MCAT CHEM (gen and orgo)

Half-life problem: 1.5 half lives have passed, what is the concentration?

1 half life: concentration /2

0.5 half life= 1/sqrt 2 =0.707 MEMORIZE .7 (multiply this)

*Another easy way is to look at starting concentration, divide that by half see where that gets you on the x axis, and then see how much you need to add on based on that to get the second part of the half life then divide the answer to that by the concentration to get the concentration

Alpha Decay

2 protons + 2 electrons lost

→ mass # is -4

→ atomic # is -2

*Away with 4

Beta(-) decay

1 electron lost

→ mass stays the same

→ atomic # is + 1

→ goes from neutron to protron

Beta(+) decay

1 positron lost

→ Mass # is the same

→ atomic # is - 1

→ protron to neutron

Gamma Decay

High energy PHOTON comes out, mass and atomic # stays the same,

Radioactive Decay - where does it occur

Nucleus

Finding the amount of IONS present in a sample

Must use AVOGADRO’S NUMBER (6.02 × 10^23)

Mass number

Protons + Neutrons (Mass number + protons)

Valence Electron problem

Say it gives you 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p1

You would look for the highest energy level → 4

Count those electrons: 1 + 2 = 3

thats how many valence electrons are in that element

If you wanna see how many electrons are left if those are lost, find the number of protons because if its neutral then that number will be the same as the electrons, then you just subtract from that number

Ionic Radii Trend

Division between the metals and non metals

What do you look at when trying to compare two elements to see if they have similar chemical behavior?

You look at the number of VALENCE ELECTRONS, because those determine the chemical behavior.

Look at the same column, this will tell you same number of valence electrons and therefore the stability

Alkaline-Earth Metals (IIA)

2, Down, H2

• 2= forms +2 ions

• Down → reactivity increases down the group

• H2 → react with water to form hydrogen gas

• They are group 2 metals

Representative Elements

Elements OUTSIDE of the periodic table (middle groups 3-12 is just boring transition metals)

The elements that show the most predictable periodic trends and represent the major families of the periodic table

Groups 1,2,13-18

Group 1 → alkali metals

Group 2 → alkaline-earth metals

Group 17 → halogens

Group 18 → noble gases

Pauli Exclusion Principle

The “spin” principle

No two identical particles with half integer spin can occupy the same quantum state at the same time, also each orbital can only contain a maximum of 2 electrons (with opposite spin)

Nuclear Charge (nuclear decay)

Z eff - Z - S

Z= atomic number

S= shielding constant or number of core electrons → to find this, subtract the number of valence electrons based on what group it’s in

Ex. 18F vs 18O

Same # of core electrons bs they are both in period 2

Fluroine has one more proton, so it has a higher Zeff

Diamagnetic vs Paramagnetic

Diamagnetic: all electrons are paired in their orbitals (filled)\

Paramagnetic: contains at least one unpaired electron

Polarizability

the extent to which an electron cloud of an atom can be distorted by an external charge or by an applied electric field to produce a dipole

Electronegativity

Tendency of an atom to attract electrons within a bond

Electron affinity

assesses the tendency of an atom to accept an additional electron by measuring the energy change when an electron is added to an atom

Ionization energy

(opposite of electron affinity)

measures the energy required to REMOVE an electron from an atom

Which group on the periodic table has the LOWEST 2nd ionization energy?

Group 2 → first ionization removes the ns1, second will remove the remaining valence electron, which is relatively easy to remove

NOT group 1, it would have a LARGE 2nd ionization because after losing its first electron, it has a noble gas configuration and the second electron would have to come from the core

What volume does 1 mol of gas occupy at STP (standard temperature and pressure)

22.4 L/mol (using the ideal gas law)

Finding Mass Percent (%)

Mass of solute / total mass of solution *100

total mass will be solute + solvent mass

ethyl alcohol in water - mass of ethyl alcohol / mass of ethyl alcohol + mass of water

If a liquid is cooled

Its volume decreases

If an object is less dense than the surrounding fluid

It will float