Atoms: The Building Blocks of Matter

CHAPTER 3

Origins of the Atom• Democritus: Greek philosopher

(460 BC - 370 BC)

• Coined the term atom from the Greek word “atomos”

• Democritus believes that atoms were indivisible and indestructible.

Let’s Get Ready to Rumble• The idea of the atom was met

with great skepticism, especially among great thinkers.

• The most vocal critic of Democritus’s idea was Aristotle.

• Aristotle’s chief argument was that there was no proof of the existence of atoms. Democritus’s claim was based purely on philosophical argument.

Aristotle’s Theory of the Atom

• Aristotle’s theory centers around the idea that everything is made up of only 4 elements: earth, wind, fire, and water.

• THERE ARE NO INDIVISIBLE PARTICLES!

Bringing Atoms Back!• During the 17th century,

scientists quietly revive the idea of the atom.

• One of the chief supporters include Sir Isaac Newton.

• At the time, most scientists are concerned with trying to explain the properties of gases.

Detour: The Three Basic Laws of Chemistry

• In their quest to discover proof of atoms, scientists began to propose three basic laws that explain all of the behavior in chemistry.

• Law of Conservation of Mass (or Matter)

• Law of Definite Proportions (or Constant Composition)

• Law of Multiple Proportions

Law of Conservation of Mass (or Matter)

• 1789: French chemist Antoine Lavoisier discovers that during an experiment involving red mercury oxide that the mass of the oxide before heating was equal to the mass of the newly formed mercury metal and oxygen gas.

• Matter cannot be created or destroyed.

Law of Definite Proportions (or Constant Composition)

• 1797-1804: French chemist Joseph Proust proposes the Law of Definite Proportions based on results from experiments using copper carbonate.

• Proust finds that all samples of copper carbonate had the same fixed composition.

• A chemical compound contains the same elements in the same proportions regardless of sample source or size.

Law of Multiple Proportions• 1803: John Dalton creates an explanation for the

Law of Conservation of Mass and the Law of Definite Proportions.

• As a result, Dalton creates the Law of Multiple Proportions.

• If 2 or more different compounds are made up of the same elements, then the ratio of the masses of elements is always a small, whole number.

Johnny D and the AT• 1808: English schoolteacher John Dalton proposes

his explanation of 2 of the 3 basic laws of chemistry.

• In his explanation, Dalton proposes proof of the atoms existence.

• Dalton’s Atomic Theory has 5 main points.

• You will have a quiz over Dalton’s Atomic Theory on Thursday!

Dalton’s Atomic Theory• 1. All matter is composed of

atoms.

• 2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.

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

Dalton’s Atomic Theory

• 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.

Does Dalton’s Theory Still Hold?

• Not all portions of Dalton’s Atomic Theory are still valid.

• Now we know that atoms can be subdivided into smaller subatomic particles such as electrons, protons, and neutrons.

• We also know that a given element can have atoms with different masses.

ATOMIC STRUCTURE

Discovery of the Electron• 1897: J.J. Thomson uses

a cathode ray tube to deduce the presence of a negatively charged particle: the electron.

How Heavy Is an Electron?• 1916: American scientist

Robert Millikan determines the mass of an electron to be 1/1837 the mass of a hydrogen atom.

• In addition, Millikan discovers that an electron has a negative one unit charge.

Is There Anything Else…• Many conclusions were made after the discovery of

the electron.

• 1. All elements must contain identically charged electrons.

• 2. There has to be positively charged particles if the atom is neutral.

• 3. There have to be heavier particles since electrons have very little mass.

On Another Note• 1886: German physicist

Eugen Goldstein discovers the proton, a positively charged particle, using anode rays.

• 1932: English scientist James Chadwick discovers the neutron, a particle with no charge but with a mass slightly larger than a proton.

Thomson’s Atomic Model• Thomson believed that the

electrons were like plums embedded in a positively charged pudding, thus calling his model the Plum Pudding model.

• This model has also been called the Blueberry Muffin Model, Chocolate Chip Cookie Model, or the Pepperoni Pizza Model.

Ernest Rutherford’s Gold Foil Experiment

• 1911: Ernest Rutherford, Hans Geiger, and Ernest Marsden fire alpha particles at a thin piece of gold foil.

Rutherford’s Results

• Most of the alpha particles passed right through; whereas, a few were either deflected or greatly deflected.

• Rutherford concluded that the nucleus is small, dense, and positively charged.

The Rutherford Atomic Model• Based on his experiment, we now

know…

• The atom is mostly empty space.

• All of the atom’s mass and positive charge is in the nucleus.

• The nucleus is composed of protons and neutrons.

• The majority of the atom’s volume is the electron cloud.