AP Chemistry Unit 1: Atomic Structure and Properties
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1.1 Moles and Molar Mass
Avogadro’s Number - 6.022 × 10²³
Named in honor of Avogadro, who is not the one who discovered it.
Molar Mass (Mass per Mole)
A substance’s molecular weight is defined by the ratio between mass and the amount of substance measured in moles.
1 mole is also Avogadro’s Number (6.022 × 10²³), the weight of an element on the periodic table, and 22.4L (only for gases at standard temperature and pressure or STP).
1.2 Mass Spectroscopy of Elements
Atomic Mass Unit (amu)
Equal to the mass of 1 proton or 1 neutron
Atomic masses on the periodic table are the average mass of all the isotopes and their percent abundance.
1/12 the mass of a carbon-12 atom, an atom that has 6 protons and 6 neutrons.
Isotopes
Elements with the same number of protons and electrons (same element) but
differing numbers of neutrons.
Isotopes have the same atomic number but different atomic masses.
Mass Spectroscopy
The most accurate method for identifying the amount and element present in a sample.
Mass Spectrum
Graph that shows the percent abundance of the ions of certain isotopes and their mass/charge.
X-Axis: Labeled as Atomic Mass (u) or Mass-To-Charge Ratio. Essentially, the mass number.
Y-Axis: Percent Abundance or the percent of the isotope in nature.
To figure out what element is shown in the graph, multiply each mass number by its percentage.
Example (very basic image below): ~0.5(79) + ~0.5(81) = ~80 amu, which would be Bromine.
1.3 Elemental Composition of Pure Substances
Empirical Formula
Simplest whole number ratio of each element in a compound.
Does not always show the actual number of elements in a compound.
Example: CH2O
Molecular Formula
Shows the exact number (mols) and types of elements present in a compound
Example: C6H12O6
1.4 Composition of Mixtures
Matter
Mixture
Pure
Substance
Homogeneous
Heterogeneous
Compound
Element
Matter: Anything that has mass and occupies “space.”
Mixtures: Physically made up of 2 or more substances
Homogeneous Mixtures (A Solution)
Composed of 2(+) substances and is Uniform in Composition (made with the same thing throughout)
Usually inseparable
Heterogeneous Mixtures
Composed of 2(+) elements or compounds and is Ununiform in Composition (not made with the same thing throughout, like air)
Usually separable
Pure Substance: Has consistent composition and properties
Element: A substance that cannot be broken into simpler substances
Compound: Two or more elements chemically combined that can be broken into simpler substances
Separating Mixtures (Explained in depth in 3.9)
Physical changes that can be made to separate mixtures into pure substances:
Distillation
The process of separating two or more liquids in a solution through vaporization of the liquid with the lowest boiling point, then condensing it back to its pure liquid state. This would separate the liquid with the lowest boiling point from the rest of the component(s)
Filtration
This method is used for mixtures that contain undissolved (insoluble) solids in the liquid. The solids are left on the filtering agent after filtering the mixture through a filtering agent (e.g., mesh).
Chromatography
Method of separation where components are distributed between two phases: mobile and stationary.
1.5 Atomic Structure and Electron Configuration
Coulomb’s Law: Calculates the force between particles
F= Electric Force between the nucleus and an electron
k = Coulomb’s Constant
q1, q2 = Charge of nucleus and electron (respectively)
r = Distance between the nuclei of the nucleus and electron
For AP, memorizing the formula is not required. Just understand it conceptually.
The least energy required to remove a valance election.
On the PES graph, the x-axis labels the ionization energy. It starts, for example, at 1000 eV, and as you move to the right, it decreases to, for example, 1eV. This means as you move to the right, it requires less energy to remove an electron.
Photoelectron Spectroscopy (PES)
Peak heights on the PES graph correspond to the relative number of electrons in each sublevel.
Shows the different orbital levels and electron configuration
X-Axis: Labeled as Binding Energy (eV), basically the ionization energy, and is labeled backward. Example: It is 1000 near the y-axis and 1 at the end of the graph.
Y-Axis: Shows the relative number of electrons within a sublevel.
1.7 Periodic Trends
Ionization Energy
First Ionization Energy
The energy required to remove one electron from an atom in the gaseous state.
Second Ionization Energy, etc.
The energy required to remove the second electron from an atom
Moving → or ↑ (Ionization Energy Increases)
Protons are being added to the nucleus, which increases the attraction, making it harder to remove an electron.
Effective Nuclear Charge: An Explanation of Ionization Energy
Net positive charge experienced by valance electrons, reduced by the shielding effect of the other electrons.
Moving → or ↑ (Effective Nuclear Charge Increases)
Zeff = Z − S
Z = Number of Protons (Atomic Number)
S = Average number of shielding electrons
You do not need to know the formula for the AP Exam, but it will help you understand the nuclear charge.
Atomic Radius
The distance between the nucleus of an atom to its valence electrons in picometers (pm).
Moving → (Atomic Radius Decreases)
Protons are added to the nucleus, creating a stronger attraction to the nucleus and attracting valance electrons closer to the nucleus, thus decreasing the atomic radius.
Moving ↓ (Atomic Radius Increases)
The number of occupied electron shells increases, causing valance electrons to move further away from the nucleus and increasing the atomic radius.
Ionic Radius (Size of Ions)
Cations (+): Smaller than Anions
When electrons are removed, electron-electron repulsions decrease, allowing the valence electrons to move closer to the nucleus, decreasing the atomic radius.
Sometimes, ions can remove an entire electron shell, which significantly decreases Ion size.
Anions (-): Larger than Cations
When electrons are added, electron-electron repulsions increase, allowing the valence electrons to move further to the nucleus, increasing the atomic radius.
Electronegativity
The attraction for a shared electron by the nucleus of an atom across a bond.
Moving → or ↑ (Electronegativity Increases)
Protons are being added to the nucleus, which increases its positive charge, making the nucleus more effective at attracting electrons.
Since most noble gases do not usually form bonds, they have zero electronegativity, but in our case, they are just not discussed.
Electron Affinity
The energy released when an electron is added to a gaseous atom.
Moving → or ↑ (More Negative)
The more negative, the more energy is released (exothermic).
Metals tend to have positive Electron Affinities, and nonmetals tend to have negative Electron Affinities.
1.8 Valence Electrons and Ionic Compounds
Valance Electrons: Outermost electrons in an atom, the ones that form bonds.
We skip the transition metals because they are unpredictable, but most of them have two valance electrons, with there are some exceptions.
Ionic Compounds
Formed between a metal and a nonmetal that does not share electrons but instead transfers them.
