass of iridium-191 is 191.0 amu and iridium-193 is 193.0 amu. The percentage abundance for each is 37.58% (iridium-191) and 62.62% (iridium-193). Calculate the average atomic mass of iridium.

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