The Atom & Electromagnetic Radiation
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|Fundamental Subatomic Particles||Electromagnetic Radiation|
|Light & Other Forms of Electromagnetic Radiation|
The number of protons, neutrons, & electrons in an atom can be determined from a phối of simple rules.The number of protons in the nucleus of the atom is equal lớn the atomic number (Z). The number of electrons in a neutral atom is equal to lớn the number of protons. The mass number of the atom (M) is equal khổng lồ the sum of the number of protons and neutrons in the nucleus. The number of neutrons is equal to the difference between the mass number of the atom (M) & the atomic number (Z).
Examples: Let"s determine the number of protons, neutrons, & electrons in the following isotopes.
The different isotopes of an element are identified by writing the mass number of the atom in the upper left corner of the symbol for the element. 12C, 13C, và 14C are isotopes of carbon (Z = 6) & therefore contain six protons. If the atoms are neutral, they also must contain six electrons. The only difference between these isotopes is the number of neutrons in the nucleus.
12C: 6 electrons, 6 protons, & 6 neutrons
13C: 6 electrons, 6 protons, và 7 neutrons
14C: 6 electrons, 6 protons, and 8 neutrons
|Practice Problem 1: |
Calculate the number of electrons in the Cl- và Fe3+ ions.
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Much of what is known about the structure of the electrons in an atom has been obtained by studying the interaction between matter & different forms of electromagnetic radiation. Electromagnetic radiation has some of the properties of both a particle and a wave.
Particles have a definite mass and they occupy space. Waves have no mass and yet they carry energy as they travel through space. In addition khổng lồ their ability lớn carry energy, waves have four other characteristic properties: speed, frequency, wavelength, & amplitude. The frequency (v) is the number of waves (or cycles) per unit of time. The frequency of a wave is reported in units of cycles per second (s-1) or hertz (Hz).
The idealized drawing of a wave in the figure below illustrates the definitions of amplitude & wavelength. The wavelength (l) is the smallest distance between repeating points on the wave. The amplitude of the wave is the distance between the highest (or lowest) point on the wave & the center of gravity of the wave.
If we measure the frequency (v) of a wave in cycles per second and the wavelength (l) in meters, the hàng hóa of these two numbers has the units of meters per second. The sản phẩm of the frequency (v) times the wavelength (l) of a wave is therefore the tốc độ (s) at which the wave travels through space.
vl = s
|Practice Problem 2: |
What is the tốc độ of a wave that has a wavelength of 1 meter and a frequency of 60 cycles per second?
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|Practice Problem 3: |
Orchestras in the United States tune their instruments khổng lồ an "A" that has a frequency of 440 cycles per second, or 440 Hz. If the tốc độ of sound is 1116 feet per second, what is the wavelength of this note?
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Light và Other Forms of Electromagnetic Radiation
Light is a wave with both electric & magnetic components. It is therefore a khung of electromagnetic radiation.
Visible light contains the narrow band of frequencies and wavelengths in the portion of the electro-magnetic spectrum that our eyes can detect. It includes radiation with wavelengths between about 400 nm (violet) & 700 nm (red). Because it is a wave, light is bent when it enters a glass prism. When white light is focused on a prism, the light rays of different wavelengths are bent by differing amounts và the light is transformed into a spectrum of colors. Starting from the side of the spectrum where the light is bent by the smallest angle, the colors are red, orange, yellow, green, blue, và violet.
As we can see from the following diagram, the energy carried by light increases as we go from red to xanh across the visible spectrum.
Because the wavelength of electromagnetic radiation can be as long as 40 m or as short as 10-5 nm, the visible spectrum is only a small portion of the total range of electromagnetic radiation.
The electromagnetic spectrum includes radio & TV waves, microwaves, infrared, visible light, ultraviolet, x-rays, g-rays, and cosmic rays, as shown in the figure above. These different forms of radiation all travel at the speed of light (c). They differ, however, in their frequencies & wavelengths. The sản phẩm of the frequency times the wavelength of electromagnetic radiation is always equal khổng lồ the tốc độ of light.
vl = c
As a result, electromagnetic radiation that has a long wavelength has a low frequency, & radiation with a high frequency has a short wavelength.
|Practice Problem 4:|
Calculate the frequency of red light that has a wavelength of 700.0 nm if the tốc độ of light is 2.998 x 108 m/s.