Counting Atoms in Compounds

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 H 2 O

H2OH2O This small number is called a subscript. This tells you how many atoms of that element are in the formula. Subscript is always behind the element it represents. If there is no subscript after an element in a formula then there is only one atom of that element. Elements How to Count Atoms

H2OH2O What elements make up this compound? –H–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 Al 2 O 3. 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 CaCO 3. 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.Elements oB – boron oO – oxygen oH - Hydrogen 2.Atoms of each element oB – 1 oO – 3 ( the subscript behind the parentheses represents every element in the parentheses) oH - 3

Count the number of atoms in K 2 CrO 4. 1.Elements: oKoK – potassium oCoCr – Chromium oOoO – oxygen 2.Atoms of each element: oKoK – 2 atoms oCoCr – 1 atom oOoO - oxygen

Look at the following compound model: Si O O What is the formula for this compound model? A. Si 2 O 3 C. SiO B. SIO 2 D. SiO 2

Which of the following models represents the compound: NaCl Na Cl B. A. NA CL C. Na Cl D. Na 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

Elements The 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 _______________________________.