FutureStepsA quick starting check.
- Recognize that an element contains one kind of atom.
- Read a periodic table to match an element symbol with its name.
- Add and multiply small whole numbers.
Build the method, one step at a time.
Read each element symbol as a unit
An element symbol begins with a capital letter and may include a lowercase letter. H is hydrogen, O is oxygen and C is carbon. Co is one symbol for cobalt, while CO contains the separate symbols C and O. Capitalization changes what the formula says.
This lesson uses simple molecular formulas. A molecule contains atoms bonded together. A compound contains more than one kind of element chemically combined; a molecule such as O₂ contains only oxygen and is not a compound. Not every substance is made of separate molecules.
A subscript applies to the symbol before it
A subscript is the smaller number following an element symbol. In H₂O, the 2 applies to H, so one water molecule contains two hydrogen atoms. O has no written subscript, which means one oxygen atom.
Make a separate count for each element before adding a total. In CO₂, count one carbon and two oxygen atoms, for three atoms altogether. The formula contains two kinds of element, even though it contains three atoms in one molecule.
A coefficient repeats the whole formula
As a next step, read a number written before a molecular formula. In 2H₂O, the coefficient 2 represents two water molecules. Multiply every atom count in one molecule by 2: four hydrogen atoms and two oxygen atoms in total.
Changing a coefficient changes how many molecules are represented. Changing a subscript changes the formula itself. H₂O and H₂O₂ are different substances, so you cannot change a subscript just to make a counting problem easier. Formulas with parentheses and ionic compounds need additional conventions beyond this introduction.
Two worked examples.
Count the atoms in one methane molecule, CH₄
- Identify the symbols: C is carbon and H is hydrogen. There are two kinds of element.
- C has no subscript, so count one carbon atom. H has a subscript 4, so count four hydrogen atoms.
- Add the counts: atoms in one molecule. Methane is a compound because it contains chemically combined carbon and hydrogen.
Check your reasoning: The subscript 4 belongs to H only. It does not make four carbon atoms or mean that there are four molecules.
Count the atoms represented by 3H₂O
- Start with one water molecule: two hydrogen atoms and one oxygen atom, or three atoms altogether.
- The coefficient 3 repeats the whole molecule. Count hydrogen atoms and oxygen atoms.
- Add the element totals: atoms. As a check, three molecules with three atoms each also gives nine.
Check your reasoning: Apply the coefficient to both elements. Counting six hydrogen atoms but only one oxygen atom would repeat only part of the molecule.
Try it before you reveal it.
Write down your thinking. Open a hint if you get stuck, then compare your method with the explanation.
Practice 1
How many carbon atoms, oxygen atoms and total atoms are in one CO₂ molecule?
Show a hint
An unwritten subscript means one.
Show the answer and explanation
One carbon atom, two oxygen atoms and three atoms total.
C contributes one atom and O₂ contributes two. Add the counts: . There are two kinds of element.
Practice 2
How many nitrogen and hydrogen atoms are represented by 2NH₃? What is the total atom count?
Show a hint
First count one molecule, then multiply both element counts by the coefficient.
Show the answer and explanation
Two nitrogen atoms and six hydrogen atoms, for eight atoms total.
One NH₃ molecule contains one N and three H atoms. Two molecules contain nitrogen atoms and hydrogen atoms. Their sum is eight.
Practice 3
Is O₂ a compound because it has two atoms? Explain using the kinds of element present.
Show a hint
A compound needs different elements, not just more than one atom.
Show the answer and explanation
No. O₂ contains only the element oxygen.
An O₂ molecule has two oxygen atoms bonded together. It is a molecule of an element, whereas a compound contains different elements chemically combined.
What to take with you.
Read symbols carefully, count each element using its subscript, and apply any coefficient to the whole molecule. State whether your answer counts atoms, kinds of element or molecules.
Try explaining one example without looking at the steps. If a step is hard to explain, revisit that part before moving on.
Give me a simple molecular formula without parentheses. Ask for each element’s atom count, then add a coefficient and ask what changes.
Eligible students ages 13 and older with Study access can sign in and use this prompt for more practice. Check AI explanations against your class materials.
Continue in StudyWritten by FutureSteps for study and review. This lesson expands a topic in our Texas starter collection. Course reference: Texas TEKS · Grade 7 Science (§112.27(b)(6)(A); §112.27(b)(6)(B)). These references do not imply endorsement, complete standard coverage or assessed mastery. Follow your school’s materials and course sequence.
Explore more in the 7th grade learning guide.