Chemistry: Introduction

Atomic number = protons
Atomic mass = protons + neutrons (aka, 'atomic weight' or 'molar mass')

Isotopes - Same element, different number of neutrons

Ion - Atoms with different number of electrons 

Periodic Table: 

Row = Period 

Group = Family

Metalloids

1. Atoms lose electrons from the highest energy shell first. This means that in transition metals, electrons are lost from the s-subshell first, and then from the d-subshell. 
2. Ions seek symmetry. Representative elements form noble gas electron configuration when they make ions. For example, group 1 atoms form 1+ cations and group 17 atoms form 1- anions. Transition metals try to "even-out" their d-orbitals so that each orbital has the same number of electrons. Whenever possible, an ion will have a half-filled or completely filled orbital. Important reference points in the periodic table include the periods 1 and 2, which have half-filled and completely filled s orbitals, periods 7, and 12, which have half-filled and completely filled d orbitals, and periods 15 and 18, which have half-filled and completely filled p orbitals. 

Periodic Trends:

• Effective Nuclear Charge (Zeff) - The amount of charge felt by the most recently added electron. (Net positive charge experienced by an electron in a mult-electron atom.)
• Ionization Energy - Increase up and across (to the right)
• Electronegativity - Increase up and across (to the right) 
• Electron Affinity - Increase up and across (to the right) 
• Atomic Radius - Increase down and across (to the left) 

n = shell

l = subshell (0 = s, 1 = p, 2 = d, 3 = f)

ml = magnetic 

ms = spin

Only electrons from the s and p subshells are considered valence electrons. (Valence electrons = electrons in the outermost shell.) 

Paramagnetic Elements-  Elements with unpaired electrons 

Diamagnetic Elements - Elements with no unpaired electrons (with subshells completely filled) 

ΔE = hf , where 'h' is the Planck's constant (6.6 x 10^-34 Js, and 'f' is frequency. 

λ = h/mv , where 'm' is mass and 'v' is velocity 

Photoelectric effect = the observation that many metals emit electrons when light shines upon them.

     

Covalent bonds - Atoms share electrons 

Ionic bonds - Electrons are transferred from one atom to another

Bond energy (or dissociation energy) - The energy necessary for a complete separation of the bond

Break bond = Energy absorption

Make bond = Energy release

*Don't forget that hydrogen bonds are not true bonds. They are an intermolecular force. Remember that intermolecular bonds are broken when a compound moves from the liquid to gas phase (a physical chnage). While hydrogen bonds are the strongest intermolecular force, they are still much weaker than covalent bonds. 

*Whenever you see a metal on the MCAT, write a + sign by it to remind yourself that the other part of the molecule is negative (-) and that the bond is ionic. Then negative part of the species is strongly basic. 

Physics reaction - Examples include melting, freezing etc.

Chemical reaction - A reaction that changes the molecular structure into a different compound.

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The Arrhenius Equation is an equation that expresses the temperature dependence on reaction rates.

k = zpe^(-Ea/RT) or k = Ae^(-Ea/RT), where 'k' is the rate constant, 'R' is the gas constant 8.314, 'z' is the product of the collision frequency, and 'p' is the fraction of collisions, also called steric factor. 'A' replaces 'zp' 

     -As Ea increases, the rate constant decreases 

*Do not confuse the rate constant with the rate of reaction. They are proportional, but they are not identical. 

Rate Law:

Alpha and beta are the reaction order of each reactant, and added together, they are the overall order of the reaction. 

     -For example, k[A][B]^2 is a third order reaction, because 1 + 2 = 3

*Only the slow steps contribute to the rate law of a reaction 

To get the rate law experimentally, follow the equation (initial rate/initial concentration)

   -If rate does not change, the number for that reactant is 0, and any number to the exponent of 0 is 1.

Catalysts affect the kinetics but not the thermodynamics of a reaction. 

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Half-life 

At = Aoe^(-kt), or ln(At/Ao) = -kt

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