Medicine: Chemistry Electronic Structure:
Absorption Spectra

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### Electronic Structure

ELECTRONIC STRUCTURE

Matter is composed of different elements, molecules and compounds which contains millions of atoms. The different arrangements of these atoms thus provides different materials with different electrical properties.

An atom consists of a tiny heavy nucleus which contains protons and neutrons and a vast region of outer space where

electrons are located.

Figure 15 Diagrammatic representation of an atom

The s, p, d, f are the names given to the orbitals that holds the electrons. These orbitals have different shapes and possesses particular amounts of energy (quantized)

Electronic structure is therefore the location and the state of motion of electrons in an electric field created by a stationary nuclei.

Some principles provides guidelines on how electron structures are built up these are;

Aufbau’s principle; Electrons first go in to orbitals with the lowest energy.

Figure 16: The filling up of electrons following Aufbau’s principle

1.    Pauli’s principle; an orbital can contain a maximum of 2 electrons in order words, 2 electrons in the same orbital cannot have the same spin quantum number.
2.    Hund’s rule; Electrons will fill a set of orbitals by keeping their spins parallel.

Figure 17 Antiparallel electrons within atomic orbitals

ABSORPTION AND EMISSION SPECTRA

When atoms gain energy, they become excited and move to higher energy levels. A line spectrum corresponding to different wavelengths and frequencies is called an absorption spectrum. These excited electrons are very unstable so in order to gain stability the electrons falls back to their lower or initial energy level emitting radiations. The line spectrum corresponding to the different wavelength and frequencies as the excited electron returns to its ground state is called an emission spectrum.

Figure 18 Absorption and emission spectra

The energy difference between the energy levels as the electrons move is calculated as follows.

ΔE=$E_1-E_2$

where   ΔE=Energy difference

$E_1$ =initial energy

$E_2$ =final energy

ΔE hf         h= Planck’s constant

h=6.626$\times&space;10^-^3^4$

The quantized energy of an energy level is given by

$E_n=\frac{-k}{n^2}$

There are primarily 5 different types of emission spectra lines. These are the Lyman, Balmer, Paschen, Bracket and Pfund.

The Lyman series of spectral lines run from higher energy levels to energy level 1($n_1$) , the Balmer series runs from higher energy levels to energy level 2 ($n_2$ ), the Paschen series runs from higher energy levels to energy level 3 ($n_3$) , the Bracket series runs from higher energy levels to energy level 4 ($n_4$)  and the Pfund series runs from higher energy levels to energy level 5 ($n_5$) .

 Figure 19 Series of Spectral lines

QUANTUM NUMBERS

A quantum number is very important since it specifies the shape of an atomic orbital and strongly influences chemical bonds and bond angles.

There are four types of quantum numbers which are; the principal quantum number, the azimuthal quantum number (angular momentum quantum number), the magnetic quantum number and the electron spin quantum number.

MAGNETIC QUANTUM NUMBER (m): This tells us the magnetic and electric properties of an atom.

ELECTRON SPIN QUANTUM NUMBER ($m_s$) :This tells us the direction of the magnetic fields of the electrons. An electron has either a spin up or spin down from Hund’s rule. The values assigned are $\frac{1}{2}$ and $-\frac{1}{2}$  for a spin up and spin down respectively.

Figure 20

v  PRINCIPAL QUANTUM NUMBER (n): It governs the energy and stability of an atom. It also tells us how many degenerate orbitals they are at a particular energy level.

The number of orbitals at a particular energy level is given by ($n^2$) where n is the energy level.

Also, the maximum number of electrons is gotten by $2n^2$   where n still remains the energy level.

v  AZIMUTHAL QUANTUM NUMBER (l): It is also called the angular momentum quantum number and depends on the principal quantum number (n). It tells us the type and the shape of the orbitals and also determines the angular momentum of the orbitals. It runs from 0 to n-1.

l =n-1

When n=1 (K shell), l=0

When n=2 (L shell), l=1

When n=3 (N shell), l=2

 Figure 21

CONCLUSION:

Atomic and electronic structure deals with the study of the atom. Atoms form the building block of matter. An atom as we already know consists of a well defined nucleus which is very massive and accounts for the mass of the atom. Around the nucleus are found orbits where electrons are located. These electrons account for the chemical reactivity of an element.

Hence the study of atomic and electronic structure is indispensable in the mastery of chemistry.