Name ___________________________________________________ Period _______
G6C: Practice Counting Atoms 1
Directions: Read the brief background then answer the questions that follow.
Brief Background: Molecules are made up of atoms, which are the smallest units of elements. Each molecule has a
specific combination of elements that define its properties and behavior.
Remember that the periodic table has all the elements known listed. Elements are represented by their chemical symbol.
Chemical symbols have either one or two letters in them. An element with one as a symbol will be capitalized/upper case.
(Example N for nitrogen) If there are two letters the first is always capitalized/upper case and the second is lower case.
For example Br is the chemical symbol for bromine.
This makes it easier to name the atoms in a molecule. Look for the capital letters. When you come to a capital letter, you
know that it is an atom in that molecule.
Let’s Practice: Put a box around all the atoms in
each molecule listed below.If there is a subscript,
a number that follows a letter and is smaller and
placed to the bottom, include that in your box for
that element. A few examples are shown to help
you.
Brief Background: In chemistry, understanding the composition of molecules is crucial. A molecule is made up of atoms,
and the number of atoms of each element within a molecule is indicated by subscripts and coefficients. The subscript in a
chemical formula tells you how many atoms of each element are in a molecule. For example, in H2O , the subscript '2'
means there are two hydrogen atoms.
A coefficient in front of a chemical formula indicates how many molecules are present. For example, 3H2O, the coefficient
'3' means there are three molecules of water, making a total of six hydrogen atoms and three oxygen atoms.
Fill in the Blank: Fill in the blank with the correct words. Word bank: molecules, hydrogen, oxygen, carbon, coefficient
1. In the chemical formula CO2, the subscript '2' indicates there are two _________________________ atoms.
2. A ____________________________ appears in front of a chemical formula to indicate the number of molecules.
3. In the formula 4NH3 the coefficient '4' means there are four _________ of ammonia.
4. The subscript in H2O shows the number of ____________________________ atoms in a water molecule.
5. A molecule of CH4 contains four ____________________________ atoms, as indicated by the subscript.
Multiple Choice Questions: Choose the correct answer from the choices for each question.
6. What does the subscript in a chemical formula represent?
A. The type of atoms
C. The number of atoms
B. The number of molecules
D. The total mass
7. In the formula 2H2O, how many hydrogen atoms are there in total?
A. 2
B. 4
8. What does the coefficient in front of a chemical formula indicate?
A. The number of molecules
B. The type of atoms
9. In the chemical formula C6H12O6, how many oxygen atoms are present?
A. 3
B. 6
C. 6
D. 8
C. The charge of the molecule
D. The weight of the molecule
C. 12
D. 1
10. Which part of the chemical formula NaCl indicates the number of sodium atoms?
A. Coefficient
C. Both subscript and coefficient
B. Subscript
D. Neither subscript nor coefficient
Brief Background: Parentheses in chemical formulas are used to indicate that a group of atoms within the parentheses is
repeated a certain number of times. When counting atoms, the subscript inside the parentheses is multiplied by the
subscript outside the parentheses. This subscript outside the parentheses applies to only what is inside the parentheses.
Fill in the Blank: Fill in the blank with the correct words. Word bank: multiply, molecules, multiple, outside
11. In a chemical formula, parentheses are used to show that a group of atoms is repeated _______________ times.
12. To find the total number of atoms, you must _________________________ the subscript of each atom by the
subscript outside the parentheses.
13. In the formula Al2(SO4)3, the number of oxygen atoms is calculated by multiplying the subscript in the parentheses
with the subscript __________________________.
14. Parentheses help in simplifying the representation of complex _______________________ in a chemical formula.
Multiple Choice Questions: Choose the correct answer from the choices for each question.
15. How many oxygen atoms are in the formula Ca(NO3)2?
A) 2
C) 6
B) 3
D) 8
16. What does the subscript outside the parentheses affect in a chemical formula?
A) Only the atoms outside the parentheses
B) Only the atoms inside the parentheses
C) All the atoms in the formula
D) None of the atoms
17. In the formula K3(PO4) how many potassium atoms are present?
A) 1
C) 3
B) 2
D) 4
18. If a molecule's formula is Fe(OH)3, how many hydrogen atoms are there?
A) 1
C) 3
B) 2
D) 6
19. Which of the following best describes the role of parentheses in a chemical formula?
A) Indicate a chemical reaction
B) Group atoms that form a repeated unit
C) Separate different molecules
D) Highlight important atoms
Open Ended Questions: Answer the following questions. :
20. Explain the difference between a molecule and an atom.
21. How does the chemical formula help in understanding the composition of a molecule?
22. Describe how you would determine the number of each type of atom in a molecule using its chemical formula.