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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 are the emission line spectrum . The emission line spectrum for hydrogen gas shows three lines: 434 nm , 486 nm , and 656 nm .
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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.
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.
Wavelength vs. Frequency • The longer the wavelength of light, the lower the frequency. • The shorter the wavelength of light, the higher the frequency.
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.
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.
Radiant Energy Spectrum • The complete radiant energy spectrum is an uninterrupted band, or continuous spectrum.
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.
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.
“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”.
Bohr Model • Colors from excited gases arise because electrons move between energy states in the atom.
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.
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.
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.
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.
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.
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.
s-orbitals p-orbitals d-orbitals
Orbitals and their Energy H-Atom Other atoms
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
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.
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.
