ATOMS
SUMMARY NOTES
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EEM-4
 Elements are pure substances that cannot be broken down into other
substances by chemical or physical means.
 Atoms are the smallest part of an element that has the properties of that
element. They are the building blocks of all matter.
 Elements are classified by their atomic number.
 The atomic number indicates the number of protons in the nucleus of an
atom of that element.
 The number of protons equals the number of electrons.
 The nucleus of an atom contains positively charged particles called protons
and neutral particles called neutrons.
 Electrons are found circling the nucleus in an area called the electron cloud.
They travel close to the speed of light.
 Electrons have very little mass.
 Most of the mass of an atom is due to the protons and the neutrons, and is
therefore found in the nucleus.
 The atomic weight of an atom is roughly equal to the number of protons
plus the number of neutrons.
 When two or more different types of atoms combine a compound is formed.
 The smallest part of a compound is a molecule.
Ex 1. Gold has an atomic number of 79. That means one atom of gold has 79
protons in its nucleus and 79 electrons in its electron cloud. It also has 118
neutrons in its nucleus. Therefore its atomic weight is 79 + 118 or 197. One
atom of gold is the smallest amount of gold that still acts like gold. Pure gold is
made up of only gold atoms. Gold cannot be broken down into other substances.
Therefore gold is an element.
Ex 2. The chemical formula for water is H2O. That means that the smallest
amount of water that acts like water consists of two atoms of hydrogen chemically
combined with 1 atom of oxygen. This is called a water molecule. There are
billions of water molecules in a small amount of water. Since a water molecule is
made up of several types of atoms chemically combined, water is known as a
compound.