Name: ______

Development of the Atomic Theory
Reading Comprehension Guide

Development of the Atomic Theory (p.280)

  • An atom is the smallest particle that an element can be divided into and still be the same substance.
  • Atoms are called the “building blocks of matter” because all elements are made of atoms.
  • The atomic theory has been around for more than 2,000 years. A theory is an explanation for hypothesis and observations. Theories are supported with testing.

Democritus Proposes the Atom (p.280-281)

  • 440 BC Democritus, a Greek philosopher said that everything is made of “uncuttable” particles and he called these atoms.
  • Democritus says that all atoms are:
  • Small
  • Hard particles made of one kind of material that is formed into different shapes and sizes.
  • Always moving
  • Aristotle Disagrees- Aristotle was another Greek philosopher who said that you could never end up with an uncuttable particle. Aristotle was wrong, but many people believed him instead of Democritus.

Dalton Creates an Atomic Theory Based on Experiments (p.281)

John Dalton (1803) performed experiments to find out why elements combine in specific proportions to form other substances. His experiments helped him to come up with his own atomic theory.

  • Dalton’s Theory:
  • All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed
  • Atoms of the same element are exactly alike
  • Atoms join with other atoms to make new substances
  • As new discoveries are made and we learn new information, the atomic theory changes.

COMPREHENSION CHECK – Use what you just read to answer question 2 in the “Review” section on p. 283 of your textbook:

What did Dalton do in developing his atomic theory that Democritus did not do when he came up with his ideas about the atom?

Thompson Finds Electrons in the Atom (p.282-283)

  • J.J. Thompson (1897) performed an experiment and discovered that there are small particles insides an atom, so atoms CAN be divided into smaller parts.
  • See figure 3 on p. 282 – Thompson’s experiment helped us to discover that there are negatively charged particles in an atom. He called them corpuscles, but now they are called electrons.
  • Thompson performed his experiment many times and got the same results each time. This helped him know that electrons must be in all atoms.
  • Thompson created a model to show a picture of his atomic theory. It is often called the “Plum-Pudding Model.”
  • Thompson’s Theory:
  • Atoms have no overall charge
  • Atoms are made of mostly positive charged material
  • There are small, negatively charged particles located throughout the positively charged material

COMPREHENSION CHECK – Use what you just read to answer questions 1 & 3 in the “Review” section on p. 283 of your textbook:

1) The discovery of the ______by ______showed that atoms are not the smallest particle.

3) Thompson needed to repeat his experiment many times because he needed to get ______results each time before he
(the same or different)
could say that ______are in ______.
(name the particle) (all kinds of atoms or one type of atom)

Rutherford Opens an Atomic “Shooting Gallery” (p. 284-285)

  • Ernest Rutherford tested Thompson’s Theory of the atomic model in 1909
  • See Figure 5 on p. 284 of the textbook to learn more about the experiment (it was very similar to our marble experiment from class!)
  • Rutherford revised the atomic theory in 1911 and created a new atomic model:
  • Every atom has a tiny, very dense nucleus located in the center of the atom
  • Most of the mass in an atom is in the nucleus
  • An atom is mostly empty space
  • Electrons travel around the nucleus in the empty space

COMPREHENSION CHECK – Use what you just read to answer questions1 & 2 in the “Review” section on p. 286 of your textbook:

1) In an atom, most of the mass is located ______.

2) What are 2 differences between Thompson’s Atomic Theory and Rutherford’s Atomic Theory?

*Think about where the electrons are located in each

* Think about the new discovery that Rutherford made

Bohr States That Electrons Can Jump Between Levels (p.286)

  • In 1913 Neils Bohr said that electrons travel around the nucleus in definite paths and electrons can jump from one path to another.

The Modern Theory: Electron Clouds Surround the Nucleus (p. 286)

  • Erwin Schrodinger and Werner Heisenberg said that the exact path an electron takes cannot be predicted like Neils Bohr thought.
  • There are regions inside the atom where electrons are likely to be found. These are called electron clouds.

COMPREHENSION CHECK – Use what you just read to answer questions3 in the “Review” section on p. 286 of your textbook:

Bohr’s Theory about where electrons are located and how they move is different from the Modern Day Theory that Schrodinger and Heisenberg discovered.

How are the two theories different?