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)