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How to Count Atoms

How to Count Atoms. Coefficient. If an element or molecule has a LARGE number (called a coefficient) in front of it, this is how many of each atom there are. The LARGE number (coefficient) applies to every element that follows. Example: 4 Na – FOUR Sodium atoms

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How to Count Atoms

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  1. How to Count Atoms

  2. Coefficient • If an element or molecule has a LARGE number (called a coefficient) in front of it, this is how many of each atom there are. • The LARGE number (coefficient) applies to every element that follows. • Example: 4Na –FOUR Sodium atoms 2HCl –TWO Hydrogen atoms and TWO Chlorine atoms

  3. Subscript • If an element has a small number directly after it, this is how many of that particular atom there are. • Example H2O – TWO Hydrogen atoms and ONE Oxygen atom C2H4 – TWO Carbon atoms and FOUR Hydrogen atoms

  4. REMEMBER • IF AN ATOM DOES NOT HAVE A NUMBER IN FRONT OR BEHIND, IT IS ALWAYS ONE

  5. Putting the Two Together • If you have coefficients AND subscripts, multiply the coefficient by the subscript. • Example: 2MnO4 –TWO Manganese atoms and EIGHT(2 x 4) Oxygen atoms 3HPO4 –THREE Hydrogen atoms and THREE Phosphorous atoms and TWELVE(3 x 4) Oxygen atoms

  6. Brackets • If there are brackets within your molecule... • A) The coefficient STILL applies to all atoms within the molecule • B) a subscript still applies to the atom it DIRECTLY follows. • C) A subscript directly OUTSIDE the bracketapplies to ALL atoms INSIDE the bracket

  7. EXAMPLES Al(NO3)3 –ONE Aluminum atom, THREE Nitrogen atoms (1 x 3), and NINE Oxygen atoms (3 x 3) 3Au2(SeO4)3 –SIX Gold atoms (3 x 2), NINE Selenium atoms (3 x 3), and THIRTY-SIX Oxygen atoms (3 x 4 x 3).

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