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1 Chapter 9 – Electrons in Atoms and the Periodic Table • Electromagnetic Radiation • Bohr Model • The Quantum Mechanical Model • Electron Configurations • Periodic Trends
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Chapter 9 – Electrons in Atoms and the Periodic Table
• Electromagnetic Radiation• Bohr Model• The Quantum Mechanical Model• Electron Configurations• Periodic Trends
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Electrons in Atoms and the Periodic TableElectromagnetic Radiation
The electromagnetic radiation spectra
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Electrons in Atoms and the Periodic TableBohr Model
If white light is passed through a prism, a continuous spectrum of light is found. However, when the light emitted by excited hydrogen atoms is passed through a prism the radiation is found to consist of a number of components or spectra lines.
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Electrons in Atoms and the Periodic TableBohr Model
• Electrons exist in quantized orbits at specific, fixed energies and specific fixed distances from the nucleus
• When energy is put into an atom, electrons are excited to higher-energy orbits
• When electrons fall from higher-energy orbits to lower-energy orbits, atoms emit light
• The energy (and therefore the wavelength) of the emitted light corresponds to the difference in energy between the two orbits in the transitions. Since these energies are fixed and discrete, the energy (and therefore the wavelength) of the emitted light is fixed and discrete
Bohr Model:
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Electrons in Atoms and the Periodic TableThe Quantum Mechanic Model
Constructive Interference: When the peaks of a wave coincideand the amplitude of the wave is increased
Destructive Interference: When the peak of one way coincideswith the trough of another wave lowering the amplitude
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Electrons in Atoms and the Periodic TableThe Quantum Mechanic Model
When light passes though a pair of closely spaced slits, circular waves are generated at each slit. These waves interfere with each other. Where they interfere constructively a bright line is seen on the screen behind the slits; where the interference is destructive, the screen is dark. If the same experiment is done with electrons the same interference patters are observed.
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Electrons in Atoms and the Periodic TableThe Quantum Mechanic Model
When designating shells, the first number is the principal quantum number and the second letter is the orbital. The simplest way of drawing an atomic orbital is as a boundary surface, a surface with in which there is a high probability (typically 90%) of finding the electron. The shaded region within the boundary surfaces is an approximate indication of the electron density at each point. All s-orbitals are spherically symmetric.
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Electrons in Atoms and the Periodic TableThe Quantum Mechanic Model
The boundary surfaces of the p-orbitals
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Electrons in Atoms and the Periodic TableThe Quantum Mechanic Model
The boundary surfaces of the d-orbitals
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Electrons in Atoms and the Periodic TableElectron Configurations
Order of Orbital Filling1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p
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Electrons in Atoms and the Periodic Table
Effective Nuclear Charge: (Zeff) The net nuclear charge after taking into account the shielding caused by other electrons in the atom.
Periodic Trends
Why: Going across the periodic table, the number of core electrons stays the same but the number of protons increases. The core electrons are responsible for most of the shielding, therefore the Zeff gets larger as you go across a period. Although going down a group adds more core electrons it also adds more protons therefore Zeff is pretty much constant going down a group.
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Electrons in Atoms and the Periodic Table
Atoms do not have sharp boundaries therefore you can not measurethe exact radius of an atom
Electrons in Atoms and the Periodic Table
Metals (Mainly Solids):Measure the distance between nuclei
Nonmetals and Metalliods (Mainly Liquids and Gasses):Covalent Radius: The contribution of an atom to the length
of a covalent bondExamples:
Cl2 has a bond length of 198 pm therefore Cl’s atomic radii is 99 pm
Nobel Gasses (Elements that do not Bond):Van der Waals Radius: Half the distance between the
centers of nonbonded, touching atoms in a solid
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Electrons in Atoms and the Periodic Table
Atomic Radii: Half the distance between the centers of neighboring atoms in a solid of a homonuclear molecule.
Periodic Trends
Why: Going across a period the Zeff increases, therefore the pull on the electrons increases and the atomic radii decrease. Going down a group, more atomic shells are added and the radii increase.
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Electrons in Atoms and the Periodic Table
First Ionization Energy: The minimum energy required to remove the first electron from the ground state of a gaseous atom, molecule, or ion.
Periodic Trends
Why: Going across a period the Zeff increases, therefore it is harder to remove an electron and the first ionization energy increases. However, going down a group the electrons are located farther from the nucleus and they can be removed easier.
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Electrons in Atoms and the Periodic Table
Properties of Metals:
•Easily lose electrons•Luster (shininess)•Good conductors of heat and electricity•High density (heavy for their size)•High melting point•Ductile (most metals can be drawn out into thin wires)•Malleable (most metals can be hammered into thin sheets)•Corrode easily. Corrosion is a gradual wearing away.
Periodic Trends
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Electrons in Atoms and the Periodic TablePeriodic Trends
Why: Metallic characteristics follow the same trend as ionization energies because one of the most important aspects of being a metal is the willingness of the atom to give up electrons.
