Spectra

Embedded in an atom which consists of a nucleus that is positively charged and which revolves round the orbit is an electron which is negatively charged.

When the atom or metal is heated and it absorbs energy to a particular intensity, the electrons starts to collide with one another and with the electron shell until the electrons are able to liberate themselves from the orbit. They create an atom with high energy level which teams up with the low energy level. When this occurs, radiation is emitted. Some of the particles and energies in the radiation are equal to the difference in energy of the atom between initial and final stage.

E = E₀ – E₁

The study of spectra is referred to as spectroscopy.

Types of Spectra

(i) Emission spectra:

These are spectra observed due to light emitted when the temperature of an atom, gas or metal is raised, e.g. incandescent solid or vapour , a spark discharge, arc discharge, the discharge of electricity through a gas or vapour contained in a discharged tube, etc.

(ii) Line Spectra

Line spectrum is obtained from atoms in gases such as hydrogen or neon at low pressure in a discharge tube. This spectrum mainly occurs in sodium, mercury vapours and hydrogen gas. When a light is incidented on a vapour-like mercury or hydrogen, a particular temperature is reached when or vapour starts to emit light of different wavelengths which depends on the nature of gas or metal. The emission can be viewed with the aid of a spectrometer in a slit. Differences are observed to be discontinuous but not overlapping, with equal distance between them, i.e. they are regularly spaced but do not form a series.

Line spectra occur when there is transmission of electron from one region to another usually from higher to lower energy level or vice-versa. Line spectra observed can be represented by:

∆E = hf = E_n – E_o

Where hf is the change of energy level (eV)

E_n = higher energy level or excitation energy level (eV)

E­_o = lower energy level or ground state (eV)

Where h Planck’s constant with value at 6.6 x 10^-34J

(iii) Continuous Spectrum

The spectrum formed from white light contains all colors, or frequencies, and is known as a continuous spectrum. Continuous spectra are produced by all incandescent solids and liquids and by gases under high pressure. A gas under low pressure does not produce a continuous spectrum but instead produces a line spectrum, i.e., one composed of individual lines at specific frequencies characteristic of the gas, rather than a continuous band of all frequencies. If the gas is made incandescent by heat or an electric discharge, the resulting spectrum is a bright-line, or emission, spectrum, consisting of a series of bright lines against a dark background. A dark-line, or absorption, spectrum is the reverse of a bright-line spectrum; it is produced when white light containing all frequencies passes through a gas not hot enough to be incandescent. It consists of a series of dark lines superimposed on a continuous spectrum, each line corresponding to a frequency where a bright line would appear if the gas were incandescent. The Fraunhofer lines appearing in the spectrum of the sun are an example of a dark-line spectrum; they are caused by the absorption of certain frequencies of light by the cooler, outer layers of the solar atmosphere. Line spectra of either type are useful in chemical analysis, since they reveal the presence of particular elements.

(iv) Band Spectrum

It consists of a number of bright bands with a sharp edge at one end but fading out at the other end. Band spectra are obtained from molecules. It is the characteristic of the molecule. Calcium or barium salts in a bunsen flame and gases like carbon-di-oxide, ammonia, and nitrogen in molecular state in the discharge tube give band spectra. When the bands are examined with high resolving power spectrometer, each band is found to be made of a large number of fine lines, very close to each other at the sharp edge but shaped out at the other end. Using band spectra the molecular structure of the substance can be studied.

(v) Absorption Spectrum

When a cool gas is placed in the path of a continuous spectrum of light, dark absorption lines will appear in the resulting spectrum. Conversely, if we observe the gas in an oblique angle, we can see emission lines produced by the gas too. The most interesting thing about the spectrum of an element is that, the wavelengths of absorption lines and emission lines produced by an element match exactly each other. This is the one of the evidences for electrons in an atom are situated in some kind of resonant columns.

When a photon falls on an electron and the resonant frequency of the shell (electron shell or transitory shell) in which the electron exists matches with the frequency of the photon, the electron will oscillate with the frequency of the photon and the photon will be efficiently absorbed by the atom. In the resultant spectrum, the absorbed photon will not be present. This is the way an atom produces its absorption lines.

When an electron oscillates, it will emit a radiation with the frequency of its oscillation. Therefore an atom produces emission lines with exactly matching wavelengths of the absorption lines that produced by the atom.

Questions

1. Calculate the energy in joules of ultraviolet light of wavelength 3 x 10⁷ m. Take the velocity of light as 3 x 10⁸m/s and Planck’s constant as 6.6 x 10^-34 Js
2. 6.6 x 10^-19 B. 9.8 x 10^-2 C. 3 x 10^-8 D. 4.5 x 10⁸.
3. An electron jumps from one energy level to another in an atom radiating 4.5 x 10^-19Joules. If Planck’s constant is 6.6 x 10³⁴ Js, what is the wavelength of the radiation? Take velocity of light = (3 x 10⁸m/s).
4. 3.4 x 10^-7m B. 4.4 x 10^-7 C. 5.9 x 10^-8 D. 2.5 x 10^-6
5. If an electron in the ground state were to absorb a photon with energy greater than 10 eV, what would happen?
6. the electron will stick to the atom B. the electron would be ejected from the atom C. the electron wil be neutralised   D. there will be increase in the electron in the atom.
7. Which of the spectra Line spectra occur when there is transmission of electron from one region to another usually from higher to lower energy level or vice-versa?
8. Line spectra B. Continuous spectra C. Absorption spectra D. Emission spectra
9. Which of these is not correct?
10. Thus Bohr suggests that electrons in the atom exist in discrete (or quantized) energy states.
11. The discreteness of atomic energy levels was first shown directly in the Franck-Hertz experiment.
12. Franck-Hertz experiment is one of the classic demonstrations of the quantization of atomic energy levels
13. To increase the energy of the electron, to move the electron into a higher energy level—energy of the electron must be stable.