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theodore-cummings
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Big Idea:
Given a chemical formula be able to count the number of atoms for each element in the compound or molecule.
Counting Atoms in Compounds or Molecules
Parts of a Chemical Formula
2 H2O
4 Pb(OH)4
CoefficientSubscript
Element Symbol
3 Parts of a Chemical Formula1. 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
Counting AtomsRules for counting atoms in a molecule or compound.
1. A subscript shows how many atoms of that element are bonded in the molecule.
2. If there is no subscript then one atom of the element is present. Group all like atoms!!
3. If the subscript is outside the brackets, all the atoms inside the brackets are multiplied by the subscript outside the brackets.
4. Coefficients are multiplied last. The total number of atoms in the formula is multiplied by the coefficient.
Count the Atoms
Oxygen – 1 atom
Hydrogen – 2 atoms
Formula - H2O
Count the Atoms
Calcium – 1 atom
Oxygen – 2 atoms
Hydrogen – 2 atoms
Ca(OH)2
CoefficientsThe number of water
molecules is shown with a coefficient.
Coefficients are written in front of the chemical formula.
3 H2O
Count all the atoms
3 H2O
The coefficient multiplies all the atoms after it.
Hydrogen 3 x 2 = 6
Oxygen 3 x 1 = 3
Test YourselfFormula Atom 1
presentHow Many?
Atom 2present
How Many?
Atom 3present
How Many?
Pb(OH)4
Mg3(PO3)2
As(IO4)5
2Cu(ClO2)2
1lead 4Oxygen 4Hydrogen
Counting atomsFormula` Atom 1
presentHow Many?
Atom 2present
How Many?
Atom 3present
How Many?
Pb(OH)4
Mg3(PO3)2
As(IO4)5
2Cu(ClO2)2
4 4
3magnesium 2phosphorus 6oxygen
1arsenic 5iodine 20oxygen
2copper 4chlorine 8oxygen
lead 1 Oxygen Hydrogen
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