Transcript atoms and the periodic table

Early models of the
Atom
• Democritus of Abdera,
• 4th century BC
• Atoms are indivisible and
indestructible
• Coined the term “atom”
John Dalton
1766- 1844
• All elements are
composed of tiny
indivisible particles
called atoms
• Atoms of one element are
identical. The atoms of any one
element is different from atoms of
any other element.
Lavoisier: (1743-1795)
• When a chemical reaction occurs mass is
neither created or destroyed
– The mass of the reactants always equals the
mass of the products
–Law of conservation of mass
John Dalton
• Atoms of different
elements can chemically
combine with one
another in simple whole
number ratios to form
compounds
• Chemical reactions occur when bonds
between atoms break, and new bonds form.
Atoms are never changed into atoms of
another element
J.J. Thomson1856-1940
An electrical current
passed through a
gas at low pressure
“bent” the cathode
ray toward a (+)
charged metal plate
Atoms have Electrons
lectdemoscrtube
• Rutherford 1871-1937
• Gold foil experiment
• Atoms are mostly empty space
surrounding a dense nuclei
Chemistry Movie
Atomic Number
• The number of protons in the
nucleus
• The number of positive charges in
the nucleus
• Equal to the number of electrons
• A WHOLE NUMBER
Mass Number
• Number of protons + the
number of neutrons
• For an element, the mass
number is a whole number
Isotopes
• Atoms that all have the same number of
protons, but have different numbers of
neutrons are called isotopes
• Isotopes have different atomic mass –
because they have different numbers of
neutrons
Isotope examples
• Hydrogen-1 has 1 proton, no neutrons
• Hydrogen-2 has 1 proton, 1 neutron
• Hydrogen-3 has 1 proton, 2 neutrons
• All isotopes are identical in terms of chemical
behavior
• For a population of atoms, the Average atomic
mass is a decimal number – and depends on the
relative proportions of isotope
Average Atomic Mass
• This is not a whole number
• Multiply the relative atomic mass for each
isotope by the relative abundance.
• Add the products together
• Do not divide by the number of
isotopes!!
Average atomic mass examples
Practice problems
• Pg 120 14-17
• General Chemistry I
Periodic Table
• Dimitr Mendeleyev (1834-1907) listed
elements in columns in order of increasing
atomic mass
• He then arranged the elements according
to similarities in their properties
• Mendeleyev left spaces for elements that
he predicted would be found
Henry Moseley 1887-1915
• Arranged elements in a table based on
atomic number instead of atomic mass
http://www.webelements.com/webelements/elements/text/p
eriodic-table/key.html
The modern Periodic table
• Periods – rows
• Families: columns
• Group A elements are representative
elements
• Group B elements are transition metals
• At the bottom, are the inner transition
metals
Basic structure- you should know
• Metals on the left
• Nonmetals on the right
• Semimetals or metalloids separate the
metals from the non metals
Common families
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•
•
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Group IA – Alkaline metals
Group IIA- The Alkaline earth metals
Group VII A – The halogens
Group 0 or IIX – the noble gases