Electrons
Models of the Atom
Models of the Atom
Electrons
• Electrons do not orbit the nucleus like the planets orbit the
sun.
• Electrons are located in the electron cloud around the
nucleus.
• The fixed energies an electron can have are called energy
levels.
• The closer an electron is to the nucleus, the less energy it has.
Energy Levels
• Electrons either gain or lose
energy to move between
energy levels.
• A quantum of energy is the
amount of energy required to
move an electron from one
energy level to another.
• Electrons cannot exist between
energy levels.
• Energy levels are not evenly
spaced in atoms.
Atomic Orbitals
• An atomic orbital is a region of space where there is a high
probability of finding an electron.
• Orbitals hold at most two electrons.
• Each type of orbital has a specific shape
• There are four types of atomic orbitals: s, p, d, and f
Atomic Orbitals
Atomic Orbitals
• The principal quantum number (n) indicates the energy level
of an electron.
• The number of electrons allowed in each of the first four
principle energy levels are shown here.
Atomic Orbitals
The numbers and kinds of atomic orbitals depend on the energy
sublevel.
Electron Configurations
The ways in which electrons are arranged in various orbitals
around the nuclei of atoms are called electron configurations.
• Sodium (Na): 1s22s22p63s1
There are three rules used for writing electron configurations.
• The Aufbau Principle
• The Pauli Exclusion Principle
• Hund’s Rule
The Aufbau Principle
The Aufbau principle states that electrons occupy the orbitals of
lowest energy first.
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p
The Pauli Exclusion Principle
The Pauli exclusion principle states that atomic orbitals may at
most describe two electrons. Those two electrons must have
opposite spins.
correct
incorrect
Hund’s Rule
Hund’s rule states that electrons spread out before they pair up
in orbitals of the same energy.
Electrons that are by themselves in an orbital are called unpaired
electrons.
Electron Configurations
The most stable electron configurations are those that are
completely filled:
• He: 1s2
• Ne: 1s22s22p6
• Ar: 1s22s22p63s23p6
• Kr: 1s22s22p63s23p64s23d104p6
• Xe: 1s22s22p63s23p64s23d104p65s24d105p6
• Rn: 1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p6
Each of the above elements are noble gases. The noble gases
are stable because they all have full principle energy levels.
Electron Configurations
Half-filled sublevels are not as stable as filled sublevels, but they
are more stable than other configurations. These are called
pseudo noble gas configurations.
Some elements deviate from their predicted electron
configuration to achieve pseudo noble gas configuration.
• Predicted: Chromium (Cr): 1s22s22p63s23p64s23d4
• Actual: Chromium (Cr): 1s22s22p63s23p64s13d5
• Predicted: Copper (Cu): 1s22s22p63s23p64s23d9
• Actual: Copper (Cu): 1s22s22p63s23p64s13d10
Ground State vs. Excited State
The lowest potential energy arrangement of electrons in an
atom is called the ground state.
• The ground state of carbon is: 1s22s22p2
An excited state electron configuration is any electron
configuration for an atom that contains the correct total number
of electrons but has a higher total electron potential energy
than the ground state electron configuration.
• One possible excited state of carbon is: 1s22s12p3