POLYATOMIC GROUPS!
Groups of atoms that act like they’re just one atom.
AN EXAMPLE
• Sodium
hydroxide is a very common chemical (it’s in Draino).
It is made of sodium, oxygen and hydrogen and it has the
formula NaOH.
• Chemists
this:
will often draw a diagram of sodium hydroxide like
Na
OH
This is weird!
Why are the oxygen and hydrogen atoms
drawn together like they’re one atom!? Why do
they have a dashed line around them?
HYDROXIDE
Na
OH
• It
turns out that oxygen and hydrogen atoms often hang out
together. It’s like they’re best friends.
• An
analogy: Do you know a group of friends at school that’s SO
CLOSE that they almost act like 1 person? They’re such good
friends that you can’t really tell them apart? Oxygen and
hydrogen are kind of like that!
• When
oxygen and hydrogen are together like this, they’re called
a “hydroxide” group. Hydroxide groups always make one bond.
HYDROXIDE
K
OH
Na
OH
Ba
OH
OH
• All
of the chemicals above are examples of oxygen and
hydrogen sticking together and acting like hydroxide.
hydroxide groups are drawn with a dashed line
because they’re not actually one atom, they’re just acting like it.
• The
• Notice
how each time, the hydroxide group is making 1 bond?
EXAMPLE!
•
What would the formula be for a chemical made of aluminum
atoms and hydroxide groups?
•
(Don’t cheat and flip the page before you try this! Get out your
periodic table, figure out how many bonds aluminum atoms make
and draw it!)
EXAMPLE!
•
Seriously, did you do the example on the last page?
•
Really?
•
Really, really?
•
Ok, if you really did, then look at the next page to see the answer!
EXAMPLE!
• Aluminum
atoms make three bonds, and hydroxide groups
make one bond, so it must look like this:
OH
Al
OH
OH
THERE’S MORE!
• It’s
actually not just oxygen and hydrogen that do this. Here
are some common atoms that group together and act like a
single atom:
Name
Atoms
# of bonds
ammonium
NH4
1
carbonate
CO3
2
hydroxide
OH
1
nitrate
NO3
1
sulfate
SO4
2
Memorize this!!!
These are the most common polyatomic groups - there are lots more! Check out
the resource in this playlist to see a longer list. (These are the only ones you need
to memorize though!)
EXAMPLE #2
• What
chemical compound would you get if calcium atoms
bonded with sulfate groups?
EXAMPLE #2
• What
chemical compound would you get if calcium atoms
bonded with sulfate groups?
• According
• Sulfate
to the periodic table, calcium atoms make 2 bonds.
groups make 2 bonds.
• So:
Ca
SO4
PARENTHESES
• In
the chemical formula, if there’s more than one polyatomic
group scientists use parentheses.
•A
subscript after the parentheses means that there’s 2 of
everything inside the parentheses...
Ca(SO4)
or
CaSO4
Al(OH)3
Ba(OH)2
Na(OH)
or
NaOH
FORMULAS & PARENTHESES
Correct:
Al(OH)3
This means that
there’s 3 OH
groups!
Incorrect:
AlOH3
This is wrong! The way
this formula is written, it is
saying there is 1 Al atom, 1
O atom and 3 H atoms.
EXAMPLE #3
• What
chemical compound would you get if magnesium atoms
bonded with nitrate groups? Draw the chemical and write the
formula.
EXAMPLE #3
• What
chemical compound would you get if magnesium atoms
bonded with nitrate groups? Draw the chemical and write the
formula.
Mg
NO3
Mg(NO3)2
NO3