Transcript Quantum Mechanical Model

Electrons in Atoms
Up until now, the model of the atom presented
considered atoms as combinations of protons &
neutrons that make-up the nucleus, which is
surrounded by electrons.
This model does not explain enough about their
properties
The Development of Atomic
Models
Dalton’s Atomic Theory:
Atom is indivisible
Thomson: Discovered existence of electrons.
Plum Pudding Model: Solid sphere w/+ charge with
negatively charged electrons embedded.
Rutherford: Discovered nucleus with gold foil
experiment. Atom was made of dense nucleus with +
charge. Nucleus was surrounded by empty space and
electrons.
PROBLEM: The trouble with Rutherford’s model is
that opposites attract. Why didn’t the electrons
collapse into the nucleus?
Bohr: Proposed that electrons were arranged in
concentric paths (orbits) around the nucleus.
Electrons had a fixed energy & do not lose energy
and fall into nucleus. The energy level of the
electron is the region in which the electron is
found.
12.2 The Quantum Mechanical Model
Schrodinger (1926): wrote a mathematical equation to describe an atom
using all the information available to date. This modern theory called the
Quantum Mechanical Theory describes the positions of the electrons
mathematically.
The theory is based on the probability of finding an electron in a certain
area.
If the area around the nucleus is described as a cloud, then the areas
with highest probability have a denser cloud and the lowest less fuzzy.
Energy levels are like rungs on a ladder.
Lowest rung has lowest energy/Highest rung has
highest energy.
As you climb each rung of the ladder the energy
goes up.
A person can’t stand between rungs/neither can an
electron.
To move from level to level, an electron must gain or
release just the right amount of energy.
Quantum: The energy required to move an electron
from its present energy level to a higher one.
The amount of energy gained or lost is not always the
same.
The energy levels are not evenly spaced like the
rungs in a ladder.
The levels are closer together at the top.
n-1
n-2
12.3 Atomic Orbitals
The Quantum Mechanical Model restricts the electrons to certain energy
levels as does Bohr, but designates electrons by principle quantum
numbers. (n)
Each principle quantum number refers to a principle energy level in the
atom.
You may want to find electrons within the sublevels..but remember:
The Quantum Theory describes “probability clouds”; these are called
atomic orbitals.
Atomic Orbitals are designated by letters. (Draw models) (w/periodic
table)
Principle Energy
Level
Number of
Sublevels
Type of Sublevel
Orbitals
Maximum # of
Electrons in level
N=1
1
1s (1 orbital)
2
N=2
2
2s (1 orbital)
2p (3 orbitals)
8
N=3
3
3s), 3p, 3d (5 orbitals)
18
N=4
4
4s,4p,4d, 4f (7 orbitals)
32
12.4 Electron Configurations
In all natural phenomena, change proceeds toward the lowest
possible energy state. High energy systems are unstable.
In atoms, the electrons and nucleus interact to make the most stable
arrangement possible.
The way in which they (electrons) are arranged around the nuclei of
atoms is called electron configuration.
We use three rules to find electron configuration.
The Aufbau Principle
The Pauli Exclusion
Hund’s Rule
Aufbau Principle
Electrons enter the lower energy levels first.
All 3 p orbitals and all 5 p orbitals have same energy levels
p is not equal to d
7s
7p
7d
7f
6s
6p
6d
6f
5s
5p
5d
5f
4s
4p
4d
4f
3s
3p
3d
2s
2p
1s
Pauli Exclusion Principle
An atomic orbital may have only 2 electrons and they
must have opposite spins. Clockwise and
Counterclockwise. ↑↓
Hund’s Rule
When electrons occupy orbitals of equal energy, like 3p5d-7f; One electron enters each orbital until all orbitals
contain 1 electron of parallel spins, Then additional
electrons are added.