counting atoms in compounds. chemical formulas all the elements have symbols that are listed on the...
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Chemical Formulas
• All the elements have symbols that are listed on the periodic table.
• Some elements have one letter and some have 2 letter symbols.– Examples (Oxygen – O and Gold – Au)
• It is very important to pay attention to the capitalization of elemental symbols. Elemental symbols are case sensitive.
Can you see how this could get confusing?
C – carbon (Element)
O – oxygen (Element)
Co – cobalt (Element)
CO – carbon monoxide (Compound)
Chemical Formulas
• Combinations of element symbols are a short way to show chemical formulas.
An example of this is water or H2O
H2OThis small number iscalled a subscript. This tells you how many atoms of that element are in theformula. Subscript is always behind the element it represents.
If there is no subscriptafter an element in a formula then there is only one atom of thatelement.
Elements
How to Count Atoms
H2O• What elements make up this compound?
– H – Hydrogen and O – Oxygen
• How many atoms of each element are in this compound?This formula tells you that the compound has
2 hydrogen atoms and 1 oxygen atom.
Count the number of atoms of each element in Al2O3.
1. First figure out how many different elements we have in this compound: Al – Aluminum O – Oxygen
2. Then look at the subscript that follows each element’s chemical symbol: Al – 2 (2 atoms of aluminum) O – 3 (3 atoms of oxygen)
Count the number of atoms of each element in CaCO3.
1. First figure out the different elements in this compound: Ca – calcium C – carbon O – oxygen
2. Then look at the subscript that follows each element’s chemical symbol: Ca – 1 (no subscript means 1 atom of that
element)
C – 1 (no subscript means 1) O – 3 (3 atoms of oxygen)
Count the number of atoms in B(OH)3.
1. Elementso B – borono O – oxygeno H - Hydrogen
2. Atoms of each elemento B – 1o O – 3 (the subscript behind the parentheses represents
every element in the parentheses)
o H - 3
Count the number of atoms in K2CrO4.
1. Elements:o K – potassiumo Cr – Chromiumo O – oxygen
2. Atoms of each element:o K – 2 atomso Cr – 1 atomo O - oxygen
Look at the following compound model:
Si O O
What is the formula for this compound model?
A. Si2O3 C. SiO
B. SIO2 D. SiO2
Which of the following models represents the compound: NaCl
Na Cl
B.
A.
NA CL
C. Na Na
Cl
D.Na
Cl Cl
Elements, Compounds, or Mixtures
• Two types of substances elements and compounds
• To recognize the difference between elements and compounds you have to look closely at the atoms or molecules
Compounds• When two or more different elements
combine chemically to form a compound.• Once the elements combine, a chemical
reaction occurs and their properties change.
Element or Compound?
• The following to diagrams show the molecule of an element and a molecule of a compound. Complete the labels to show you know which is which.
1. This diagram shows ______________,
Because there is more than one type of ___________________________________.
2. This diagram shows ____________,
Because there is only one type of_______________________________.