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```ELECTRON CONFIGURATIONS
I. QUANTUM MECHANICAL MODEL OF THE ATOM
 Modern description of the electrons in atoms, derived from the mathematical solution to
the Schrodinger equation.
II. ENERGY LEVEL
 The region around the nucleus where an electron is likely to be moving.
III. QUANTUM OF ENERGY
 The amount of energy required to move an electron from its present energy level to the
next higher one.
IV. PRINCIPAL QUANTUM ENERGY LEVELS
 There are 7 principal quantum energy level (1 – 7) right now corresponding to the period
numbers on the left of the periodic table.
V. ATOMIC ORBITALS
 A region in space around the nucleus where there is a high probability of finding an
electron.
 s-orbitals

1 orbital—spherical-shaped clouds
 p-orbitals
 3 orbitals—dumbbell-shaped/peanut-shaped clouds
 d & f-orbitals
 far more complex clouds; 5 and 7 orbitals respectively
Each orbital can contain maximum of 2 electrons per orbital.
ex. 3p sublevel contains 3 orbitals and up to 6 electrons
VI. ELECTRON CONFIGURATIONS
 the ways in which electrons are arranged around the nuclei of atoms
Three rules govern the filling of atomic orbitals
1. THE AUFBAU PRINCIPLE
 Electrons enter orbitals of lowest energy first. Within a principal quantum energy level
the s-sublevel is always the lowest energy level. Yet the range of energy levels within a
principal energy level can overlap the energy levels of an adjacent principal energy
level
Example: 4s orbital is lower in energy than the 3d
Two exceptions to the Aufbau Principle
Cr
1s2 2s2 2p6 3s2 3p6 3d5 4s1
Cu
1s2 2s2 2p6 3s2 3p6 3d104s1
2. THE PAULI EXCLUSION PRINCIPLE
 An atomic orbital may describe at most two electrons. To occupy the same orbital, two
electrons must have opposite spins; that is, the electrons must be paired.
3. HUND’S RULE
 When electrons occupy orbitals of equal energy, one electron enters each orbital until
all the orbitals contain one electron with spins parallel.
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