1 Chapter 2 — Atoms - Part 1

CHAPTER 2

Atoms and Elements

© 2006 Brooks/Cole Thomson

Lectures written by John Kotz

Chemistry (Custom UMD Edition) #

Nivaldo J. Tro#

CHAPTER 2!

Atoms and Elements!Lectures notes by E. Chang!

www.almaden.ibm.com/vis/stm/stm.html#www.ieap.uni-kiel.de/surface/ag-kipp/stm/stm.htm#

•  Greeks (5th century B.C.)#•  Law of Conservation of Matter (Lavoisier, 1789)#

•  Law of Definite Proportions (Proust, 1797)#

•  Law of Multiple Proportions (Dalton, 1804)#

•  Atomic Theory (Dalton, 1808)#

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Robert Millikan Experiment (1909)#

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*Charges are expressed relative to the charge of an electron (-1.60217733 Coulombs), which is then given the value of -1.

Particle Charge Mass Electrons -1* 9.109389 x 10-28 g Protons +1 1.672623 x 10-24 g Neutrons 0 1.674286 x 10-24 g

Elements can further combine to form compounds. Note that compounds do not "inherit" the properties of the elements.#

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A molecule is a group of two or more atoms chemically bonded together.#

X = Atomic symbol of the element

A = mass number; A = Z + N

Isotope = atoms of an element with the same number of protons, but a different number of neutrons

A Z

Z = atomic number (the number of protons in the nucleus) N = number of neutrons in the nucleus

X = The symbol of the atom or isotope

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AZ

105B

10B

11B

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1 proton and 2 neutrons, tritium#radioactive#

1 proton and 1 neutron, deuterium#

1 proton and 0 neutrons, protium#

44#35#35#

16#16#16#

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Atomic Mass/"Weight"

The existence of isotopes leads naturally to the concept of an average atomic mass, or atomic weight, associated with each element. Average atomic mass is found by taking the weighted sum of each isotope:

Avg. atomic mass = mass isotope 1 (% abund. 1

100%) + mass isotope 2 (

% abund. 2100%

) + . .

10B

11B

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Reaction of potassium + H2O

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Factors# Electrons Gained/Lost#

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•  1 atom of C has mass of 12.011 amu#

•  100 atoms C have mass of 1201.1 amu#

•  6.02214 x 1023 atoms (1 mole) of C have mass of 12.011 grams#

So, Avogadro’s number allows us to know, in grams, the mass of NA atoms of an element. It is, in fact, a unit conversion factor between atoms and moles. #

# atoms #mols#divide by NA#

Multiply by NA#