MCAT General Chemistry

Atoms

smallest unit of any element - has protons, neutrons, and electrons

• p = +1, mass = 1amu

• e = -1, mass = 0amu

• n = 0, mass = 1amu

Atomic Number

Z, the number of protons(p+)

Mass number

= P*+n

The charge on atoms

= p-e

• Cations and anions are ions

  • C>0 = cation → +

  • C<0 = anion → -

  • C=0 = atom level

Isotopes

two atoms of the same element that differ in their number of neutrons - determined by mass number

Bohr model of the atom

Electrons orbit at a fixed distance from the nucleus - the orbit decreases with distance from the nucleus - as we move away, the ends come closer and energy increases with distance from the nucleus

Electrons absorb only specific allowed E(due to fixed quantities of E)

• Current orbit = ground state

• Higher E orbit = excited state

• Ephoton = Ef-Ei

• e- in an excited state can come to a lower level to emit a photon - when dropped, the e- becomes relaxed

Hydrogen Absorption and Emission spectrum

Absorption = dark bands on a bright background(absorb, so black line)

Emission = bright bands on a dark background(emit, so bright lines)

The Energy of a photon is related to…

• its wavelength(λ lambda) and frequency(f)

  • E = hf = hc/λ → Wavelength and frequency are inversely related - when f is high, λ is low, E is high; when f is low, λ is high, E is low

E- exists in 3D orbitals and 4 quantum number describe their structures

orbitals are the areas around the nucleus where an e- is most likely to be found)

• s, p, d, f → s being lowest in E and F in highest E

• ex. boron= 1s2 2s2 2p1 → goes by block and row of the periodic table

3 basic rules for Electron filling:

• Pauli principle: there can be no more than 2 e- in any given orbital(spin up and down)

  • e- cannot be the same, hence, different spins

• Aufbau principle: E- occupies the lowest orbitals first and is filled in increasing E

  • exception is 3d and 4s → 4s are removed before 3d

• Hund’s Rule: E- first occupy an orbital singly then pair up(no orbital left empty)

Anomalous electron configurations

when some elements prefer to be half-filled or filled by taking an e from 4s and putting it into 3d ex. Cr and Cu

Paramagnetic

at least one unpaired e-

Diamagnetic

all e- are paired ex. noble gases

Ground state e-configuration

the ground state is the lowest e configuration → correct amount of e as the element has

Excited state e-configuration

the element has jumped an orbital but it has not changed the amount of electrons as it originally had

A half-filled shell is…

more stable than one that isn’t; filled one is the most stable ex. noble gases

Valence shell configurations

determine the chemical reactivity of the elements → Elements in the same group show similar characteristics ex. noble gases as calm due to their octet shells, while halogens are reactive gases with 1 e-missing

Valence shell electrons experience electrostatic attraction due

• to the nucleus → shielding effect

  • force of electrostatic attraction is proportional to Zeff + C/r²

Atomic radius increases…

going down right to left

Ionization E increase…

going up from left to right

Electron affinity(negativity) increases…

going up from left to right

Electronegativity increases going…

up from left to right

• FON=ClBrISC=H