# 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?

Jul 10, 2016

Four.

#### Explanation:

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.

You have $\text{Ca}$, followed by a subscript of $10$. This means that you're dealing with a single element that has the chemical symbol $\text{Ca}$, and that the chemical formula contains $10$ atoms of calcium.

A quick look in the periodic table will show that this element is calcium.

Now, notice that you have two capital letters in succession, $\text{P}$ and $\text{O}$. This automatically implies that you're dealing with two distinct elements, one that has the chemical symbol $\text{P}$ and the other that has the chemical symbol $\text{O}$.

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. $6$, is distributed to both elements.

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

• calcium, $\text{Ca}$
• phosphorus, $\text{P}$
• oxygen, $\text{O}$
• hydrogen, $\text{H}$

A formula unit of hydroxyapatite looks like this