Chapter 3, Problem 3.72QP

Interpretation Introduction

Interpretation:

The four quantum numbers which is applied to characterize an electron in an atom should be described.

Concept Introduction:

The electron density gives the probability of finding an electron in a particular region in an atom.  An atomic orbital is the region of three-dimensional space, defined by ψ² (the square of the wave function, ψ), where the probability of finding an electron is high.  An atomic orbital can accommodate a maximum of two electrons.

A wave function for an electron in an atom is called an atomic orbital; this atomic orbital describes a region of space in which there is a high probability of finding the electron.  Energy changes within an atom are the result of an electron changing from a wave pattern with one energy to a wave pattern with a different energy (usually accompanied by the absorption or emission of a photon of light).

Each electron in an atom is described by four different quantum numbers. The first three (n, l, m_l) specify the particular orbital of interest, and the fourth (m_s) specifies how many electrons can occupy that orbital.

Principal Quantum Number (n)

The principal quantum number (n) assigns the size of the orbital and specifies the energy of an electron.  If the value of n is larger, then the average distance of an electron in the orbital from the nucleus will be greater.  Therefore the size of the orbital is large. The principal quantum numbers have the integral values of 1, 2, 3 and so forth and it corresponds to the quantum number in Bohr’s model of the hydrogen atom.  If all orbitals have the same value of ‘n’, they are said to be in the same shell (level).  The total number of orbitals for a given n value is n².  As the value of ‘n’ increases, the energy of the electron also increases.

Angular Momentum Quantum Number (l)

The angular momentum quantum number (l) explains the shape of the atomic orbital.  The values of l are integers which depend on the value of the principal quantum number, n.  For a given value of n, the possible values of l range are from 0 to n − 1.  If n = 1, there is only one possible value of l (l=0).  If n = 2, there are two values of l: 0 and 1.  If n = 3, there are three values of l: 0, 1, and 2. The value of l is selected by the letters s, p, d, and f.  If l = 0, we have an s orbital; if l = 1, we have a p orbital; if l = 2, we have a d orbital and finally if l = 3, we have a f orbital.  A collection of orbitals with the same value of n is called a shell.  One or more orbitals with the same n and l values are referred to a subshell (sublevel).  The value of l also has a slight effect on the energy of the subshell; the energy of the subshell increases with l (s < p < d < f).

Magnetic Quantum Number (m_l)

The magnetic quantum number (m_l ) explains the orientation of the orbital in space.  The value of m_l depends on the value of l in a subshell.  This number divides the subshell into individual orbitals which hold the electrons.  For a certain value of l, there are (2l + 1) integral values of m_l which is explained as follows:

m_l  = ‒ l, ..., 0, ..., +l

If l = 0, there is only one possible value of m_l: 0.

If l = 1, then there are three values of m_l: −1, 0, and +1.

If l = 2, there are five values of m_l, namely, −2, −1, 0, +1, and +2.

If l = 3, there are seven values of m_l, namely, −3, −2, −1, 0, +1, +2, and +3, and so on.

The number of m_l values indicates the number of orbitals in a subshell with a particular l value.  Therefore, each m_l value refers to a different orbital.

Electron Spin Quantum Number (m_s)

It specifies the orientation of the spin axis of an electron.  An electron can spin in only one of two directions.  There are two possible ways to represent m_s values. They are +½ and ‒½.  One electron spins in the clockwise direction.  Another electron spins in the anticlockwise direction.  But, no two electrons should have the same spin quantum number.