Atoms and Moles

Chapter 4

• Approximately 2500 years ago

• Philosophers

• How many times can one divide something until there is nothing left?

DEMOCRITUS

Eventually you would have the smallest possible piece of matter….hmm?

The Atom

atom (n) Gr. Atomos meaning indivisible.

• Early Greek philosopher who did NOT believe that there was a limit to the number of times matter could be divided.

• Thought all matter consisted of four elements

Aristotle’s Four Basic Elements

Law of Definite Proportions

• No matter the amount of a compound, the element proportions are exactly the same throughout.

• Joseph Proust (1754-1826) French chemist

Law of Conservation of Mass

• Matter can neither be created nor destroyed.

• Antoine Lavoisier (1743-1794)

• French Chemist

Law of Multiple Proportions

• The Law of Multiple Proportions is the third postulate of Dalton's atomic theory. It states that the masses of one element which combine with a fixed mass of the second element are in a ratio of whole numbers.

JOHN DALTON• Early 1800s

English chemist

• Developed the “Atomic Theory” in 1803

The Atomic TheoryThe Atomic Theory1) All matter is composed of atoms.

2) Atoms of the same element are exactly alike

3) Atoms of different elements are different

4) Compounds are formed by the joining of atoms of 2 or more elements

JJ ThOMSON• 1897 English

scientist• Proved atom could

be divided into smaller parts

• Discovered small negatively charged particles he called “corpuscles”

The Plum Pudding Model

ERNEST RUTHERFORD

• 1908 British physicist

• Tested Thomson’s model

• click to view biography

Thomson vs Rutherford

Rutherford’s Gold Foil Experiment

What should happen?

The particles should pass relatively easily and evenly through the foil.

The Rutherford Model

• A small dense positively charged center called the “nucleus”

• The electrons are located somewhere around the nucleus

NIELS BOHR• 1913 Danish Scientist• Said “electrons move

in definite orbits around the nucleus”

• Often known as the “solar system model”

• click to view biography

Modern Atomic Model

• It is impossible to determine the exact location of an electron

• Particles are explained according to wave mechanics and quantum mechanics

• Max Planck (1858-1947) considered the “father of quantum mechanics”

• click to view biography

Protons• Sub-atomic particle that is

positively charged• Mass of 1 amu (a small unit

used to measure the masses of atoms)

• Found in the nucleus

Electrons

• Negative charge• Very small 1/2000 the

mass of a proton• In a neutral atom

number of electrons same as number of protons

• Moving very fast

Neutrons

• Sub-atomic particle that has NO CHARGE. Neutral.

• Very similar in mass to protons; slightly larger

• Found in the nucleus

Atomic Number

• Same as the number of protons in an atom.

• Determines the identity of the element.

• Basis for the arrangement of periodic table order.

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Isotopes

• = atoms of the same element that have the same number of protons but different number of neutrons

• The arrangement of nucleus determines the isotope:

Mass Number

• =the sum of the PROTONS and NEUTRONS in the nucleus of an atom

• Example: What is the mass number of a nitrogen atom with 8 neutrons?

• Answer: 15

• MASS NUMBER = Atomic number + # of neutrons

Atomic Mass

• The average mass of all the isotopes for a particular element.

• always a decimal

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Atomic mass of an element:

Electron Cloud

• PROBABLE space around the nucleus in which electrons are most likely found.

• Similar to bees around a beehive

Electron Cloud continued:

• Location of electron depends on how much energy it possesses

• Arranged in energy levels• Lowest energy level closest to nucleus• Highest energy level furthest away• Each energy level can only hold a specific

number of electrons.• 1st level holds 2; 2nd and 3rd hold 8

Electron Configuration

• The arrangement of the electrons in an orbital

• Ground state : when all of the electrons in an atom have the lowest possible energies.

Assignment