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Element
• A substance that cannot be broken down into simpler substances
• It contains only one kind of atom
Each element is represented by a Symbol
• The first letter is capitalized
• The second letter (if present) is lower case
• C- carbon• O- oxygen• Co- cobalt
But when elements combine, they lose their unique characteristics and form a new substance with different
characteristics
= ?
We call this new substance a compound
• Compound- – a type of
substance made of atoms of two or more elements joined by chemical bonds
Compounds are represented by chemical formulas
• It will tell you:A. the elements in that compound
B. the number of atoms of that element
Writing a chemical formula for a compound
1. Write the symbol of the elements in that compound (H and O)
2. Use a subscript to identify how many atoms of each element are in the compound. (no subscript for a single atom)
Common Compound Formulas• Carbon Dioxide–CO2
• Carbon monoxide-CO - (no subscripts since only
one atom is present)• Hydrogen peroxide–H2O2
• Methane–CH4
Unit 7, 16
Molecules
• A molecule is a group of atoms joined together. • It is the smallest part of an element or
compound that can exist independently.
• There are two types of molecules:–Molecules of elements:
(All the same atoms in the molecule) e.g. Cl2, C3.–Molecules of compounds:
(Different atoms in the molecule) e.g. H2O, HCl, H2SO4.
Unit 7, 19
Elements which form molecules• Elements that end in –ine and –gen exist as
diatomic (2 atom) molecules in their natural form.
Counting atoms in compoundsNaCl
• First identify which how many different elements are in the compound (2)
• Name those elements (Na- sodium, Cl- Chlorine)• Count how many atoms of each element are
present by looking at the subscript (1 of each)• Add up the atoms to determine the total
number of atoms in that molecule (2)
Formula # of elements
Name of elements
# of atoms of each element
Total # of atoms
NaCl
2
SodiumChlorine
11
2
H2SO4
3
HydrogenSulfur
Oxygen
214
2+1+4 =7
NaOH
CHCl3
CaCO3
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