Question

how to count atoms
worksheet

1. the symbol of an element represents one atom of that element.

e.g., br=

1. a subscript is a number written at the lower right - corner behind the symbol of an element. if there is more than one atom of the element, then a subscript is used to indicate the number of atoms.

e.g., cl₂=

1. a subscript outside a bracket multiplies all the elements inside the brackets.

e.g., ca₃(po₄)₂
ca =
p =
o =

1. a coefficient is a number written in front of a chemical symbol and indicates the number of atoms of that element or number of molecules

Explanation:

Step1: Analyze single - element symbol

The symbol $Br$ represents 1 bromine atom. So the answer for $Br$ is 1.

Step2: Analyze subscript for single - element molecule

In $Cl_2$, the subscript 2 indicates there are 2 chlorine atoms. So the answer for $Cl_2$ is 2.

Step3: Analyze subscript outside brackets

In $Ca_3(PO_4)_2$:

• For $Ca$, the subscript 3 means there are 3 calcium atoms.
• For $P$, first consider the subscript inside the bracket is 1 (implied), and the subscript outside the bracket is 2. So the number of $P$ atoms is $1\times2 = 2$.
• For $O$, the subscript inside the bracket is 4 and the subscript outside the bracket is 2. So the number of $O$ atoms is $4\times2=8$.

Answer:

$Br$: 1
$Cl_2$: 2
$Ca_3(PO_4)_2$:
$Ca$: 3
$P$: 2
$O$: 8