وحدة Electrons In Atoms الكيمياء الصف الثاني عشر
Light and Quantized Energy
MaINIDEA Light, a form of electromagnetic radiation, has of a wave a particle
The Atom and Unanswered Questions
After discovering three sututomic particles in the early scientßts ccN1tinued their quest to at(Nnic structure and the rnent of electrons within atoms Rutherford that all of an atom•s vxsitive charge and vir-
tually all of its mass are concentrated in a nucleus that is surrounded by electrom. The mcxiel did mt explain how the atom's
electrons are arranged in the space around the nucleus Nor did it acHress the question of why the negatively charged electrons are not pulled into the atom's charged nucleus- Rutherfords nuclear did not to u-count for the differences similarities in &mical among the various elements
For example. consider the elements lithium scxåium, and B)tassium which are found in different on the table but have similar chemical trhaviors. All three elements metallic in nature, arui their atoms react vigorously with water to liberate hydrogen gas. In fact. as show-n in FiBre 1, tx»th scxiium and react so vio-
lently that the gas can ignite and even explcxie. In the early I scientists trgan to unravel the puzzle of chemical •Ihey that certain elements emitted visible light when in a flame. Analysis of emitted light revealed that an element's chemical behavior is related to the arrangement of the elec- trons in its atoms. To understand this relationship and nature of atomic structure. it will be Irlpful to first understand the nature of light
The Particle Nature of Light
While considering light as a wave explains much of its everyday t*hav- ior, it fails to adequately describe irnvxmant of light's interac- tions with matter. The wave rncxåel of light cannot explain why heated
objects emit only certain frequencies of light at a given or why some metals emit electrons when light of a frequency shines on them. Scientists realized that a or a revision of the wave rncxåel of light was needed to address these phenomena
The quantum concept When objects are heated, they emit glow- ing light. Figure 6 illustrates this phenomenon with iron. A piece of iron dark gray at rcx•rn glows red when heated sufficiently, and turns orange. then bluish in color at even higher As you will learn in later the temv•rrature of an is a measure of the average kinetic energy of its particles As the iron gets hotter, it B)ssesses a greater amount of energy and emits different colors of light. These different colors corresB»nd to different frequencies and wavelengths
The wave could not explain the emission of these different wavelengths In German physicist Max Planck (185& 1947) searching for an explanation of thß phenomenon as he studied tlr light emitted by heated objects. His study led him to a startling conclusion: matter can gain or lose energy only in small. amounts called quanta. A quantum is the minimum amount of energy that can be gained or by an atom
READING CHECK Explain why the color of heated objects changes with temperature
Planck and other physicists of the time thought the concept of quantized energy was revolutionary, and some found it disturbing. Prior exvrrierre had led scientists to think that energy could absortkd and in continually varying quantities, with no minimum limit to the amount. For example, think atx»ut heating a cup of water in a micro- wave oven. It eems that you can add any amount of thermal energy to the water by regulating the B)wer and duration of the microwaves Instead. the water's increases in infinitesimal steps as its molecules absorb quanta of energy. Because these steps are so small. the temvrrature seems to rise in a continuous, rather than a stepwise,
Atomic Emission Spectra
Have you ever wondered how light is in the gbwing tutrs of neon sigrw' Illis prcxes is anotlrr phenomenon that cannot explained by the wave of light The light of the neon sign is produced by passing electricity thrmzgh a tube filled with neon gas. Neon atoms in the tul* absorb and excited These excited atoms retum to their stable state by emitting light to release that energy If the light emitted by the neon passed through a glass prism, neon's atomic emission spectrum is prcxiuced The atomic emission spectrum of an element is the set of frequencies of the electromagnetic waves emitted by atoms of the element. Neon's atomic emission spectrum consists of several individual lines of color corresB)nding to frequencies of the radiation emitted by the atoms of neon. It is not a continuous range of colors, as in the visible of white light
READING CHECK Explain how an emission spectrum is produced
Each element's atomic emission sFrctrum unique and can to identify an or &termirr whether that element is part of an unknown com- B)und For example, when a platinum wire is into a strontium nitrate M)lution and then inserted into a burner flame, the strontium atoms emit a character iStic red color. You can a series of flame tests by doing the MiniLab
FWre 8 shows an illustration of the characteristic purple-pink glow by excited hydrogen atoms the visible portion of emision trum responsible for the glow. Note how the line nature of hydrogen's atomic emission trum differs from that of a continuous spectrum
)Although the of all electromagnetic waves vacuum is same. waves can have different wavelengths and frequencies. As you can see from the equation on the previous page, wavelength and frequerry are inversely related in other words. as one quantity increases the other decreases. To understand this relationship. examine the two waves illustrated in Fiwre 3. Although tX)th waves travel at the of light, you can see that the red wave has a longer wavelength and lower frequerry than the wave
Electromagnetic Sunlight, which is one example of white light, contains a nearly continuous range of wavelengths and frequencies. White light passing through a prism separates into a continuous spectrum of colors similar to the spectrum in Figure 4
These are the colors of the visible The is called continuous each B)int of it corresB)nds to a unique wavelength and frequency. You might be familiar with the colors of the visible spectrum. If you have ever seen a rainbow, you have seen all of the visible colors at once. A rainlxyw is formed tiny drops of water in the air disvrrse the white light from the Sun into its component colors, producing a spectrum that arches across the sky