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Atomic Structure and Isotopes
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Atomic StructureAn atom is the smallest particle of an element that retains its identity in a chemical reaction.
The ancient Greek philosopher Democritus believed that atoms were indivisible and indestructible.
(Chapter 4)
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Democritus's ideas lasted until the late 1700s when John Dalton proposed Dalton’s Atomic Theory:
> All elements are composed of atoms.
> Atoms of the same elements are the same.
> Atoms of different elements can combine together to make compounds.
> Chemical reactions occur when atoms are separated, joined, or rearranged.
Atomic Structure and Isotopes
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There have been many models of the atom since Dalton introduced his atomic theory.
1. Dalton's Theory The "Billiard Ball" Model
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2. Thomson’s Theory– The “Raisin Bun” Model
Atomic Structure and Isotopes
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3. Rutherford’s Theory – “beehive” or “nuclear” model.
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Bohr Theory – The ‘Planet’ Model
Bohr stated that the nucleus contains both protons and neutrons with electrons in fixed energy levels around the nucleus.
Atomic Structure and Isotopes
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The Structure of the Nuclear Atom
Subatomic Particles
• Electrons (e) are negatively charged subatomic particles that surround the nucleus of the atom.
• Protons (p+) are positively charged subatomic particles and are found within the nucleus of an atom.
• Neutrons (n0) are subatomic particles with no charge (neutral). They are found within the nucleus of an atoms.
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Atomic Structure and Isotopes
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Distinguishing Among Atoms
Atomic Number
Each element in the periodic table has a unique atomic number.
The atomic number corresponds to the number of protons in the nucleus.
For an element, the number of electrons will equal the number of protons.
Atomic Number(Number of protons
and electrons)
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Mass Number
The mass number is the total number of protons and neutrons in the nucleus.
Mass Number = protons + neutrons
Not found on the periodic table!
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Isotopes
Isotopes are atoms that have the same number of protons, but different numbers of neutrons.
Therefore, isotopes of the same element will have different mass numbers.
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Special notation is used to distinguish between different isotopes:
6C 6C
Carbon12 Carbon13
12 13
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Atomic Mass
The atomic mass is the weighted average mass of the atom’s naturally occurring isotopes. It is not the mass number!
Atomic Mass
Atomic Structure and Isotopes
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Fill in the following table:
Element Name Number e Number of p+ Number of n0 Shorthand
Carbon12
25 25
Chlorine35
14 16
5024Cr