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The nature
of the atom
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A model of the atom
The atom contains a very dense nucleus, formed by protons
and neutrons, surrounded by moving electrons.
Quantum mechanics and the
atomic orbital model are used
to describe the behavior of
electrons in atoms.
p+
n
e–
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The uncertainty principle
According to the uncertainty principle, it is impossible to
precisely measure the simultaneous position and speed of a
small particle like an electron.
In fact, it is impossible to study the electron without giving
energy, thus altering its speed or its position.
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Energy levels
Electrons can have only specific energy states, called energy
levels. They can never have intermediate energy states
between two levels.
Excited states
Ground state
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The atomic orbital model /1
The atomic orbital represents a possible energy state of the
electron and refers to the physical region where there is a 90%
probability of locating the electron.
Each atomic orbital is characterized by three quantum
numbers.
The principal quantum number (n) defines the energy level of
the orbital. It can only be an integer, positive number: 1, 2, 3…
All electrons with the same value of n form the energy level.
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The atomic orbital model /2
The secondary quantum number (l) describes the shape of
the atomic orbital and its energy sublevel. It can only be an
integer, positive number, from 0 to (n–1).
Value of l
Letter
Shape
0
s
sphere
1
p
dumbbell
shape
d
double
dumbbell
shape
2
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The atomic orbital model /3
The magnetic quantum number (m) describes the different
spacial orientations of orbitals that have the same l value.
It can be an integer number from –l to +l (including 0).
Value of n
Value of l
Value of m
Sublevel
Number of
orbitals
1
0
0
1s
1
2
0
1
0
–1, 0, 1
2s
2p
1
3
3
0
1
2
0
3s
–1, 0, 1
3p
–2, –1, 0, 1, 2 3 d
1
3
5
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The orbital energy diagram
In this diagram, orbitals are displaced according to an
increase in energy.
4p
Energy
4s
3d
3p
3s
2p
2s
1s
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The spin quantum number
Each electron is associated with a a spin quantum number
that can have a value of +½ and –½.
According to the Pauli exclusion principle, it is impossible to
have electrons with the same quantum numbers in an atom.
Each orbital can therefore have a maximum of two electrons
with antiparallel spin.
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Electron configurations
In order to write the electron configuration of an atom,
electrons are displaced in the orbital energy diagram starting
from the orbital with lower energy.
C
1s2 2s2 2p2
2p
2s
1s
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Valence electrons
The electrons in the outermost orbital are called valence
electrons. All the other electrons represent the core.
valence electron
The distribution of electrons in the
most external orbital is called the
external electronic configuration.
The elements that belong to a
chemical group have the same
external electronic configuration.
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The size of atoms
The atomic radius is the average distance between a
nucleus and the outermost electrons. It is half the distance
between two equal atoms linked together.
2r
In the periodic table, the atomic
radius usually decreases in periods
from left to right and increases in
groups from top to bottom.
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Ionization energy
The ionization energy is the energy required to remove an
electron from a neutral gaseous atom.
It is expressed in kJ/mol.
In the periodic table, these values increase along each period
and decrease along each group.
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The electron affinity
The electron affinity is the energy that is emitted or absorbed
when an electron is added to a neutral, gaseous atom, that
becomes a negative ion.
As the same as ionization energy, in the periodic table these
values increase along each period and decrease along each
group.
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From atoms to ions
The electron configuration of atoms of noble gases is
particularly stable, making them inert with respect to chemical
reactivity.
Metals easily form cations with the electron configuration of
the nobel gas preceding them in the periodic table.
Non-metals easily form anions with the electron configuration
of the noble gas proceding them in the periodic table.
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