Periodic Trends
Mr. Lee says:
Atomic radius increases as you go DOWN a group because you are adding more energy levels (orbitals) of electrons... 1s, 2s, 3s, 4s, etc.
Atomic radius decreases as you go from LEFT TO RIGHT because there are more protons, but the same energy levels. Those extra protons pull the electrons in closer and closer as you move right.
First ionization energy is how "easy it is to steal 1 valence electron" and it's easier to steal a valence electron from larger atoms, because the valence electrons are farther away from the nucleus, and thus they're not held on as tightly. Francium is the largest atom, so it has the lowest ionization energy.
Electronegativity is how much an atom wants to "hog" or attract electrons. Fluorine is the most electronegative atom, and electronegativity increases as you go toward it (left to right and bottom to top). The noble gases, being so fancy, rich, and satisfied, are NOT electronegative and do NOT care to hog electrons.
Atomic radius increases as you go DOWN a group because you are adding more energy levels (orbitals) of electrons... 1s, 2s, 3s, 4s, etc.
Atomic radius decreases as you go from LEFT TO RIGHT because there are more protons, but the same energy levels. Those extra protons pull the electrons in closer and closer as you move right.
First ionization energy is how "easy it is to steal 1 valence electron" and it's easier to steal a valence electron from larger atoms, because the valence electrons are farther away from the nucleus, and thus they're not held on as tightly. Francium is the largest atom, so it has the lowest ionization energy.
Electronegativity is how much an atom wants to "hog" or attract electrons. Fluorine is the most electronegative atom, and electronegativity increases as you go toward it (left to right and bottom to top). The noble gases, being so fancy, rich, and satisfied, are NOT electronegative and do NOT care to hog electrons.
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This chart to the right demonstrates that cations (+) are SMALLER than their neutral atoms. Why? This is because when you LOSE electrons (and thus gain a + charge), the atoms loses the outer shell that the electrons were in... and now it's smaller.
Anions are LARGER than their neutral atoms. Why? This is because when you GAIN electrons (and thus gain a - charge), there are more electrons... and electrons repel each other and get more spaced out, therefore expanding the outer shell. |
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Lewis Structures & Electron Dot Structures (2-Dimensional)
Mr. Lee says: To draw a molecule in Lewis / Electron Dot structure...
1) Add up all the valence electrons from the atoms. This is your "valence electron bank"
2) Draw the naked (meaning electron-free) atoms: Identify and draw the central atom with all the outside atoms around it.
3) Draw the minimum number of bonds need to connect. Each bond line uses 2 electrons from your bank.
4) Extra electrons go as lone pairs on the outside atoms.
5) If you still have leftover electrons, they go on the central atom as lone pairs.
6) If the central atom is not "happy" (with a full octet) after adding the lone pairs, then the outside atoms have to "share in". Sharing in means they will use their lone pairs to create a double or triple bond with the central atom.
7) Refine and re-draw your Lewis structure molecule for clarity and neatness.
1) Add up all the valence electrons from the atoms. This is your "valence electron bank"
2) Draw the naked (meaning electron-free) atoms: Identify and draw the central atom with all the outside atoms around it.
3) Draw the minimum number of bonds need to connect. Each bond line uses 2 electrons from your bank.
4) Extra electrons go as lone pairs on the outside atoms.
5) If you still have leftover electrons, they go on the central atom as lone pairs.
6) If the central atom is not "happy" (with a full octet) after adding the lone pairs, then the outside atoms have to "share in". Sharing in means they will use their lone pairs to create a double or triple bond with the central atom.
7) Refine and re-draw your Lewis structure molecule for clarity and neatness.
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Left: Naked atoms. Valence electron bank is B+F+F+F = 24 v.e.
Center: Minimum bonds used 6 v.e. from the bank.18 v.e. remained. Right: Remaining 18 v.e. are spread among outside atoms. No remnants had to go on central atom. Boron is an octet rule exception: It is OK with 6 shared valence electrons. |
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Valence Shell Electron Pair Repulsion (VSEPR) Theory (3-Dimensional)
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Mr. Lee says: For general high school chemistry, the shapes you should be concerned about are tetrahedral, bent, trigonal planar, trigonal pyramidal, and linear. (You can disregard higher-order geometries you may see, such octahedral, trigonal bipyramidal, T-shaped, see-saw, etc.)
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Polarity
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Mr. Lee says: Nearly all molecules with lone pairs on the central atom are polar. All bent molecules (like water) and all trigonal pyramidal molecules (like ammonia) are polar.
Polar substances mix well with other polar substances (for example, water and ammonia are both polar and they mix well.) Likewise, non-polar substances will mix well with other non-polar substances (such as oil and bromine, both of which are non-polar). A polar substance will not mix with a non-polar substance; oil and water, for example, do not mix. To determine polarity, you must draw the molecule and consider both electronegativity of the atoms, as well as geometry symmetry or asymmetry of the entire molecule |
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Intermolecular Forces
Mr. Lee says: Intermolecular forces are the attractive forces that exist between molecules. They are what keep materials together by keeping molecules attracted to each other.
The THREE that matter for our class:
- London Dispersion Force (LDF): It's the weakest force, and exists between all atoms and molecules. The larger an atom or molecules, the more electrons it has... and the stronger the LDF is.
- Dipole-Dipole Force: Next in strength is DP-DP, which exists only between polar molecules.
- Hydrogen Bonds: These are a special type of dipole force that exists only between molecules with H-F, H-N, or H-O bonds.
The THREE that matter for our class:
- London Dispersion Force (LDF): It's the weakest force, and exists between all atoms and molecules. The larger an atom or molecules, the more electrons it has... and the stronger the LDF is.
- Dipole-Dipole Force: Next in strength is DP-DP, which exists only between polar molecules.
- Hydrogen Bonds: These are a special type of dipole force that exists only between molecules with H-F, H-N, or H-O bonds.
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