Early Atomic Theory and the Structure of the Atom

Chemistry

Structure of an Atom Discovery

1. Compare and contrast the different atomic models.

2. Distinguish between the subatomic particles.

3. Describe the structure of the nuclear atom.

4. Define atom

5. Explain the role of atomic number in determining the identity of an atom.

6. Define isotope and explain why atomic masses are not whole numbers.

7. Calculate the number of electrons, proton, neutrons.

8. Explain the relationship between unstable nuclei and radioactive decay.

9. Characterize the different types of radiation.

Atomic Structure Outline

Make a timeline for the discovery of the structure of an

Part 1: Find all 8 scientists involved with discovering the atom (Democritus, Aristotle, Dalton, Crookes,

Thomson, Millikin, Rutherford, Chadwick)

-give their name, approximate date, and discovery

-explain the discovery in 1-2 sentences

-show to teacher before doing Part 2

Part 2: In chronological order, put the scientists, with their date, and one sentence explaining what they discovered

-put this on 8 ½ x 14 paper (with title) - Be Neat!!

Early Theories of Matter

Democritus (460-370 BC)- first person to propose the idea matter was not infinitely divisible and that matter was made of tiny individual particles called the atom

Aristotle (384-322 BC)- rejected Democritus’ view and that atoms existed

-since Democritus could not prove his theory,

Aristotle’s view become widely known

John Dalton (1803)-revised Democritus’ view based on scientific research

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; atoms of different elements vary.

3. Atoms cannot be subdivided, created, or destroyed.

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.

Changes to Dalton’s Atomic Theory

Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts.

Eventually, Dalton’s theory had to be revised as technology and new information became available:

-atoms are divisible (made up of smaller particles)

-all atoms of a given element do not have

identical properties (isotopes)

William Crookes (1880’s)-discovered the cathode ray, a ray of radiation that travels down a cathode ray tube.

-accidental discovery

JJ Thomson (1890’s)-continued

cathode tube work and

concluded the electron

was smaller than even H

-cathode ray was negative (because it

traveled away from the negative plate

and attracted to the positive plate)

-renamed the electron

Robert Millikan (1909)-confirmed the charge of an electron and determined its mass

Matter is not negatively charged, so atoms can’t be negatively charged either.

If atoms contained extremely light, negatively charged particles, then they must also contain positively charged particles—probably with a much greater mass than electrons.

Ernest Rutherford (1911)- most of the mass of an atom is in the nucleus, which is positively charged; most of the space comes from the electrons

-by 1920 he concluded the positive charge was actually a proton (+1 charge)

-“gold foil experiment”: a beam of positively charged subatomic particles toward a thin piece of gold foil

~Because most particles

passed through the foil

they concluded the atom

is nearly all empty space. ~Because so few particles

were deflected, they

proposed that the atom

has a small, dense,

positively charged central

core

James Chadwick (1932)- nucleus contains another subatomic particle, the neutron, but does not carry a charge (neutral)

Notes: -the proton and neutron are about the same size; each

is about 1836 times larger than the electron -mass of an atom comes from the nucleus, but the

volume (size) comes from the electron cloud

What is an Atom? Activity

Objectives

1. Define atom, atomic number, and mass number.

2. Determine the number of electrons, protons, and neutrons in an atom.

3. Differentiate between atoms and isotopes.

4. Determine the charge on the atom based on if it gains or loses electrons.