Why are there sets of lines in an emission spectrum

The speed of atoms in a gas depends on the temperature. When the temperature is higher, so are the speed and energy of the collisions. The hotter the gas, therefore, the more likely that electrons will occupy the outermost orbits, which correspond to the highest energy levels. This means that the level where electrons start their upward jumps in a gas can serve as an indicator of how hot that gas is. In this way, the absorption lines in a spectrum give astronomers information about the temperature of the regions where the lines originate.

We have described how certain discrete amounts of energy can be absorbed by an atom, raising it to an excited state and moving one of its electrons farther from its nucleus. If enough energy is absorbed, the electron can be completely removed from the atom—this is called ionization. The atom is then said to be ionized. The minimum amount of energy required to remove one electron from an atom in its ground state is called its ionization energy. Still-greater amounts of energy must be absorbed by the now-ionized atom called an ion to remove an additional electron deeper in the structure of the atom.

Successively greater energies are needed to remove the third, fourth, fifth—and so on—electrons from the atom. If enough energy is available, an atom can become completely ionized, losing all of its electrons.

A hydrogen atom, having only one electron to lose, can be ionized only once; a helium atom can be ionized twice; and an oxygen atom up to eight times. When we examine regions of the cosmos where there is a great deal of energetic radiation, such as the neighborhoods where hot young stars have recently formed, we see a lot of ionization going on. An atom that has become positively ionized has lost a negative charge—the missing electron—and thus is left with a net positive charge.

It therefore exerts a strong attraction on any free electron. Eventually, one or more electrons will be captured and the atom will become neutral or ionized to one less degree again. During the electron-capture process, the atom emits one or more photons. Which photons are emitted depends on whether the electron is captured at once to the lowest energy level of the atom or stops at one or more intermediate levels on its way to the lowest available level.

Just as the excitation of an atom can result from a collision with another atom, ion, or electron collisions with electrons are usually most important , so can ionization.

The rate at which such collisional ionizations occur depends on the speeds of the atoms and hence on the temperature of the gas—the hotter the gas, the more of its atoms will be ionized. The rate at which ions and electrons recombine also depends on their relative speeds—that is, on the temperature. In addition, it depends on the density of the gas: the higher the density, the greater the chance for recapture, because the different kinds of particles are crowded more closely together. From a knowledge of the temperature and density of a gas, it is possible to calculate the fraction of atoms that have been ionized once, twice, and so on.

It is the strongest atomic emission line from the sun and drives the chemistry of the upper atmosphere of all the planets, producing ions by stripping electrons from atoms and molecules. It is completely absorbed by oxygen in the upper stratosphere, dissociating O 2 molecules to O atoms which react with other O 2 molecules to form stratospheric ozone.

B This wavelength is in the ultraviolet region of the spectrum. Calculate the wavelength of the second line in the Pfund series to three significant figures. In which region of the spectrum does it lie? The following are his key contributions to our understanding of atomic structure:. Unfortunately, Bohr could not explain why the electron should be restricted to particular orbits.

Scientists needed a fundamental change in their way of thinking about the electronic structure of atoms to advance beyond the Bohr model. Thus far we have explicitly considered only the emission of light by atoms in excited states, which produces an emission spectrum a spectrum produced by the emission of light by atoms in excited states.

The converse, absorption of light by ground-state atoms to produce an excited state, can also occur, producing an absorption spectrum a spectrum produced by the absorption of light by ground-state atoms. When an atom emits light, it decays to a lower energy state; when an atom absorbs light, it is excited to a higher energy state.

If the light that emerges is passed through a prism, it forms a continuous spectrum with black lines corresponding to no light passing through the sample at , , , and nm.

Any given element therefore has both a characteristic emission spectrum and a characteristic absorption spectrum, which are essentially complementary images. Emission and absorption spectra form the basis of spectroscopy , which uses spectra to provide information about the structure and the composition of a substance or an object. In particular, astronomers use emission and absorption spectra to determine the composition of stars and interstellar matter.

Superimposed on it, however, is a series of dark lines due primarily to the absorption of specific frequencies of light by cooler atoms in the outer atmosphere of the sun. By comparing these lines with the spectra of elements measured on Earth, we now know that the sun contains large amounts of hydrogen, iron, and carbon, along with smaller amounts of other elements.

During the solar eclipse of , the French astronomer Pierre Janssen — observed a set of lines that did not match those of any known element. Alpha particles are helium nuclei. Alpha particles emitted by the radioactive uranium pick up electrons from the rocks to form helium atoms.

Similarly, the blue and yellow colors of certain street lights are caused, respectively, by mercury and sodium discharges. In all these cases, an electrical discharge excites neutral atoms to a higher energy state, and light is emitted when the atoms decay to the ground state. In the case of sodium, the most intense emission lines are at nm, which produces an intense yellow light.

The spectral lines of a specific element or molecule at rest in a laboratory always occur at the same wavelengths. For this reason, we are able to identify which element or molecule is causing the spectral lines.

If the emitter or absorber is in motion, however, the position of the spectral lines will be Doppler shifted along the spectrum. There are two types of spectral lines in the visible part of the electromagnetic spectrum: Emission lines — these appear as discrete coloured lines, often on a black background, and correspond to specific wavelengths of light emitted by an object.

The energy that is gained by the atom is equal to the difference in energy between the two energy levels. The electron energy level diagram for the hydrogen atom. Monsif Sabadell Teacher. What is meant by hydrogen spectrum?

Rochdi Kleine Teacher. What is the significance of the line spectrum of hydrogen? Line Spectra in Hydrogen. By looking at the specific wavelengths of light that are either absorbed or emitted from a sample of H atoms, we discover something about the energy of the electrons in the atom.

This means that there are discrete energy levels that the electron is moving between. Rashpal Arenache Teacher. What causes a discrete spectrum? Discrete Spectrum. Berdaitz Schwenkenbecher Reviewer. Why do different elements give off different wavelengths of light? Heating an atom excites its electrons and they jump to higher energy levels. When the electrons return to lower energy levels, they emit energy in the form of light.

Every element has a different number of electrons and a different set of energy levels. Thus, each element emits its own set of colours. What are emission lines used for? Spectral lines are often used to identify atoms and molecules. These "fingerprints" can be compared to the previously collected "fingerprints" of atoms and molecules, and are thus used to identify the atomic and molecular components of stars and planets, which would otherwise be impossible.

Brigitte Polka Reviewer. What emission spectrum tells us? Each element has a different atomic spectrum.