n - principle quantum number - determines distance from the nucleus and energy l - orbital angular momentum quantum number l = 0, 1, 2,… n-1 m l
- magnetic quantum number m l
= l, …. 0, …. +l (2l+1) values m s
- spin quantum number m s
= +1/2 or -1/2

Electron configuration - a list of all occupied orbitals of an atom, with the number of electrons that each contains
H 1s 1
The electronic structure of an atom determines its chemical properties
In many- electron atoms, because of shielding by other electrons and different degree of “penetration” of orbitals, electrons feel an “effective” nuclear charge.
Order of the energy of orbitals in a given shell is typically s < p < d < f.

Atom
Li
B
C
N
O
F
Z*(2s)
1.28
2.58
3.22
3.85
4.49
5.13
Z*(2p)
2.42
3.14
3.83
4.45
5.10
Note that value of Z* increases across the period
Pauli Exclusion Principle : No more than two electrons may occupy any given orbital. When two electrons occupy an orbital their spins must be paired.
No two electrons in an atom can have the same set of quantum numbers

Subshell energies of multi-electron atoms depend on both n and l
Electrons are assigned to subshells in order of increasing
“n+l” value
For two subshells with the same value of “n+l” electrons are assigned first to the subshell of lower n
Experimentally determined order of subshell energies

Building Up: fill orbitals starting with the lowest energy
( aufbau principle ), pairing electrons as determined by the
Pauli principle.

Using the “n+l” rule to which of the following subshells should an electron be assign first?
a) 4s or 4p b) 5d or 6s c) 4f or 5s

H - one electron, occupies the 1s orbital
Electron configuration: 1s 1 n = 1, l = 0, m l
= 0, m s
= +1/2 or -1/2
He - two electrons, both occupy the 1s orbital with paired spins (opposite spins)
Electron configuration: 1s 2 n = 1, l = 0, m l n = 1, l = 0, m l
= 0, m s
= 0, m s
= +1/2
= -1/2
He has a CLOSED shell; a shell containing the maximum number of electrons allowed by the exclusion principle

Li - three electrons. Two in 1s and one is 2s
Electron configuration: 1s 2 2s 1 n = 1, l = 0, m l n = 1, l = 0, m l n = 2, l = 0, m l
= 0, m s
= 0, m s
= 0, m s
= +1/2
= -1/2
= +1/2 or -1/2
Li has a “core” made up of an inner helium-like closed shell and an outer shell, valence shell , containing a higher energy electron.
Can denote Li electron configuration as [He] 2s 1
Electrons in the outer most shell are the valence electrons and are the ones involved in bonding and in chemical reactions.

If more than one orbital in a subshell is available, add electrons with parallel spins to different orbitals of that subshell rather than pairing electrons in one of the orbital -
Hund’s rule

Closed shell unpaired electrons - paramagnetic
All electrons paired: diamagnetic

In the presence of an external magnetic field, the electron aligns itself along the direction of the field or against the direction of the field - results in two spin states, +1/2 or -1/2
Atoms with unpaired electrons paramagnetic - attracted to a magnet
Atoms with paired electrons diamagnetic

An atom of any element can be considered to have a noble gas core surrounded by electrons in the valence shell , the outermost occupied shell.
The valence shell is the occupied shell with the largest n.
All the atoms in a given period have a common valence shell, and the principle quantum number of that shell is equal to the period number.
The valence shell of elements in Period 2 (Li to Ne) is the shell with n = 2
Atoms in the same period have the same type of core
Atoms in Period 2: have a helium-like core (1s 2 )
Period 3 atoms: Ne-like core (1s 2 2s 2 2p 6 )
TABLE

Atoms in the same group: have analogous valence shell configurations that differ in the value of n
Group I: valence configuration is ns 1
Group IV: valence configuration ns 2 np 2
TABLE

Use the position of element in the periodic table to determine electron configuration

Electron configuration of Ti
Determine position of Ti on the periodic table
Second member of the 3d block
Electron configuration: 1s 2 2s 2 2p 6 3s 2 3p 2 4s 2 3d 2 or [Ar] 3d 2 4s 2
Bi: [Xe] 4f 14 5d 10 6s 2 6p 3
TABLE

Excited states- when an atom absorbs energy an electron can occupy a higher energy orbital; energy absorbed must equal to the difference in energy between the excited and ground state
Ground state of C: [He] 2s 2 2p 2
Excited state of C: [He] 2s 1 2p 3
C: [He] 2s 1 2p 3
D
E
C: [He] 2s 1 2p 3

Elements in the same group have the same number of valence electrons and related electron configurations; hence have similar chemical properties.
The ground state electron configuration of the elements vary periodically with atomic number; all properties that depend on electron configuration tend to vary periodically with atomic number

The variation of effective nuclear charge through the periodic table plays an important role in determining periodic trends
Z eff increases from left to right across a period, but drops every time an outer electron occupies a new shell.