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Science · Grades 8–11

Chemical equations: atom bookkeeping

Change coefficients in front of formulas, not the subscripts that identify a substance.

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Understand the idea

Balance coefficients

Change coefficients in front of formulas, not the subscripts that identify a substance.

Count each element

Count atoms on each side separately before deciding whether an equation is balanced.

Examples and reasoning

How many hydrogen atoms are represented by 3H₂O?

3 × 2 = 6

6

How many oxygen atoms are represented by 3H₂O?

3 × 1 = 3

3

Check your understanding

  1. How many oxygen atoms are in one formula unit of Ca(NO₃)₂?

  2. Balance using the smallest whole-number coefficients: __H₂ + O₂ → 2H₂O. What fills the blank?

  3. In 2Mg + O₂ → 2MgO, how many magnesium atoms are represented on the product side?

  4. Why should H₂O not be changed to H₂O₂ merely to balance an equation?

    1. Subscripts can never be read.
    2. That changes the identity of the substance.
    3. Oxygen cannot appear twice.
    4. Coefficients represent temperature.

Answers and marking notes

Answers and marking notes
  1. 6

    The outside 2 multiplies the 3 oxygen atoms in each nitrate group.

  2. 2

    2H₂ provides the 4 hydrogen atoms in 2H₂O.

  3. 2

    2

  4. That changes the identity of the substance.

    That changes the identity of the substance.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Learning preview · teacher review required. Grade bands are approximate, not official curriculum alignment.