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Ions: Formula Writing

Elements or compounds with a  +  or –  charge are called ions.

The  +  or –  charge is because they gained or lost electrons.

Losing an electron makes them positive (cation). Gaining an electron makes them negative (anion).

For example,
write the formula for:
  • Sodium ion
  • Chloride ion
  • Nitrate ion

Practice: The key to success.

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Keys for Single Element Ions (like Sodium ion, Calcium ion, Nitrate ion):

  • Write the element symbol as found on the Periodic Table.
  • Find the charge (oxidation number) for the element using the Periodic Table.
  • Write the charge as a superscript above and to the right of the element symbol.

For Single Element Ions of Transition Metals

  • Write the element symbol and then write ionic charge based on the Roman Numeral in the name. For example, the Iron (III) ion would be Fe3+.
  • You'll also want to do this for Pb, Sn as well.

Keys for Polyatomic Ions (like NO3-,  PO43-,  NH4+):

  • Polyatomic ions have two or more elements. Most often they will have a negative charge but there a few with positive charges.
  • Either memorize the formulas of look them up on a Common Ion Table.

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