Download 4.9 Bohr`s Theory of the Atom

4.9 Bohr’s Theory of the Atom
(Sec 7.2 pg 209)
• The last atomic model we learned about was
Rutherford's. Rutherford's model proposed that
electrons orbit the nucleus like planets around the
sun.
• While this was a 'pretty good' model, even
Rutherford knew he had a problem. For example,
celestial (planet's) orbits decrease with time; in his
model the electrons would eventually hit the
nucleus.
• Time for a new theory! In order for a new theory to
emerge, more data was necessary.
• Einstein found that data. He examined the light
given off by pure elemental gas viewed through a
prism. What he found was surprising. Instead of a
steady rainbow he found that there were bright
spots and gaps.
• This pattern is called the 'emission spectrum', and it
turns out each element has its own unique emission
spectrum.
• Bohr used Einstein’s observations to come up with a
theory about where electrons orbit around the
nucleus.
• Bohr suggested that electrons only orbit at defined
distances from the nucleus. Bohr suggested that
when an electron moves from one orbit to another,
it absorbs or emits light.
• Bohr’s model (summary):
– Electrons are located in defined shells, which are located
certain distances from the nucleus.
– Electrons cannot exist between the defined shells.
– Electrons can gain energy to move to a higher shell, or
they can lose energy to move down to a lower shell.
– Electrons are more stable when they are closer to the
nucleus.
• It was later found that each shell can only hold a
certain amount of electrons before it is full.
– 1st shell = 2 electrons
– 2nd shell = 8 electrons
– 3rd shell = 8 electrons
• It was also found that electrons fill the shells in a certain
order, from inside to outside. Also, the number of shells
that an element contains is predicted by the row it is found
on the periodic table. The periodic table is just filled with
useful information for chemists!
• Please turn to page 210 of your text.
• Let’s review ‘The Atomic Theory of Matter’ – this is
the theory of the atom we’ll be working with in
Science 9 (we will apply this theory in our
coursework – you need to know it).
• Standard Atomic Notation (see Fig.4 pg.211)
• Standard Notation for Ions (see Fig. 5 pg.212)