Counting Atoms in Compounds. Chemical Formulas All the elements have symbols that are listed on the periodic table. Some elements have one letter and.

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_______________________________.