The Modern Atom. Emission Line Spectra When an electrical voltage is passed across a gas in a sealed...
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Transcript of The Modern Atom. Emission Line Spectra When an electrical voltage is passed across a gas in a sealed...
![Page 1: The Modern Atom. Emission Line Spectra When an electrical voltage is passed across a gas in a sealed tube, a series of narrow lines is seen. These lines.](https://reader035.fdocuments.net/reader035/viewer/2022062713/56649cee5503460f949bc2db/html5/thumbnails/1.jpg)
The Modern Atom
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Emission Line Spectra
• When an electrical voltage is passed across a gas in a sealed tube, a series of narrow lines is seen.
• These lines are the emission line spectrum. The emission line spectrum for hydrogen gas shows three lines: 434 nm, 486 nm, and 656 nm.
![Page 3: The Modern Atom. Emission Line Spectra When an electrical voltage is passed across a gas in a sealed tube, a series of narrow lines is seen. These lines.](https://reader035.fdocuments.net/reader035/viewer/2022062713/56649cee5503460f949bc2db/html5/thumbnails/3.jpg)
Wave Nature of Light• Light is made of particles (photons) with no mass
carrying energy.
• Photons travels through space as a wave, similar to an ocean wave.
• A wave has characteristics:
a) Wavelength is the distance light travels in one cycle.
b) Frequency is the number of wave cycles completed each second.
c) Speed: Light has a constant speed: 3.00 × 108 m/s.
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Wavelength vs. Frequency
• The longer the wavelength of light, the lower the frequency.
• The shorter the wavelength of light, the higher the frequency.
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Energy and Frequency• There is a relationship between the energy
and the frequency of photons: E = h x , where E is the energy, h is called Planck’s constant (h= 6.62x 10-34 J.s), and , is the frequency.
• The frequency of an electromagnetic radiation determines the color of the light.
• Photons with high frequency carry more energy than photons with low frequency.
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Visible Spectrum• Radiation composed of only one wavelength is called
monochromatic.
• White light is made of photons of different wavelengths.
• These photons can be separated into a continuous spectrum of colors.
• The visible spectrum is the range of wavelengths between 400 and 700 nm.
• Radiant energy that has a wavelength lower than 400 nm and greater than 700 nm cannot be seen by the human eye.
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Radiant Energy Spectrum• The complete radiant energy spectrum is an
uninterrupted band, or continuous spectrum.
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Bohr Model of the Atom
• Niels Bohr speculated that electrons orbit about the nucleus in fixed energy levels.
• Electrons are found only in specific energy levels, and nowhere else.
• The electron energy levels are quantized.
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Evidence for Energy Levels• The electric charge temporarily excites an electron
to a higher orbit. When the electron drops back down, a photon is given off.
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“Atomic Fingerprints”• The emission line spectrum of each element is
unique.
• We can use the line spectrum for the identify of elements, using their “atomic fingerprint”.
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Bohr Model• Colors from excited gases arise because electrons
move between energy states in the atom.
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The Wave/Particle Nature of Light
• In 1900, Max Planck proposed that radiant energy is not continuous, but is emitted in small bundles. This is the quantum concept.
• Radiant energy has both a wave nature and a particle nature.
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The Photoelectric Effect
• The photoelectric effect provides evidence for the particle nature of light -- “quantization”.
• If light shines on the surface of a metal, there is a point at which electrons are ejected from the metal.
• The electrons will only be ejected once the threshold frequency is reached.
• Below the threshold frequency, no electrons are ejected.
• Above the threshold frequency, the number of electrons ejected depend on the intensity of the light.
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The Photoelectric Effect
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The Quantum Concept• The quantum concept states that energy is present
in small, discrete bundles.
• For example:– A tennis ball that rolls down a ramp loses potential
energy continuously.
– A tennis ball that rolls down a staircase loses potential energy in small bundles. The loss is quantized.
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Energy Levels and Sublevels
• It was later shown that electrons occupy energy sublevels within each level.
• These sublevels are given the designations s, p, d, and f.
– These designations are in reference to the sharp, principal, diffuse, and fine lines in emission spectra.
• The number of sublevels in each level is the same as the number of the main level.
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Quantum Mechanical Model• An orbital is the region of space where there is a
high probability of finding an atom.
• In the quantum mechanical atom, orbitals are arranged according to their size and shape.
• The higher the energy of an orbital, the larger its size.
• s-orbitals have a spherical shape.
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Shapes of p-Orbitals
• There are three different p sublevels.
• p-orbitals have a dumbbell shape.
• Each of the p-orbitals has the same shape, but each is oriented along a different axis in space.
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s-orbitals
p-orbitals
d-orbitals
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Orbitals and their Energy
H-Atom Other atoms
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Energy Levels and Sublevels• The first energy level has 1 sublevel:
– 1s
• The second energy level has 2 sublevels:– 2s and 2p
• The third energy level has 3 sublevels:– 3s, 3p, and 3d
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Electron Occupancy in Sublevels
• The maximum number of electrons in each of the energy sublevels depends on the sublevel:
– The s sublevel holds a maximum of 2 electrons.
– The p sublevel holds a maximum of 6 electrons.
– The d sublevel holds a maximum of 10 electrons.
– The f sublevel holds a maximum of 14 electrons
• The maximum electrons per level is obtained by adding the maximum number of electrons in each sublevel.
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Conclusions• Light has both the properties of waves and
particles.
• The particles of light are referred to as photons.
• The energy of photons is quantized.
• Electrons exist around the nucleus of atoms in discrete, quantized energy levels.
• Electrons fill energy sublevels starting with the lowest energy sublevel and filling each successive level of higher energy.