Atoms,Molecules,

and Ions

Atomic Theory of Matter

The theory that atoms are the fundamental building blocks of matter reemerged in the early 19th century, championed by John Dalton.

Atoms,Molecules,

and Ions

Dalton’s Postulates

Each element is composed of extremely small particles called atoms.

Atoms,Molecules,

and Ions

Dalton’s Postulates

All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.

Atoms,Molecules,

and Ions

Isotopes

• Atoms of the same element with different masses.• Isotopes have the same number of protons but a

different numbers of neutrons.

Atoms,Molecules,

and Ions

Dalton’s Postulates

Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.

Atoms,Molecules,

and Ions

Law of Conservation of Mass

The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place.

Atoms,Molecules,

and Ions

Dalton’s Postulates

Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

Atoms,Molecules,

and Ions

Law of Constant CompositionJoseph Proust (1754–1826)

• Also known as the law of definite proportions.

• The elemental composition of a pure substance never varies.

Atoms,Molecules,

and Ions

Lecture Problem

• A pure compound is composed the carbon and oxygen. Given the following data, illustrate the law of definite proportions.Sample #1: 1.201 grams carbon & 3.200 grams of

oxygen.Sample #2: 60.05 grams carbon & 160.00 grams

of oxygenSample #3: 27.29% carbon & 72.71% oxygen

Atoms,Molecules,

and Ions

Lecture Problem

• A compound of phosphorus and chlorine used in the manufacture of a flame-retardant treatment for fabrics contains 1.20 grams of phosphorus for every 4.12 grams of chlorine. Suppose a sample of this compound contains 6.22 grams of chlorine, how many grams of phosphorus does it contain?

Atoms,Molecules,

and Ions

Law of Multiple Proportions

• Whenever two elements form more than one compounds, the different masses of one element that combine with a fixed mass of the other element are in ratio of small whole numbers.

Atoms,Molecules,

and Ions

Lecture Problem

• Compound #1: 6.005 grams carbon & 8.000 grams

oxygen.

• Compound #2:6.005 grams carbon & 16.000 grams

oxygen.

Atoms,Molecules,

and Ions

Lecture Problem

• Two different compounds of made of phosphorus and chlorine exist. One compound consist of 6.194 grams of phosphorus and 21.27 grams of chlorine. The second compound consist of 2.32 grams of phosphorus and 13.29 grams of chlorine. Illustrate the law of definite proportions.

Atoms,Molecules,

and Ions

The Electron

• Streams of negatively charged particles were found to emanate from cathode tubes.

• J. J. Thompson is credited with their discovery (1897).

Atoms,Molecules,

and Ions

The Electron

Thompson measured the charge/mass ratio of the electron to be 1.76 108 coulombs/g.

Atoms,Molecules,

and Ions

Millikan Oil Drop Experiment

Once the charge/mass ratio of the electron was known, determination of either the charge or the mass of an electron would yield the other.

Atoms,Molecules,

and Ions

Millikan Oil Drop Experiment

Robert Millikan (University of Chicago) determined the charge on the electron in 1909.

Atoms,Molecules,

and Ions

Radioactivity

• The spontaneous emission of radiation by an atom.

• First observed by Henri Becquerel.

• Also studied by Marie and Pierre Curie.

Atoms,Molecules,

and Ions

Radioactivity• Three types of radiation were discovered by

Ernest Rutherford:

Atoms,Molecules,

and Ions

The Atom, circa 1900

• “Plum pudding” model, put forward by Thompson.

• Positive sphere of matter with negative electrons imbedded in it.

Atoms,Molecules,

and Ions

Discovery of the Nucleus

Ernest Rutherford shot particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

Atoms,Molecules,

and Ions

The Nuclear Atom

Since some particles were deflected at large angles, Thompson’s model could not be correct.

Atoms,Molecules,

and Ions

The Nuclear Atom• Rutherford postulated a very small,

dense nucleus with the electrons around the outside of the atom.

• Most of the volume of the atom is empty space.

Atoms,Molecules,

and Ions

Other Subatomic Particles

• Protons were discovered by Rutherford in 1919.

• Neutrons were discovered by James Chadwick in 1932.

Atoms,Molecules,

and Ions

Subatomic Particles

• Protons and electrons are the only particles that have a charge.

• Protons and neutrons have essentially the same mass.

• The mass of an electron is so small we ignore it.

Atoms,Molecules,

and Ions

Symbols of Elements

Atoms,Molecules,

and Ions

Atomic Number

All atoms of the same element have the same number of protons

Atoms,Molecules,

and Ions

Mass Number