ELECTRONS IN THE ATOM
ELECTRON CONFIGURATIONS
“THE ADDRESSES OF ELEMENTS”
EQ’s
• What do the chemical properties of atoms
depend on?
• What is the quantum mechanical model?
• How is the quantum mechanical model
organized?
• What is the Aufbau Principle?
ELECTRONS:
The chemical properties of atoms,
ions,
and molecules
are related to the arrangement of electrons.
EVOLUTION OF THE ATOM
To understand this concept, let’s take a look
at the history of the atomic models.
Dalton – solid indivisible mass
EVOLUTION OF THE ATOM
Thomson - The atom is a ball of positive
charge with electrons stuck into the ball.
Electron
Positive
charge
EVOLUTION OF THE ATOM
Rutherford – Most of an atom’s mass is
concentrated in the small, positively
charged nucleus. The electrons surround
the nucleus and the rest of the atom is
empty space.
EVOLUTION OF THE ATOM
Bohr – Electrons are arranged in concentric
circular paths around the nucleus.
EVOLUTION OF THE ATOM
Quantum Mechanical Model – Modern
atomic theory describes the electronic
structure of the atom as the probability of
finding electrons within certain regions
of space.
90% probability of finding
the electron within this
space
WHERE ARE THE ELECTRONS?
The quantum mechanical model is a theoretical
mathematical approach to the study of atomic
and molecular structure – a very complex
theory! So let’s not go there. Instead we will
learn some of the basic concepts using a visual
that we can all relate to: an apartment building.
Vs.
ELECTRON APARTMENTS
The apartment building has different floors
[principal energy level], different
apartments on each floor [sublevel], and
rooms [orbitals] within each apartment.
PRINCIPAL ENERGY LEVELS
There are seven “floors” in our building.
Each of these “floors” is assigned a
number. These are called the principal
quantum numbers (n). Principal energy
levels are assigned values in order of
increasing energy: n = 1, 2, 3, 4, and so
forth.
PRINCIPAL ENERGY LEVELS
Take a look at your periodic table. How many
periods are there?
Hummmm – do you think there may be a
connection here?
Seven levels, seven periods!
So floor 2 (n = 2)
would be the
second period;
Floor 5 (n = 5)
would be the 5th
period
SUBLEVELS
The “apartments” (sometimes called blocks or
shells) within our Principal Energy Levels are
identified with a letter: s, p, d or f.
Principal energy level n = 1 is a bit
strange, because the sublevel s is
split into two areas.
If you are looking
for apartment 6s, it
would be found
here
Apartment 4p,
would be found
ORBITALS
Each sublevel (apartment) contains a very
specific number of rooms (orbitals):
s – blocks contain 1 orbital
p – blocks contain 3 orbitals
d – blocks contain 5 orbitals
f – blocks contain 7 orbitals
Each orbital can contain a maximum of 2
electrons.
Orbitals are difficult to show until we learn a
couple of principles and one rule.
A good way to remember the number of orbitals
is to count (horizontally) the number of elements
in a block and divide that by 2.
s-block; 2 elements = 1 orbital
p-block; 6 elements = 3 orbitals
d-block; 10 elements = 5 orbitals
f-block; 14 elements = 7 orbitals
ELECTRON CONFIGURATION
The electron configuration actually gives us the
location of any element on the periodic table. We
simply have to be able to count as we fill in
boxes!
The way we read the configuration is to account
for every electron in the atom – time to
remember that as elements progress across the
periodic table, the number of protons and
electrons increase by one.
A little practice is all it takes.
Hydrogen: s1
Helium:1s2
Chlorine:
1s22s22p63s23p5
Iron:
1s22s22p63s23p63d6
PAULI EXCLUSION PRINCIPLE
An atomic orbital may describe at most
two electrons.
To occupy the same orbital, two electrons
must have opposite spins.
Arrows are used to indicate the electron and
its direction of spin (↑ or ↓).
An orbital containing paired electrons is
written as ↑ ↓
HUND’S RULE
When electrons occupy orbitals of equal energy,
one electron enters each orbital until all the
orbitals contain one electron with parallel spins.
p orbitals
When the 4th electron
is needed, it will
occupy the first
orbital and so on - - -
AUFBAU PRINCIPLE
Electrons enter orbitals of lowest energy first
7p
6d
5f
7s
6p
5d
Increasing
Energy
4f
6s
5p
4d
5s
4p
3d
4s
3p
3s
2p
2s
1s
Hydrogen (H):
Helium (He):
Beryllium (Be):
Aluminum (Al):
Sulfur (S):