Chemical Formulas

Chemical Formulas

Middle School

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Learning Objectives

• Define what chemical symbols and chemical formulas are and what they represent.

• Identify elements and their proportions in a compound using its chemical formula.

• Explain the roles of subscripts and coefficients in a chemical formula.

• Count the total number of atoms for each element in a chemical formula.

Key Vocabulary

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Chemical Symbol

A one- or two-letter designation for a chemical element, with the first letter always capitalized.

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Chemical Formula

An expression showing the elements in a compound and the relative proportions of those elements.

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Compound

A substance formed from the chemical combination of two or more different elements.

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Subscript

A number written to the lower right of a symbol showing the atom count for that element.

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Coefficient

A number placed in front of a chemical formula that multiplies every atom in the formula.

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Chemical Symbols and Formulas

Chemical Symbols

• A chemical symbol is a shorthand used to represent a specific element, like O for oxygen.

• The first letter is always capitalized, while the second letter, if present, is always lowercase.

• Some symbols are from English names (Zn for zinc), while others are from Latin (Fe for iron).

Chemical Formulas

• A chemical formula represents a compound by showing the elements and the number of atoms of each.

• For example, the formula for water is H₂O, indicating it has two hydrogen atoms and one oxygen atom.

• The formula H₂SO₄ shows that sulfuric acid contains hydrogen, sulfur, and oxygen atoms.

Multiple Choice

What is a chemical formula?

• An expression that shows the elements in a compound and their relative proportions.

• A one- or two-letter designation of an element.

• A symbol for an element that is derived from its Latin name.

• The name of a compound based on the elements it contains.

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Reading Formulas: What Are Subscripts?

• Chemical formulas use symbols and subscripts to show a compound's elements.

• A subscript shows the number of atoms for a specific element.

• For example, in H₂O, the ₂ means two hydrogen atoms.

• No subscript means one atom, like the S in H₂SO₄.

Multiple Choice

In the chemical formula C₁₂H₂₂O₁₁, how many atoms of hydrogen (H) are present?

• 22

• 12

• 11

• 1

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Coefficients and Parentheses

Coefficients

• A coefficient is a number in front of a chemical formula showing how many molecules there are.

• For example, in 3Na₂SO₄, the coefficient 3 multiplies every atom in the formula.

• This gives you 6 sodium atoms, 3 sulfur atoms, and 12 oxygen atoms.

Parentheses

• A subscript outside parentheses multiplies all the elements inside the parentheses.

• In Ca₃(PO₄)₂, the subscript 2 multiplies the phosphorus and oxygen atoms.

• This means the formula has 2 phosphorus atoms and 8 oxygen atoms.

Multiple Choice

What does the coefficient '2' in the expression '2H₂O' signify?

• There are two molecules of water.

• There are two atoms of hydrogen.

• There are two atoms of oxygen.

• The molecule is twice as large.

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Common Misconceptions

Misconception	Correction
'Co' and 'CO' are the same thing.	'Co' is the symbol for cobalt; 'CO' is the formula for carbon monoxide.
A subscript and a coefficient do the same job.	A subscript (e.g., H2) shows atoms; a coefficient (e.g., 2H2O) shows molecules.
No number by an element means there are zero atoms.	No subscript after a symbol means there is one atom of that element.

Multiple Choice

Which of the following is the correct chemical formula for a molecule containing three hydrogen (H) atoms, one sulfur (S) atom, and four oxygen (O) atoms?

• H₃SO₄

• HSO₄

• 3HSO₄

• H₃S₁O₄

Multiple Choice

How is the element 'iron' represented by its chemical symbol, and what is the origin of this symbol?

• Fe, from its Latin name 'ferrum'

• Ir, from its English name 'iron'

• I, from its English name 'iron'

• Fe, from its Greek name 'ferros'

Multiple Choice

Calculate the total number of atoms in the compound represented by the formula Pb(NO₃)₂.

• 9

• 6

• 7

• 5

Multiple Choice

Analyze the expression 4CaCO₃. What is the total number of oxygen atoms present?

• 12

• 7

• 4

• 3

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Summary

• Chemical symbols are one or two-letter codes for elements.

• Chemical formulas show the elements and their proportions in a compound.

• Subscripts indicate the number of atoms; no subscript means one atom.

• Coefficients multiply all atoms in a formula; parentheses group atoms.

Poll

On a scale of 1-4, how confident are you about counting atoms in chemical formulas?

• 1

• 2

• 3

• 4