Download Atomic Theory - Aurora City Schools

Atomic Theory
• Atom
– smallest part of an element with
that element’s properties
• 2 parts
– Nucleus
• Protons (positive charge, weigh 1 amu*)
• Neutrons (neutral charge, weigh 1 amu*)
– Orbitals (inside electron cloud)
• Electrons (negative charge, negligible mass)
• (* amu = atomic mass unit)
A note about electrons
• They don’t all have the same amount of
energy.
• The farther away from the nucleus they are,
the more energy they have.
• We don’t know exactly where they are, we
just have an idea. They move fast, like
the blades of a fan. We know they’re
there somewhere.
• Outermost ones are called valence electrons.
They are responsible for how elements
react with each other and the physical
and chemical properties.
Electrons
• Electron cloud is split into energy
levels that hold different
amounts of electrons.
– 1st one holds 2 (maximum)
– 2nd one holds 8
– 3rd one holds 18
– 4th one holds 32
• Electrons fill the lowest one first
• The energy levels are split into
orbitals, called s, p, d and f.
So it gets complicated
•Really the 8 electrons
in the second
energy level are 2 in
the s orbital and 6
in the p orbital
•And the 18 in the third
energy level are 2
in s, 6 in p and 10
in d orbital…
s, p, d, f orbitals
• You can see this pattern on the
table!
Do we have to know this?
• But you’ll hear more about that in
chemistry. For now realize that
each energy level is made up of
sublevels that hold specific
amounts of electrons.
• The sublevels are called orbitals
and are named s, p, d, f
• You can see these patterns in the
periodic table.
The Periodic Table
• Was organized according to the chemical &
physical properties of the known
elements
• Protons, neutrons and electrons had not yet
been discovered
• Dmitri Mendeleev noticed that when the
elements were put into order by their
atomic mass, some of their properties
repeated periodically (at regular
intervals)
• This is because the pattern of valence
electrons repeats as orbitals are filled
What it tells you
•
Atomic number =
the number of
protons
•
Atomic mass =
average mass for
all the forms of that element
• Name of the element
• Symbol (first letter always capital)
Ions
• Atoms aren’t always neutral (no
charge)
• Lose electrons
– More protons, positive ion
• Gain electrons
– More electrons, negative ion
• Has 1 charge for each electron lost or
gained
• Written above the symbol, e.g. Na+, O2-
Isotopes
• Some forms of the atom naturally have
more or less neutrons
• Because it has the same number of
protons it’s still the same atom, but
called an isotope.
• That’s why atomic mass is not a whole
number, it averages all the isotopes
• Write the mass number and atomic
number to left of symbol
14
• 6 C is carbon-14, used to carbon date
Patterns on the table
• Rows are called periods
• As you go across a period:
– Atomic number (#p+) increases by 1
– So does #e- for neutral atoms
– So does # valence electrons
• Columns are called families or groups
– Similar properties (density, melt pt, etc.)
– React chemically the same way
– Because have same valence electrons
How you use it
• #p+ = to Atomic Number (by
definition)
• #e- = # protons (in a neutral atom)
• #n0 = atomic mass, rounded off –
atomic number
Bohr Model
• Shows protons
and neutrons
in nucleus
• Shows electrons
in the cloud
in layers
called energy
levels
Lewis Diagram
• Also called dot
diagrams
• Only shows
valence electrons
(outer most)
• Valence are only
ones involved in
chemical
reactions