Chapter 9 Electrons in Atoms and the Periodic Table

Transcription

1 Chapter 9 Electrons in Atoms and the Periodic Table Electromagnetic Radiation Bohr Model The Quantum Mechanical Model Electron Configurations Periodic Trends 1

2 Electromagnetic Radiation 2 The electromagnetic radiation spectra

3 Bohr 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. 3

4 Bohr Model Bohr 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 higherenergy 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 4

5 The Quantum Mechanic Model Constructive Interference: When the peaks of a wave coincide and the amplitude of the wave is increased Destructive Interference: When the peak of one way coincides with the trough of another wave lowering the amplitude 5

6 The Quantum Mechanic Model 6 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.

7 The Quantum Mechanic Model 7 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.

8 The Quantum Mechanic Model 8 The boundary surfaces of the p-orbitals

9 The Quantum Mechanic Model 9 The boundary surfaces of the d-orbitals

10 Electron Configurations 10 Order of Orbital Filling 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p

11 Periodic Trends Effective Nuclear Charge: (Z eff ) The net nuclear charge after taking into account the shielding caused by other electrons in the atom. 11 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 Z eff gets larger as you go across a period. Although going down a group adds more core electrons it also adds more protons therefore Z eff is pretty much constant going down a group.

12 Atoms do not have sharp boundaries therefore you can not measure the exact radius of an atom 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 bond Examples: Cl 2 has a bond length of 198 pm therefore Cl s atomic radii is 99 pm 12 Nobel Gasses (Elements that do not Bond): Van der Waals Radius: Half the distance between the centers of nonbonded, touching atoms in a solid

13 Periodic Trends Atomic Radii: Half the distance between the centers of neighboring atoms in a solid of a homonuclear molecule. 13 Why: Going across a period the Z eff 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.

14 Periodic Trends First Ionization Energy: The minimum energy required to remove the first electron from the ground state of a gaseous atom, molecule, or ion. 14 Why: Going across a period the Z eff 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.

15 Periodic Trends 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. 15

16 Periodic 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. 16