Chemistry Notes - Ch 4

Chemistry Notes

• Chapter Four
• The Structure of the Atom

4.1 Early Theories of Matter

• Democritus - (460-370 B.C.)
• believed matter was made up of tiny individual particles called atomos, from which the word atom is derived
• he could not explain how atoms were held together
• Aristotle - (384-322 B.C.)
• Believed matter was formed of air, earth, fire, and water
• Captain Planet - Intro Theme (Embed)

Slide 3

• John Dalton - (1766-1844)
• Believed all matter is composed of atoms
• **All atoms of a given element are identical having the same size, mass, and chemical properties
• **Atoms are indivisible (cannot be divided into smaller particles)
• Atoms combine in simple whole-number ratios to form compounds
• **These ideas were later proven incorrect

Slide 4

• Atom - the smallest particle of an element that retains the properties of the element
• the diameter of a single copper atom is 1.28 x 10^-10
• therefore, 6 billion copper atoms side by side, = less than 1 meter

4.2 Subatomic Particles and the Nuclear Atom

• Discovering the electron
• Sir William Crookes - performed experiments using cathode ray tubes
• Cathode ray tube (CRT) - glass tube with most of air removed, and a cathode (negative electrode) at one end and an anode (positive electrode) at the other through which electricity is passed

Slide 6

• Scientists doing CRT research determined that the cathode ray emitted was negatively charged
• it was deflected toward the positive pole of a magnet as well as the positively charged electric plate (opposite charges attract)
• it was deflected away from the negative pole of a magnet as well as the negatively charged electric charge (like charges repel)
• these negatively charged particles became known as electrons
• Cathode Ray Tube (Embed)

Slide 7

• J.J. Thomson - (1856-1940)
• determined the mass of an electron
• the mass was much less than that of a hydrogen atom, the lightest known atom
• therefore disproving Dalton's theory that atoms were indivisible
• atoms could be divided into smaller subatomic particles!
• Plum pudding model - negatively charged electrons were distributed throughout a uniform positive charge
• Plum Pudding Model (Embed)

Slide 8

• Robert Millikan - (1868-1953)
• Determined the charge of an electron to be negative one (-1)
• determined the mass of an electron to be equal to 9.1 x 10^-28g with oil drop experiment
• Millikan Oil Drop Experiment (Embed)

Slide 9

• Ernest Rutherford - (1871-1937)
• gold foil experiment
• bombarded thin sheet of gold foil with alpha particles (positively charged particles)

Slide 10

• Rutherford's Experiment: Nuclear Atom (Embed)

Slide 11

• Most Alpha particles passed through the foil with little or no deflection
• indicating that an atom is mostly empty space
• some alpha particles were deflected at small angles
• because the + alpha particle came close to the + nucleus
• A few were deflected straight back to the source
• because the + alpha particle collided with the + nucleus

Slide 12

• Rutherford's model of the atom:
• A tiny, dense positively charged nucleus at the center of the atom
• negatively charged electrons rapidly moving through empty space outside the nucleus
• an atom is mostly empty space
• if the diameter of an atom were the size of two football fields, the nucleus would be the size of a nickel

Completing the Atom The Third Subatomic Particle

• after the nucleus defined, Rutherford's co-worker, James Chadwick, found that the nucleus contained the neutron
• neutrons carry no electrical charge, therefore are neutral, but a mass relatively close to a proton

Slide 14

• Scientists credited for discovery of subatomic particles:
• Thomson - electron
• Rutherford - proton
• Chadwick - neutron

4.3 How Atoms Differ

• Atomic Number
• the number of protons in an atom (the periodic table is arranged in order of atomic numbers)
• also the number of electrons in a neutral atom

Slide 16

• Atomic Number Examples:
• What is the atomic number of chlorine? - 17
• How many protons are in a chlorine atom? - 17
• How many electron are in a chlorine atom? - 17

Slide 17

• Isotopes - atoms with the same number of proton s but different number of neutrons
• Mass Number - # protons + # neutrons
• isotopes are designated by the atomic symbol - mass number
• ex.) K-39 and K-40 are two isotopes of potassium

Slide 18

• Atomic # and mass # Example:
• element: Carbon
• Atomic #: 6
• Mass #: 14
• # protons = 6
• # electrons = 6
• # neutrons = 8

Slide 19

• Atomic Mass Unit - the unit used to describe the mass of atoms. It is equal to 1/12 the mass of a carbon-12 atom.
• Atomic Mass - the weighted average mass of hte isotopes of that element

4.4 Unstable Nuclei and Radioactive Decay

• Nuclear reactions - reactions that invoke a change in an atom's nucleus. (unlike chemical reactions which only involve an atom's electrons)

Slide 21

• Radioactivity - a process whereby substances spontenously emit radiation
• Radiation - ray and particles emitted by the radioactive materials

Slide 22

• By emitting radiation, atoms of one element can change into atoms of another element!!!
• Chemical reactions never do this!!!!
• Radioacrive atoms emit radiation because their nuclei are unstable
• Radioactive Decay - the process whereby unstable atoms gain stability by losing energy -- i.e. emitting radiation