Ch 4 Section 3 Flashcards
The arrangement of electrons in an atom is known as the atoms
Electron configuration.
Because atoms of different elements have different numbers of electrons a unique
Electron configuration exists for the atoms of each element
Like all systems in nature electrons in atoms tend to
Assume arrangements that have the lowest possible energies
The lowest energy arrangement of the electrons for each element is called they
Elements ground state electron configuration
To build up electron configurations for the ground state of any particular atom
First the energy levels of the orbitals are determined
Then electrons are added to orbitals one by one according to three basic rules
According to the aufbau principle an electron occupied the
Lowest energy orbital that can receive it
Shows the order in which electrons occupy orbitals
Beginning with the third main energy level n = 3, the energies of sublevels in different main energy levels
Begin to overlap
The 4s sublevel is lower in energy to the
3d sublevel
4s sublevel is filled before an electrons
Enter the 3d orbitals
Less energy is required for two electrons to pair up in the 4s orbital than for
Those two electrons to occupy a 3d orbital
The second rule reflects
Importance of spin quantum number
According to the Pauli exclusion principle no two electrons in the same atom can
Have the same set of four quantum numbers
The two values of he spin quantum number reflect the fact that for
2 electrons to occupy the same orbital they must have opposite spin states
Third rule requires placing as many
Unpaired electrons as possible in separate orbitals in the same sublevel
In this way electron electron repulsion is
Minimized so that the electron arrangements have the lowest energy possible
According to hunds rule orbitals of equal energy are each occupied by one electron before any orbital he
Occupied by a second electron and all electrons in singly occupied orbitals must have the same spin state
Applying hunds rule shows that one electron will enter each of the three
P orbitals in a main energy level before a second electron entered any of them
3 methods or notations are used to indicate
Electron configurations
Third notation is mostly used with elements of the third period and higher
(Orbital notation) an unoccupied orbital is represented by a
Line with the orbitals name written underneath the line
(Orbital notation) the lines are labeled with the
Principal quantum number and sublevel letter.
Ex: orbital notation for helium
He ⬆️⬇️
-------
1sElectron configuration notation eliminated the
Lines and arrows of orbital notation
(Electron configuration notation) instead the number of k
Electrons in a sublevel is shown by
Adding a superscript to the sublevel designation
In the first period elements (hydrogen and helium) electrons occupy
The orbital of the first main energy level
According to the aufbau principle after the 1s orbital is filled the next electron occupies the
S sublevel in the second main energy level
The highest occupied energy level is the electron containing main energy level with the
Highest principal quantum number
Inner she’ll electrons are electrons that are not in the
Highest occupied energy level
According to the aufbau principle the next electron must pair with another electron in one of the
2p orbitals as long as the spins of the paired electrons are opposite
Atoms such as those of neon which have the s and p sublevels of their highest occupied level filled with eight electrons are said to
Have an octet of electrons
After the outer octet is filled in neon the next electron enters the
S sublevel in the n= 3 main energy level
The first 10 electrons in an atom of each of the third period elements have the
Same configuration as neon which allows us to use a shorthand notation for the configurations
(Noble gas notation) the group 18 elements are called
Noble gases
(Noble gas notation) to simplify notation of third period elements the symbol for neon is
Enclosed j square brackets and the rest of the configuration follows
A noble gas configuration refers to an outer main energy level occupied
In most cases by eight electrons
The fourth period begins by filling the
4s orbital, the empty orbital of lowest energy
With the 4s sublevel filled the
4p and 3d sublevels sre the next available vacant orbitals
The 3d sublevel is lower in energy than the
4p sublevel. Therefore the five 3d orbitals are next to be filled
A total of 10 electrons can occupy the
3d orbitals
Fourth period element configurations begin with
[Ar]
According to hunds rule one electron is added to each of the three
4p orbitals before electrons are paired in any 4p orbital
In the 18 elements of the fifth period sublevels fill in a similar manner as in
Elements of the fourth period but start at the 5s orbital instead of the 4s
Successive electrons are added to the first to the 5s orbital then to the
4d orbitals and finally to the 5p orbitals
The sixth period consists of
32 elements
To build up electron configurations for the sixth period electrons are added first to the
6s orbital in cesium and barium. Then jn lanthanum an electron is added to the 5d orbital
With cerium 4f orbitals
Begin to fill
Next 5d orbitals are filled and the period is completed by filling the
6p orbitals
Because the 4f and 5d orbitals are very close in energy number our deviations from the simple rules
Occur as these orbitals are filled
The seventh period is
Incomplete and consists largely of synthetic elements
The quantum model of the atom improves on the Bohr model because it
Describes the arrangements of electrons in atoms other than hydrogen
