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CHAPTER 3
Atoms
&
Atomic Theory
Foundations of Atomic Theory
• Element: cannot be broken down by
ordinary chemical means
– Elements combine to form compounds
• Chemical Reaction: transformation of a
substance (or substances) into one or more
new substances
Laws of Matter
• Conservation of Mass
– Matter can not be created or destroyed in a chemical
reaction
• Law of Definite Proportions
– A chemical compound contains the same elements in
the same proportions regardless of the size of the
sample
• Ex: Salt is NaCl,
• its composition is always 39.4 % Na & 60.6% Cl
• Law of Multiple Proportions
– Atoms combine in whole # ratios
• Ex: CO2, H2O, C6H12O6
Conservation of Mass
Dalton’s Atomic Theory
• In 1808 an English school teacher proposed an
atomic theory that he created using the laws of
matter and previously known atomic theory
1. All matter is composed of atoms
2. All atoms of a given element are identical in size, mass,
and other properties
3. Atoms can not be divided, created or destroyed
4. Atoms of different elements combine in simple whole #
ratios to form chemical compounds
5. Chemical reactions cause atoms to combine, separate,
and rearrange
Ex: Fe + O2  Fe2O3
(iron) (oxygen) (iron oxide aka rust)
Modern Atomic Theory
Not all aspects of Dalton’s atomic
theory have proven to be correct.
We now know that:
1) Atoms are divisible into even smaller particles
(subatomic particles)
2) A given element can have atoms with different masses
(isotopes)
Atomic Structure
• Atoms- atoms are the smallest particles of an
element that retain the properties of the element
• Subatomic Particles:
– Protons: positively charged(+), located in the nucleus
of an atom
– Neutrons: neutral particles(o), located in the nucleus of
an atom
– Electrons: negatively charged(-), located in the
electron cloud region surrounding the nucleus
Properties of Subatomic
Particles
Discovery of the Nucleus
• A scientist named Ernest Rutherford shot
Alpha-Particles at a thin sheet of gold foil
Rutherford’s Experiment
• 99.9% of the alpha-particles went directly
through the gold foil
• It was previously believed that atoms were
solid dense spheres
• The experiment had 3 main conclusions:
– 1) Atoms are made mostly of empty space
– 2) The nucleus is positively charged
– 3) The nucleus is very small and very dense
Nuclear Forces
• Nuclear forces hold the positively (+)
charged protons together in the nucleus
Nuclear Numbers
• Atomic #- the number of protons in the nucleus in
the atom
– Identifies the element
– Periodic table is organized by increasing atomic #
– Also tells you the # of electrons surrounding the
nucleus
• Mass #- The total number of protons + neutrons in
the nucleus of an atom
– the mass # that appears on the periodic table is an
average of all masses of all isotopes of a given element
• Isotopes- atoms of the same element with
different numbers of neutrons
– Nuclide: an isotope of an element
Isotopes and Nuclides
Average Atomic Mass
• The weighted average mass of all isotopes of an
atom
– Takes into account the abundance of isotopes
– Ex: Copper(Cu) has 2 Isotopes: Cu-63 & Cu-65
70% of all copper is Cu-63, 30% is Cu-65
Average Mass = 63.55 amu
Calculating Average Atomic
Masses
To find the average atomic mass you multiply
the % abundance of each isotope by the mass
of each isotope and take the sum of all masses
• Cu-63 = 63 amu x .70 = 44.10 amu
• Cu-65 = 65 amu x .30 = 19.45 amu
+ ------63.55 amu
Atomic Mass & # of Particles
• Moles (mol)- moles are a counting unit (dozen = 12)
– a mole = 6.02 x 1023 particles
• Avogadro's #: The number of particles in one
mole
– 6.02 x 1023
• Molar Mass- the mass of one mole of a substance
(numerically equivalent to atomic mass)
– Measured in grams per mole (g/mol)
***The Mole Road***
multiply (x)
Mass(g)
Molar
Mass(g/mol)
multiply (x)
Moles
divide (/)
6.02 x
1023
# of
Atoms
divide (/)
• To convert from Moles  Mass
– Multiply the moles by the molar mass of the compound
• To convert from Moles  # of Atoms
– Multiply the moles by 6.02 x 1023
Mole Road Sample
Calculations
• How many grams of He are in 2 moles He?
Mole Road Sample
Calculations
• How many mol Ag in 3.01 x 1023 atoms
Ag?
END OF
CHAPTER 3
NOTES!!!