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Understanding Counting Atoms in Compounds or Molecules: Basics, Rules, and Examples

This guide explains the principles of counting atoms in compounds or molecules using chemical formulas, symbols, subscripts, and coefficients. Learn how to identify and calculate the number of atoms for each element in a compound. Practice counting atoms with examples and test yourself to enhance your understanding.

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Understanding Counting Atoms in Compounds or Molecules: Basics, Rules, and Examples

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  1. Counting Atoms in Compounds or Molecules Big Idea: • Given a chemical formula be able to count the number of atoms for each element in the compound or molecule.

  2. Parts of a Chemical Formula Element Symbol 2 H2O 4 Pb(OH)4 Subscript Coefficient

  3. 3 Parts of a Chemical Formula 1. Chemical symbols - Written in the order they appear on the periodic table, from left to right. 2. The subscript - Gives the number of atoms - Must be written after the symbol, half size 3. The coefficient - Gives the number of compounds or molecules • Must be written regular size before the formula

  4. Counting Atoms • Rules for counting atoms in a molecule or compound. • A subscript shows how many atoms of that element are bonded in the molecule. • If there is no subscript then one atom of the element is present. Group all like atoms!! • If the subscript is outside the brackets, all the atoms inside the brackets are multiplied by the subscript outside the brackets. • Coefficients are multiplied last. The total number of atoms in the formula is multiplied by the coefficient.

  5. Count the Atoms • Oxygen – 1 atom • Hydrogen – 2 atoms • Formula - H2O

  6. Count the Atoms • Calcium – 1 atom • Oxygen – 2 atoms • Hydrogen – 2 atoms Ca(OH)2

  7. Coefficients The number of water molecules is shown with a coefficient. Coefficients are written in front of the chemical formula. 3 H2O

  8. Count all the atoms 3 H2O The coefficient multiplies all the atoms after it. Hydrogen 3 x 2 = 6 Oxygen 3 x 1 = 3

  9. Test Yourself lead 1 Oxygen 4 Hydrogen 4

  10. Counting atoms lead 1 Oxygen 4 Hydrogen 4 magnesium 3 phosphorus 2 oxygen 6 arsenic 1 iodine oxygen 20 5 copper 2 8 4 oxygen chlorine

  11. Try Grouping Like Atoms • Given the formula CH3O(CH2)2OH • How many atoms of carbon in the formula? • 3 • How many atoms of oxygen in the formula? • 2 • How many atoms of hydrogen in the formula? • 8

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