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Section 4–3:
Modern Atomic Theory
Coach Kelsoe
Physical Science
Pages 113–118
• Describe Bohr’s model of the atom and the
evidence for energy levels.
• Explain how the electron cloud model represents
the behavior and locations of electrons in atoms.
• Distinguish the ground state from excited states of
an atom based on electron configurations.
Bohr’s Model of the Atom
• Niels Bohr, a Danish physicist,
created a new and updated
model of the atom.
• His new model resembled the
solar system, with electrons
orbiting the nucleus like
planets orbit the sun.
Bohr’s Model of the Atom
• Bohr, who worked with Ernest
Rutherford, agreed with
Rutherford’s model of a
nucleus surrounded by a large
volume of space.
• Bohr’s model added to
Rutherford’s by focusing on
Energy Levels
• In Bohr’s model, electrons move with constant
speed in fixed orbits around the nucleus.
• Each electron in an atom has a specific amount of
• If an atom gains or loses energy, the energy of an
electron can change.
• The possible energies that electrons in an atom
can have are called energy levels.
Energy Levels
• Energy levels are a lot like a staircase. You can measure
your position by the distance.
• No two elements will have the same set of energy levels.
Energy Levels
• An electron in an atom can move from one energy
level to another when the atom gains or loses
• Electrons can move up multiple energy levels at a
time if it gains the right amount of energy. The
size of the jump between energy levels
determines the amount of energy gained or lost.
Evidence for Energy Levels
• Scientists can measure the
energy gained when electrons
absorb energy and move to a
higher energy level and they
can measure the energy
released when the electron
returns to a lower energy
• The movement of electrons between
energy levels explains the light you
see when fireworks explode.
• When the explosion happens,
electrons absorb the energy and the
electrons jump to higher energy
levels. When they fall to a lower
level, they emit different colors of
Electron Cloud Model
• Bohr’s model was improved over time – he was
right about energy levels, but was wrong in
assuming that electrons moved like planets in the
solar system. Electrons are much less predictable
than this.
Electron Cloud Model
• An electron cloud is a visual
model of the most likely
locations for electrons in an
atom. The cloud is denser
at those locations where the
probability of finding an
electron is high.
Electron Cloud Model
• Scientists use the electron
cloud model to describe the
possible locations of electrons
around the nucleus.
• This cloud is much like the
blur you see around a
spinning propeller.
Atomic Orbitals
• The electron cloud represents all the orbitals in an
• An orbital is a region of space around the nucleus
where an electron is likely to be found.
• Your textbook compares an orbital to the
locations you visit most at school.
Atomic Orbitals
• An electron cloud is a good approximation of how electrons
behave in their orbitals.
• Each energy level has different numbers of orbitals, and each
increases as the distance from the nucleus increases.
Energy Levels, Orbitals, and Electrons
Energy Level
Number of Orbitals
Maximum Number of Electrons
Electron Configurations
• An electron configuration is the arrangement of
electrons in the orbitals of an atom.
• The most stable electron configuration is the one
in which the electrons are in orbitals with the
lowest possible energies.
Electron Configurations
• When all the electrons in an atom have the lowest
possible energies, the atom is said to be in its
ground state.
• For example, a lithium atom with 3 electrons is in
ground state when two of its electrons are in the
orbital in the first energy level and the other is in
the first orbital in the second energy level.
Energy levels
Electron cloud
Electron configuration
Ground state