Counting Atoms in Compounds

Chemical Formulas

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

ElementsThe atoms or molecules that make up an element have only 1 type of core (or element)

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.

Compounds

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_______________________________.