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Atomic Theory
Atom, Subatomic Particles, and Isotopes
The Atom
• All the elements listed on the periodic
table are made up of atoms
• An atom is the smallest part of an
element that retains the characteristics
of that element.
• The word atom comes from the Greek
word atomo.
Dalton’s Atomic Theory
• Scientist, John Dalton (1766-1844)
developed an atomic theory that
proposed that atoms were responsible
for the combinations of elements found
in compounds
Dalton’s Atomic Theory
•
Theory States:
•
All matter is made up of tiny particles called atoms
•
All atoms of a given element are similar to one another
and different from atoms of other elements
•
Atoms of two or more different elements combine to
form compounds. A particular compound is always
made up of the same kinds of atoms and always has
the same number of each kind of atom.
•
A chemical reaction involves the rearrangement,
separation, or combination of atoms. Atoms are never
created nor destroyed during a chemical reaction.
The Atom
• Atoms are the building blocks of everything we
see around us; yet we cannot see an atom or
even a billion atoms with the naked eye.
• When billions and billions of atoms are packed
together, the characteristics of each atom are
added to those of the next until we can see the
characteristics we associate with the element.
Parts of the Atom
•
Dalton had imagined atoms as solid spheres; but by the
early twentieth century growing evidence indicated that
atoms were not solid spheres.
•
Subatomic Particles
•
•
Smaller bits of matter that compose an atom
Three Subatomic Particles
•
Proton (+)...(p+)
•
Electron ( - )...(e-)
•
Neutron (neutral-no electrical charge)...(n)
Remember!!
• Like charges repel (they push away
from each other) and opposite or unlike
charges attract
Mass
• All of the subatomic particles are
extremely small compared with the
things you see around you.
• One (p+) has a mass of 1.7x10-24g and
the (n) is about the same.
• The mass of the (e-) is even less!
Mass
• Because the mass of the subatomic particles
is so minute (small), chemists use a small unit
called the Atomic Mass Unit
• Atomic Mass Unit (amu)
• Is one-twelfth of the mass of the carbon
atom (with 6 (p+) and 6 (n); the standard for
which every other atom is compared
Mass
• The amu is the same as the Dalton in
Biology.
• The (p+) and the (n) have a mass of
about 1 amu. Because the mass of the
(e-) is so small (0.0005) it is usually
ignored in the atomic mass calculations
Structure of the Atom
• In 1911, an experiment led Ernest Rutherford
to propose that most of the mass of the atom
was contained in a small region at the center of
the atom.
• The Nucleus is the dense core of the atom
where (p+) and (n) are located; it has a (+)
charge and contains most of the mass of the
atom.
• Most of the atom is empty space, which is
occupied by only by fast moving (e-)
Structure of the Atom
Atomic Number
• Atomic Number (AN)
• is equal to the number of (p+) in the
nucleus of an atom
• It is used to identify each element.
• AN = Number of (p+) in an atom
• On the Periodic Table the AN is the
whole number above the symbol.
Atomic Number
• An atom is electrically neutral (there is
no charge)
• The number of (p+) is equal to the
number of (e-)
• This electrical balance gives the atom
an overall charge of zero
• In every atom, the AN also gives the
number of (e-)
Mass Number
• We know that (p+) and (n) determine
the mass of an atom
• Mass Number or Atomic Mass (AM)
• the sum of the number of (p+) and (n)
in the nucleus.
• AM = (p+) + (n)
Atomic Symbol
Isotopes
• Isotopes
• Atoms of the same element that have
different numbers of neutrons.
• The different number of neutrons
causes the atom to have a different
mass number but not a different
chemical behavior
Isotopes
• To distinguish between the different
isotopes of an element, we can write
the Atomic Symbol that indicates the
mass number and the AN of the atom
Isotopes of Magnesium
24
Mg
12
(n)=12
26
25
Mg
12
(n)=13
Mg
12
(n)=14
2
Ions
• An atom with a positive or negative
charge.
• Atoms of the same element have a
different number of electrons
• Can also occur with groups of bonded
atoms.