Transcript Atoms- Basic Units of Matter

Atoms- Basic Units of Matter
The atom
• Parts of an atom:
– Protons: positively charged particles; located in the
nucleus
– Neutrons: particles with no charge; located in the
nucleus
– Electrons: negatively charged particles; located
AROUND the nucleus in the electron cloud
• Regions of the atom:
– Nucleus: center of the atom that is positively charged
– Electron cloud: region surrounding an atomic nucleus
where an electron is most likely to be found; electrons
can be anywhere
Size of the atom
• As tiny as atoms are, electrons, protons and
neutrons are even smaller.
• Electrons have only about 1/2000 the mass of
protons or neutrons
• Visualize the nucleus as the size of an ant, the
atom as a whole would be the size of the
football stadium.
• How small is an atom?
The Law of Conservation of Mass
• The total mass before chemical reaction is the
same as the total mass after reaction
• A chemical reaction is a process in which
atoms in the starting materials rearrange to
form products with different properties
The Law of definite proportions
• Any pure compound always contains the same
elements in the same proportion by mass
• This law applies to any compound no matter
where the sample comes from or how large or
small it might be
• EXAMPLE: Water is always H2O if it comes
from your sink or an ice cap on Mars
History Review
• Democritus – first person to use the word atom, comes
from the Greek word atoma, means “indivisible”
• John Dalton came up with the atomic theory
• JJ Thomson did experiments using a cathode ray tube,
came up with model that included positive and
negative charges
• Ernest Rutherford did the Gold Foil experiment, which
changed the model to a positive nucleus surrounded by
electrons
• Niels Bohr determined that electrons move in specific
distances from the nucleus based on how much energy
they have