Electrons in Atoms.ppt - Electrons in Atoms Ernest Rutherfords Discovered dense Model positive piece at the center of the atom nucleus Electrons would

Electrons in Atoms.ppt - Electrons in Atoms Ernest...

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Unformatted text preview: “Electrons in Atoms” Ernest Rutherford’s Discovered dense Model positive piece at the center of the atom- “nucleus” Electrons would surround and move around it, like planets around the sun Atom is mostly empty space It did not explain the chemical properties of the elements – a better description of the electron behavior was needed Niels Bohr’s Model Why don’t the electrons fall into the nucleus? Move like planets around the sun. In specific circular paths, or orbits, at different levels. An amount of fixed energy separates one level from another. The Bohr Model of the Atom I pictured the electrons orbiting the nucleus much like planets orbiting the sun. Niels Bohr However, electrons are found in specific circular paths around the nucleus, and can jump from one level to another. Bohr’s model Energy level of an electron • analogous to the rungs of a ladder The electron cannot exist between energy levels, just like you can’t stand between rungs on a ladder A quantum of energy is the amount of energy required to move an electron from one energy level to another The Quantum Mechanical Model Energy is “quantized” - It comes in chunks. A quantum is the amount of energy needed to move from one energy level to another. Since the energy of an atom is never “in between” there must be a quantum leap in energy. The Quantum Mechanical Model Has energy levels for electrons. Orbits are not circular. It can only tell us the probability of finding an electron a certain distance from the nucleus. The Quantum Mechanical Model The atom is found inside a blurry “electron cloud” An area where there is a chance of finding an electron. Think of fan blades Atomic Orbitals Principal Quantum Number (n) = the energy level of the electron: 1, 2, 3, etc. These are called atomic orbitals regions where there is a high probability of finding an electron. Sublevels- like theater seats arranged in sections: letters s, p, d, and f Principal Quantum Number Generally symbolized by “n”, it denotes the shell (energy level) in which the electron is located. Maximum number of electrons that can fit in an energy level is: 2n2 How many e- in level 2? 3? Summary # of shapes (orbitals) Maximum electrons Starts at energy level s 1 2 1 p 3 2 d 5 6 10 f 7 14 4 3 ECN Energy level (Period ) Number of electrons 1s Orbital block 2 By Energy Level First Energy Level Has only s orbital only 2 electrons 1s2 Second Energy Level Has s and p orbitals available 2 in s, 6 in p 2s22p6 8 total electrons By Energy Level Third energy level Has s, p, and d orbitals 2 in s, 6 in p, and 10 in d 3s23p63d10 18 total electrons Fourth energy level Has s, p, d, and f orbitals 2 in s, 6 in p, 10 in d, and 14 in f 4s24p64d104f14 32 total electrons By Energy Level Any more than the fourth and not all the orbitals will fill up. You simply run out of electrons The orbitals do not fill up in a neat order. The energy levels overlap Lowest energy fill first. Increasing energy 7s 6s 5s 4s 3s 2s 7p 6p 5p 4p 6d 5d 4d 5f 4f 3d 3p 2p aufbau diagram 1s Aufbau is German for “building up” Electron Configurations… …are the way electrons are arranged in various orbitals around the nuclei of atoms. Three rules tell us how: 1) Aufbau principle - electrons enter the lowest energy first. • This causes difficulties because of the overlap of orbitals of different energies – follow the diagram! 2) Pauli Exclusion Principle - at most 2 electrons per orbital - different spins Pauli Exclusion Principle No two electrons in an atom can have the same four quantum numbers. Wolfgang Pauli To show the different direction of spin, a pair in the same orbital is written as: Electron Configurations 3) Hund’s Rule- When electrons occupy orbitals of equal energy, they don’t pair up until they have to. Lets write the electron configuration for Phosphorus We need to account for all 15 electrons in phosphorus Increasing energy 7s 6s 5s 4s 3s 2s 1s 7p 6p 6d 5d 5p 4d 4p 3p 5f 4f 3d The first two electrons go into the 1s orbital 2p Notice the opposite direction of the spins only 13 more to go... Increasing energy 7s 6s 5s 4s 3s 2s 1s 7p 6p 6d 5d 5p 4d 4p 5f 4f 3d 3p The next electrons 2p go into the 2s orbital only 11 more... Increasing energy 7s 6s 5s 4s 3s 2s 1s 7p 6p 5p 4p 6d 5d 4d 5f 4f 3d 3p 2p • The next electrons go into the 2p orbital • only 5 more... Increasing energy 7s 6s 5s 4s 3s 2s 1s 7p 6p 5p 4p 6d 5d 4d 5f 4f 3d 3p 2p • The next electrons go into the 3s orbital • only 3 more... Increasing energy 7s 6s 5s 4s 3s 2s 1s 7p 6p 5p 4p 6d 5d 4d 5f 4f 3d 3p • The last three electrons go into the 3p orbitals. 2p They each go into separate shapes (Hund’s) • 3 unpaired electrons Orbital notation = 1s22s22p63s23p3 Orbitals fill in an order Lowest energy to higher energy. Electrons don’t pair up until they have to. Increasing energy 7s 6s 5s 4s 3s 2s 7p 6p 5p 4p 6d 5d 4d 3d 3p 2p aufbau diagram 1s 5f 4f Light The study of light led to the development of the quantum mechanical model. Light is a kind of electromagnetic radiation. Electromagnetic radiation includes many types: gamma rays, x-rays, radio waves… Speed of light = 2.998 x 108 m/s, and is abbreviated “c” All electromagnetic radiation travels at this same rate when measured in a vacuum - Page 139 “R O Y Frequency Increases Wavelength Longer G B I V” Long Wavelength = Low Frequency = Low ENERGY Short Wavelength = High Frequency = High ENERGY Wavelength Table Atomic Spectra White light is made up of all the colors of the visible spectrum. Passing it through a prism separates it. If the light is not white By heating a gas with electricity we can get it to give off colors. Passing this light through a prism does something different. • These are called the atomic emission spectrum • Unique to each element, like fingerprints! • Very useful for identifying elements Atomic Spectrum Each element gives off its own characteristic colors. Can be used to identify the atom. This is how we know what stars are made of. Light is a Particle? Energy is quantized. Light is a form of energy. Therefore, light must be quantized These smallest pieces of light are called photons. Photoelectric effect? Albert Einstein Energy & frequency: directly related. The energy (E ) of electromagnetic radiation is directly proportional to the frequency () of the radiation. Equation: E = h E = Energy, in units of Joules (kg·m2/s2) (Joule is the metric unit of energy) h = Planck’s constant (6.626 x 10-34 J·s) = frequency, in units of hertz (hz, sec-1) Explanation of atomic spectra When we write electron configurations, we are writing the lowest energy. The energy level, and where the electron starts from, is called it’s ground state - the lowest energy level. Changing the energy Let’s look at a hydrogen atom, with only one electron, and in the first energy level. Changing the energy Heat, electricity, or light can move the electron up to different energy levels. The electron is now said to be “excited” Changing the energy As the electron falls back to the ground state, it gives the energy back as light Changing the energy They may fall down in specific steps Each step has a different energy { { { Ultraviolet The Visible Infrared further they fall, more energy is released and the higher the frequency. This is a simplified explanation! The orbitals also have different energies inside energy levels All the electrons can move around. What is light? Light is a particle - it comes in chunks. Light is a wave - we can measure its wavelength and it behaves as a wave If we combine E=mc2 , c=, E = 1/2 mv2 and E = hthen we can get: = h/mv (from Louis de Broglie) called de Broglie’s equation Calculates the wavelength of a particle. Wave-Particle Duality J.J. Thomson won the Nobel prize for describing the electron as a particle. His son, George Thomson won the Nobel prize for describing the wave-like nature of the electron. The electron is a particle! The electron is an energy wave! Confused? You’ve Got Company! “No familiar conceptions can be woven around the electron; something unknown is doing we don’t know what.” Physicist Sir Arthur Eddington The Nature of the Physical World 1934 The physics of the very small Quantum mechanics explains how very small particles behave • Quantum mechanics is an explanation for subatomic particles and atoms as waves Classical mechanics describes the motions of bodies much larger than atoms Heisenberg Uncertainty Principle It is impossible to know exactly the location and velocity of a particle. The better we know one, the less we know the other. Measuring changes the properties. True in quantum mechanics, but not classical mechanics Heisenberg Uncertainty Principle “One cannot simultaneously determine both the position and momentum of an electron.” Werner Heisenberg You can find out where the electron is, but not where it is going. OR… You can find out where the electron is going, but not where it is! It is more obvious with the very small objects To measure where a electron is, we use light. But the light energy moves the electron And hitting the electron changes the frequency of the light. After Before Photon Moving Electron Photon wavelength changes Electron velocity changes Fig. 5.16, p. 145 ...
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