4.10 Using Bohr’s Model of the Atom

(Sec 7.4 pg 218)

• Recall that Bohr’s model of the atom has electrons orbiting the nucleus, in defined electron shells.

• The Bohr model can be represented very well with diagrams.

• This is done by drawing circles that represent the electron shells around the element symbol.

• Then dots, representing the electrons, are added to the circles in sequential order from the inside to the outside.

• Remember each shell con only hold a certain number of electrons!– 1st ring = 2 electrons– 2nd ring = 8 electrons– 3rd ring = 8 electrons

• Let’s take this fine opportunity to draw Bohr diagrams for the first 20 elements!

• We can also represent an ion (an atom that has a different amount of electrons than protons) with a Bohr diagram.

• Normally P only has 15 electrons. This P ion has 3 additional electrons, giving it a charge of 3-.

• Bohr’s theory is supported by emission spectrum data. However, a really good theory also makes predictions about what kinds of observations you should see in future experiments (this is a good way to test a theory).

• One prediction from the Bohr model is that chemical reactivity is determined by the interaction of the outermost electrons in the atom.

• This prediction suggests that an atom is most stable when it has a ‘full’ outer electron shell. (sometimes known as the ‘stable octet’ rule)

• Further, this theory suggests that some elements like to ‘give up’ electrons, while others like to ‘gain’ electrons in order to have a ‘full’ outer electron shell.

• For example:– Lithium only needs to lose 1 electron to make the first

electron shell its outermost ring.

– Fluorine only needs to gain 1 electron to have a full outermost ring.

• This means that lithium should be able to lose its one ‘outer’ electron to fluorine.

• This will make lithium positively charged (less electrons than protons) and fluorine negatively charged (extra electrons).

• Now they have opposite charges, and should be attracted to each other!

• It turns out that this does happen! The Bohr model can predict how atoms bond! This makes it a very strong theory.

• Please note this type of attraction between atoms (‘bonding’) is called ionic bonding, and only occurs between metals and non-metals (more on this later)