Chapter 4

Atom

• Smallest particle of an element that retains its chemical identity

• Atoms of the same element cannot be separated chemically because they share identical chemical properties

Daltons Atomic Theory

• Dalton have four postulates (characteristics) of atoms

• First postulate• Second postulate• Third postulate• Fourth postulate

First Postulate

• All elements are composed of tiny indivisible particles called atoms

– Atoms cannot be divided into smaller pieces

Second Postulate

• Atoms of the same element are identical.

– Atoms of the same element have identical chemical and physical properties. This is why they cannot be separated from one another by chemical means

Third Postulate

• Atoms of different elements can physically mix together or can chemically combine in simple whole number ratios to form compounds

– Remember when elements mix they do not have to form compounds

Fourth Postulate

• Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms, however, never change during a chemical reactions

Revision of Daltons First Postulate

• Daltons first postulate was later revised to read – Atoms cannot be divided by chemical means

• No chemical reaction can divide atoms. However, nuclear weapons, and nuclear power have shown how atoms can be divided by nuclear processes

Subatomic particles

• Three sub atomic particles exist

• Protons• Electrons• Neutrons

Protons

• Discovered by Eugen Goldstein

• Positively charged particle found inside the nucleus of an atom

• Protons have a mass of approximately 1 amu

Neutrons

• Neutral particles (carry no charge)

• Found in the nucleus of an atom

• Neutrons have a mass of approximately 1 amu

Electrons

• Discovered by J. J. Thompson with the Cathode Ray Tube experiment

• Carry a negative charge

• Found outside the nucleus of the atom

• Have very little mass (approximately 1/1840 the mass of a proton)

Cathode Ray Tube Experiment

• Passed a “beam of electricity” called a cathode ray through a gas in a cathode ray tube

• The beam responded to magnetic and electric forces which indicates it was charged particles

• Those particles were later called electrons

Cathode Ray Tube Experiment

• Because the cathode ray tube experiment could be repeated with many types of anodes and cathodes it was found that electrons resided in all materials.

• Hence all elements contained electrons

Rutherford Gold Foil Experiment

• Discovered atoms had a nucleus

• Fired alpha particles into gold foil

• Results

Results

• Most particles went straight through the gold foil – Atoms are mostly space

• Some particles bounced back– Atoms have dense tightly packed core