To count the number of particles on each side of a chemical reaction, first use the balanced equation to count atoms from subscripts and coefficients, then (if needed) convert moles to particles using Avogadro’s number.

What “particles” means here

In chemistry, “particles” usually means:

  • Atoms (like O, H, Na).
  • Molecules (like H₂O, O₂, CO₂).
  • Formula units for ionic compounds (like NaCl, CaCO₃).

In a written chemical equation, you can count either:

  • The number of atoms of each element on each side.
  • The number of molecules or formula units, using the big numbers in front (coefficients).

Step 1: Make sure the equation is balanced

You can only compare particle counts meaningfully if the equation is balanced, because a balanced equation obeys the law of conservation of matter (same atoms of each element on both sides).

Example:
2H2+O2→2H2O\text{2H}_2+\text{O}_2\rightarrow 2\text{H}_2\text{O}2H2​+O2​→2H2​O This equation is balanced because:

  • Hydrogen: 2×2 = 4 H atoms on the left, 2×2 = 4 H atoms on the right.
  • Oxygen: 1×2 = 2 O atoms on the left, 2×1 = 2 O atoms on the right.

Step 2: Use coefficients to count molecules/formula units

The large numbers in front of formulas (coefficients) tell you how many molecules (or formula units) you have.

In 2H2+O2→2H2O\text{2H}_2+\text{O}_2\rightarrow 2\text{H}_2\text{O}2H2​+O2​→2H2​O:

  • Reactant side:
    • 2 molecules of H₂.
    • 1 molecule of O₂ (no number means 1).
  • Product side:
    • 2 molecules of H₂O.

So you can say:

  • 3 total molecules on the left (2 + 1).
  • 2 total molecules on the right.

Notice: the total number of molecules does not have to match; what must match is the total number of atoms of each element.

Step 3: Use subscripts to count atoms in each molecule

The small numbers inside a formula (subscripts) tell you how many atoms of each element are in one particle.

  • In H₂O:
    • 2 H atoms, 1 O atom per molecule.
  • In CO₂:
    • 1 C atom, 2 O atoms per molecule.

To count atoms on each side:

  1. Multiply the coefficient by the subscript for each element.
  1. Repeat for all substances on that side.

Example: 2H2+O2→2H2O\text{2H}_2+\text{O}_2\rightarrow 2\text{H}_2\text{O}2H2​+O2​→2H2​O Reactants:

  • H in H₂: coefficient 2 × subscript 2 = 4 H atoms.
  • O in O₂: coefficient 1 × subscript 2 = 2 O atoms.

Products:

  • H in H₂O: coefficient 2 × subscript 2 = 4 H atoms.
  • O in H₂O: coefficient 2 × subscript 1 = 2 O atoms.

So you’ve counted the particles (atoms) on both sides and checked they’re equal.

Step 4: If needed, convert moles to actual number of particles

Often, questions ask: “How many molecules (or atoms) is this?” rather than just “how many moles?”. You then use Avogadro’s number.

  • Avogadro’s number: about 6.02×10236.02\times 10^{23}6.02×1023 particles in 1 mole.

To find number of particles:

number of particles=moles×6.02×1023\text{number of particles}=\text{moles}\times 6.02\times 10^{23}number of particles=moles×6.02×1023

Example idea (like in the tutorial): If you know there are 5.43 moles of CaO formed, you use the balanced equation and mole ratios to find moles of O₂, then multiply by Avogadro’s number to get number of O₂ molecules.

HTML table: Atom inventory example

Here’s a simple atom inventory for a reaction, in HTML as you requested:

html

<table>
  <thead>
    <tr>
      <th>Side</th>
      <th>Substance</th>
      <th>Coefficient</th>
      <th>Atoms per particle</th>
      <th>Total atoms</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Reactants</td>
      <td>H₂</td>
      <td>2</td>
      <td>H: 2</td>
      <td>H: 2 × 2 = 4</td>
    </tr>
    <tr>
      <td>Reactants</td>
      <td>O₂</td>
      <td>1</td>
      <td>O: 2</td>
      <td>O: 1 × 2 = 2</td>
    </tr>
    <tr>
      <td>Products</td>
      <td>H₂O</td>
      <td>2</td>
      <td>H: 2, O: 1</td>
      <td>H: 2 × 2 = 4; O: 2 × 1 = 2</td>
    </tr>
  </tbody>
</table>

This “atom inventory” style matches how many teaching resources recommend organizing counts for each side of a reaction.

Mini story to remember it

Imagine each molecule is a “Lego figure” and each atom is a single Lego brick. On the left side of the arrow you count every brick in your starting figures, and on the right side you count every brick in the finished figures. Even if you break and rebuild the figures, you must have the same number of each color brick (each element) on both sides; the coefficients are how many figures you have, and the subscripts tell you how many bricks of each color are in one figure.

Quick TL;DR

  • Check the equation is balanced first.
  • Use coefficients to count molecules or formula units.
  • Use subscripts (times coefficients) to count atoms of each element on each side.
  • If the problem uses moles and asks for number of particles, multiply moles by 6.02×10236.02\times 10^{23}6.02×1023.

Information gathered from public forums or data available on the internet and portrayed here.