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Chemistry for Changing Times 10th edition
Chapter 2
Atoms: Are They for Real?
• Very small
• 1022 atoms in 1 penny
• Equivalent to 1 grain of sand in a sandbox the
size of Texas
Properties of Atoms
• Smallest characteristic of a given element
• Make up everything
• Different for each element
– Only 90 elements occur in nature
Ancient Greeks and the Atom
• Leucippus and Democritus postulated
tiny particles of water that could not be
subdivided further
– Called them atomos
• Each type of atom has a distinct shape
and size
Four Elements of Ancient
• Thought everything was continuous Atoms
didn’t exist
• Ancient Greeks could not test either model
• Aristotle’s view prevailed for >2000 years
• Father of modern chemistry
• Accurately weighed starting materials and
products of reactions
Law of Conservation of Mass
• Mass is neither created nor destroyed
during chemical reactions
• Make new materials by rearranging
• Basis for chemical calculations
Law of Definite Proportions
• Also called law of constant composition
• A compound has the same elements in
a certain definite proportion and no
other combinations
• Also implies compounds have constant
• Possible to tell when reaction is complete or
when too much starting material is present
End of Ancient Greek View
• Electrolysis of water was death blow to the
ancient Greeks' view of water as an element
Could separate water
into simpler things:
hydrogen and oxygen
Law of Multiple Proportions
• Elements might combine in more than
one set of proportions
– Each set makes up a new compound
Law of Multiple Proportions
Mass Ratios
Example: The gas methane (CH4) can be decomposed to
give carbon and hydrogen in a ratio of 3.00 g of carbon to
1.00 g of hydrogen. How much hydrogen can be made
from 90.0 g CH4?
Atom Ratios
Dalton’s Atomic Theory
All matter is composed of extremely small particles
– Atoms are indivisible
Atoms of a given element are alike but different from
atoms of any other element
– Atoms for any element have identical chemical and
physical properties
Compounds are formed when different elements
combine in fixed proportions
– Typically written with smallest whole numbers
4. A chemical reaction involves only a rearrangement of
Mendeleev’s Periodic Table
• Arranged in order of increasing atomic mass and by
chemical property
– Some elements don’t fit where their atomic mass
Data from Periodic Table
• Each element is represented by a box
Groups of atoms chemically bonded together
H represents a hydrogen atom
H2 represents a hydrogen molecule
How many atoms of O are in H2O2?
Be careful when writing formulas for
• H2O2  H2O
• Remember the law of definite proportions!