Human bone is made up of a compound known as calcium hydroxyapatite, #[Ca_10(PO_4)_6(OH)_2]#. How many different types of elements is found in hydroxyapatite?
The thing to remember about chemical elements is that their symbols always start with a capital letter and that you'll never find a chemical symbol that contains more than one capital letter.
If an element's chemical symbol contains two letters, the second one must be a lower-case letter.
So, grab a periodic table and let's start with the first one.
A quick look in the periodic table will show that this element is calcium.
Now, notice that you have two capital letters in succession,
The periodic table will reveal that these elements are phosphorus and oxygen. Moreover, the two elements are grouped together by parentheses, which means that the subscript that follows them, i.e.
Therefore, your chemical formula contains
#6 xx [1 xx "P"] = color(white)(a)6 xx "P"#
#6 xx [4 xx "O"] = 24 xx "O"#
Next, you have two capital letters grouped together by another set of parentheses. Once again, you know that these must be two distinct elements.
One is hydrogen and the other is oxygen. The subscript gets distributed to both elements, so your chemical formula will contains
#2 xx [1 xx "O"] = 2 xx "O"#
#2 xx [1 xx "H"] = 2 xx "H"#
So, we've identified four distinct elements that make up hydroxyapatite
A formula unit of hydroxyapatite looks like this