Chemistry Chapter 3

AtomsBuilding Blocks of Matter

Not the history of the atom itself, butthe history of the idea of the atom

Philosophical Idea

• Greeks– Only theoretical– 400 BC thought matter could be divided into

smaller particles until basic particle found– Democritus

• 460-370 BC • Coined the term atom• atoma (sg. atomon) or "indivisible units"• Believers of Democritus theory called atomists

– Aristotle• 384-322 BC • claimed that there was no smallest part of matter

and that different substances were made up of proportions of fire, air, earth, and water

• As there were of course no experimental means available to test either view, Aristotle's prevailed mainly because people liked his philosophy better.

• Followed this idea until 18th century

Robert Boyle (1600’s)• 1st true “chemist”

• Discovered a relationship between pressure and volume (Boyle’s Law)

Contributing Principles to Atom

• Antoine and Marie Lavoisier • (1700’s)Conducted experiments• that eliminated the 4 element idea

• Underlying the new chemistry is the assumption that matter is conserved.  Weight becomes the tool for determining if a product is simpler or more complex than an ingredient.

• Joseph Louis Proust

• Created Law of definite Proportions

or Proust Law

states that a chemical compound always contains exactly the same proportion of elements by mass

Robert Bunsen• Found that when heated, different

elements produced different colors in a flame

Atomic Theory

• John Dalton 1766-1844– English school teacher, hobby meteorology– 1st individual to recognize the importance of Democritus theory– Daltons Theory

• All matter is composed of extremely small particles called atoms

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

• Atoms cannot be subdivided, created, or destroyed

• Atoms of different elements can combine in simple, whole number ratios to form compounds

• In chemical rxn, atoms are combined, separated, or rearranged

• Proposed the “Billiard-ball model” of the atom

Atomic theory accepted

• Explained the Laws of conservation of mass and definite composition

• Exceptions to theory– Atoms are divisible—discovery of the

subatomic particles– An elements atoms can have different

mass***discovery of isotopes

Atom and conservation of mass

• “if atoms are indivisible and atoms of different elements can combine in chemical reactions then it must be that mass is conserved in a chemical reaction.”

Law of Multiple Proportions

• Created by John Dalton

• States: If two or more different compounds are composed of the same two elements, the masses of the second element combined with a certain mass of the first element can be expressed as ratios of small whole numbers

• Carbon Monoxide vs Carbon Dioxide

Structure of the Atom

• Defn. atom: smallest particle of an element that can exist alone or in combination with other atoms

• Defn. atomic structure: the identity and arrangement of smaller particles w/in atoms

• Regions of atom– Nucleus

• DENSE central part• Is positive in charge

– Outer energy levels (shells, rings)• Contain negative particles

Subatomic particles

• Discovery of Electron– John Joseph (JJ) Thomson

• English physicist• Measured ration of the charge of a cathode-ray

particle to mass• Found ratio always the same regardless of metal

used to make the cathode or the gas used• Concluded all cathode rays were composed of

negative particles• Called particles ELECTRONS

Plum pudding model

Charge and Mass of Electron

• Robert Millikan– American physicist– Found mass of e- to be 1/2000 of mass of H

atom– Accepted mass of an electron is 9.109 x 10-

28 grams– Electron present in all matter

Inferences

• Because atoms are neutral, must also contain a positive charge

• Because e- are so small, there must be other particles present to account for the mass

Discovery of Atomic Nucleus• Ernest Rutherford, Hans Geiger, and

Ernest Marsden– Studied the bombardment of thin metal foil

with alpha particles (+)– Rutherford concluded: The force of repulsion

must be caused by very densely packed bundles of matter with a positive charge

– Called the bundle the NUCLEUS

• Nucleus– Has a positive charge– Contains nearly all mass of the atom– Takes up an insignificant fraction of volume– Composition

• Protons– Positive charged– Equal in magnitude to the negative charge of an electron

• Nuclear Model of the Atom

Discovery of Neutron

• James Chadwick

• Chadwick made a fundamental discovery in the domain of nuclear science: he proved the existence of neutrons - elementary particles devoid of any electrical charge.

Henri Becquerel (December 15, 1852-August 25, 1908)

•Accidentally discovered radioactivity

•Alpha particles (+2 charge)

•(Also beta particles, gamma rays)

Niels Bohr

• Created first accepted modelOf the atom using the work of previous

scientistsStated that electrons orbit the nucleus in

specific energy levels Will be expanded in next chapter

Arnold Sommerfeld

• Expanded the Bohr model

Electrons travel in orbitals, but

the orbitals are not the same shape

-- this leads to the electron cloud model of the atom

• Erwin Schrödinger (1887 – 1961) and Louis Victor de Broglie

• Model: The Wave Mechanical Model (Also the Quantum Mechanical Model)

• Schrödinger used the new quantum theory to write and solve mathematical equations to describe the location and energy of an electron in an atom.

• The model is derived from mathematical solutions to the Schrödinger equation.

• Schrödinger’s model is primarily mathematical; there are few (if any) analogies in real life.

• The Quantum Mechanical Model – a modern description of electrons in atoms

• Wolfgang Ernst Pauli (25 April 1900 – 15 December 1958)– an Austrian theoretical physicist noted for his

work on spin theory, and for the discovery of the exclusion principle underpinning the structure of matter and the whole of chemistry.

• Werner Heisenberg (December 5,1901 –February 1, 1976) was a German theoretical physicist who made foundational contributions to quantum mechanics and is best known for asserting the uncertainty principle of quantum theory.

Properties of Subatomic particles

• See table 1 page 76

Important Information for Counting Atoms

• Atomic number called the Z number

• Atomic mass is the average of the atomic masses of the naturally occurring isotopes of the element– Isotopes are atoms from same element having different numbers of

neutrons• Nuclear Symbol:

• Hyphen notation: Name---mass– When using masses, round to the nearest hundredth

– Mass must be relative (ie expressed in a unit)• Formula mass in u or amu (atomic mass unit)• Ex.: Hydrogen is 1.01 u or 1.01 amu

• A mole (mol) is a counting unit that describes the amount of substance that contains as many particles as there are atoms in 12g of Carbon-12

• Molar mass: the mass of one mole of a substance expressed in grams/mol– For elements that is the atomic mass in g/mol

Amedeo AvogadroLorenzo Romano Amedeo Carlo Avogadro, conte di Quaregna e di Cerreto (1776 -

1856)

• at the same temperature and pressure, equal volumes of gases contain the same number of particles

Avogadro’s hypothesis

• Avogadro’s Number

6.022E23 atoms=1mole