Summary General Chemistry Notes on Quantum Mechanical Model and Quantum Numbers, Subshells and electron capacity
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General Chemistry: Quantum Mechanical Model
and Quantum Numbers
The Quantum Mechanical Model of Atoms represents a significant evolution from Bohr’s model, which was limited
in its ability to explain the behavior of multi-electron atoms. Bohr’s model proposed that electrons travel in fixed
circular orbits around the nucleus at specific distances. However, this approach did not account for the repulsion
between multiple electrons, leading to inaccuracies in predicting atomic structure and behavior.
In contrast, modern quantum mechanics provides a more comprehensive framework for understanding atomic
structure. It recognizes that electrons do not follow defined paths but instead exist within probabilistic regions
known as orbitals. These orbitals represent areas where there is a high likelihood of finding an electron at any given
time, reflecting the complex nature of electron movement and interactions.
The key distinction between Bohr’s model and the quantum mechanical model lies in this understanding of electron
behavior: while Bohr envisioned fixed orbits, quantum mechanics describes a dynamic and probabilistic distribution
of electrons around the nucleus.
Historical Context of Electron Location In the early 20th century, scientists, particularly those following Niels
Bohr’s model, held a strong belief that they could accurately determine the location and pathway of electrons
around an atomic nucleus. This perspective has evolved significantly with advancements in quantum mechanics.
Current Understanding of Electron Positioning The modern quantum mechanical model posits that instead of
pinpointing an electron’s exact location at any moment, we can only describe the probability of finding an electron
within a specific region surrounding the nucleus. This shift reflects a more nuanced understanding of atomic
behavior.
Heisenberg Uncertainty Principle A fundamental concept in quantum mechanics is encapsulated in the Heisenberg
Uncertainty Principle. This principle states that it is impossible to simultaneously know both the momentum and
position of an electron with perfect accuracy. If one attempts to measure the position of an electron, it must be
brought to a stop, thus eliminating its momentum. Conversely, if one measures its momentum while it is in motion,
its position becomes uncertain.
This principle highlights a crucial limitation in our ability to observe subatomic particles: knowing one property
(position or momentum) inherently leads to uncertainty in the other.
Visual Representation The Heisenberg Uncertainty Principle can be illustrated through two scenarios:
1. When momentum is known, position remains uncertain.
2. When position is known, momentum becomes uncertain.
In mathematical terms, this relationship can be expressed as:
Δx⋅Δp≥h/4 pi where Δx represents the uncertainty in position and Δp represents the uncertainty in
momentum.
In summary, our understanding of electrons has transitioned from a deterministic view to one characterized by
probabilities and inherent uncertainties as described by quantum mechanics.
, Modem Atomic Theory Overview
Modem atomic theory asserts that each electron in an atom can be described by four quantum numbers: n (principal
quantum number), l (azimuthal quantum number), m_l (magnetic quantum number), and m_s (spin quantum
number). The Pauli exclusion principle states that no two electrons in the same atom can have identical sets of these
four quantum numbers. The combination of these quantum numbers defines the energy state of an electron, which
includes its position and energy.
Quantum Number Relationships
The principal quantum number (n) determines the overall size and energy level of the orbital. It also restricts the
possible values for the azimuthal quantum number (l), which describes the shape of the orbital. For each value of n,
there are specific permissible values for l, ranging from 0 to (n-1). Similarly, for a given value of l, only certain values
for the magnetic quantum number (m_l) are allowed, which range from -l to +l. This hierarchical structure means
that as you define one quantum number, it constrains the values available for subsequent numbers.
Qualitative Information from Quantum Numbers
The four quantum numbers provide qualitative insights into various properties of orbitals:
Size: Determined primarily by n.
Shape: Defined by l.
Orientation: Specified by m_l.
Students often find understanding l and m_l particularly challenging due to their interdependent nature and how
they relate to the spatial characteristics of orbitals.
In summary, understanding these four quantum numbers is crucial for grasping atomic structure and electron
configurations in modern atomic theory.
Principal Quantum Number (n)
In Bohr’s model of the atom, the principal quantum number (n) is a key concept that can take on any positive
integer value. The significance of this number lies in its correlation with the energy levels and radii of an electron’s
shell. As n increases, both the energy level and the radius of the electron shell increase.
The maximum capacity for electrons within a given shell is determined by the formula:
Maximum number of electrons within a shell = 2n² (Equation 1.6)
Here, n represents the principal quantum number.
Energy Differences Between Shells
The difference in energy between two shells diminishes as the distance from the nucleus increases. This relationship
can be expressed mathematically as a function of 1/n². For instance, the energy difference between shells n = 3 and
n = 4 is less than that between shells n = 1 and n = 2. This trend illustrates how energy levels become closer
together at higher values of n.
Electron Motion Representation
It is important to note that while Bohr’s model simplifies electron motion by depicting it as traveling in defined
orbits, this representation does not accurately reflect reality. Electrons do not follow precise paths; instead, their
behavior is better described by quantum mechanics.
Definition and Designation
The second quantum number, known as the azimuthal (or angular momentum) quantum number, is represented by
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