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The Structure of Atoms
What is an atom??
 Atoms are made up of 3 main subatomic particles
 The nucleus is located at the center of the atom
 The nucleus contains protons that are positively charged
and neutrons which have no charge
 Electrons are located outside the nucleus in specific energy
Subatomic Particles
Mass (kg)
1.67 x 10-27
1.67 x 10-27
9.11 x 10-31
Inside nucleus
Inside nucleus
Outside nucleus
 Neutral atoms have equal numbers of protons and electrons
whose charges exactly cancel each other
 The electric force between protons and electrons is what
holds the atom together
How Do the Structure of Atoms Differ?
Atomic # (Z)
 Equal to the number of protons in an atom of the element
 Each element has a unique number of protons
Mass # (A)
 Equal to the number of protons plus the number of neutrons in an
atom of the element
 Each element has many possible mass numbers, but the most
commonly occurring mass number is determined by rounding the
atomic mass to the nearest whole number
 Atoms of the same element that have different numbers of
 Vary in mass, but are all atoms of the same element because they
have the same number of protons
 When it is important to distinguish one isotope from another, the
mass number will follow the element name
o Ex.: Carbon-12
6 p, 6 n, 6 e
6 p, 8 n, 6 e
Atomic Mass
 The weighted average of all the masses of isotopes
 More commonly found isotopes have a greater effect on the atomic