CHEM130 Midterm 1 (Chapters 1.1 -3.7)

Rounding When Adding/Subtracting

round the result to the same number of decimal places as the number with the least number of decimal places

Rounding When Multiplying/Dividing

round the result to the same number of sigfigs as the number with the least number of sigfigs

Rounding 5s

• if there are trailing non-zeros, then round up

• if it’s the last digit or it’s followed by trailing zeros, round up or down so that the last digit is an even number

Law of Conservation of Mass

mass cannot be created or destroyed in a chemical reaction

Law of Definite Proportions

a given compound always contains the same proportion of elements by mass

Law of Multiple Proportions

when two elements form more than one compound, a fixed mass of one element combines with varying masses of the other in simple whole-number ratios

Postulates of Dalton’s Atomic Theory

1. matter is composed of exceedingly small particles called atoms

2. an element consists of only one type of atom

3. atoms of one element differ in properties from atoms of all other elements

4. a compound consists of atoms of two or more elements combined in a small, whole-number ratio (law of definite proportions)

5. atoms are neither created nor destroyed during a chemical change, but are instead rearranged

Isotopes

atoms of the same element that differ in mass

Thompson and the Electron

found out that the particles in a cathode ray are…

• attracted by positive charges and repelled by negative charges (so they must be negatively charged)

• less massive than atoms and indistinguishable (so they must be subatomic constituents of all atoms)

• one gram of these particles possesses -1.76×10⁸ C

Millikan and the Electron

• using ionized oil droplets he was able to determine the charge on individual drops

• then he was able to determine the charge of a single electron as -1.60×10^-19

Nuclear Theory (Rutherford)

• by shooting a beam of high-speed, positively charged alpha particles at a very thin piece of gold foil

• he concluded that most of the atom is empty space, and the center (nucleus) contains most of an atom’s mass and all of its positive charge

Extensive Property

a physical property of a substance that depends on the amount of matter present in a sample

Intensive Property

a physical property that does not change regardless of the amount of matter present

Precision

when all outcomes are in the range of uncertainty of one another (outcomes are all similar)

Accuracy

when the average outcome is in the range of uncertainty of where it should be (outcomes are close to the true value)

Uncertainty of a Value

the last digit of a measurement is uncertain, and its place value is used in the uncertainty (eg. 80.2 g has an uncertainty of ± 0.1g)

Empirical Formula

indicates the types and number of atoms present in their simplest whole number ratio (eg. C4H5N2O)

Molecular Formula

a representation of a molecule that uses chemical symbols to indicate the types of atoms followed by subscripts to show the number of atoms of each type in the molecule, doesn’t need to be the simplest form (eg. C8H10N4O2)

Structural Formula

gives the same information as its molecular formula but also shows how the atoms are connected in the molecule

Isomer

compounds with the same chemical formula but different molecular structures

Structural Isomerism/Constitutional Isomerism

compounds in which the molecules differ in how the atoms are connected to each other

Spatial Isomerism/Stereo Isomerism

compounds that have the same atomic connectivity but differ in the three-dimensional orientation of their atoms in space

Molar Mass/Formula Mass

the mass of 1 mole of the atoms of an element (numerically equivalent to atomic weight in amu)

Photoelectric Effect

the emission of electrons from a metal when electromagnetic radiation (stream of photons) is incident on it

Threshold Frequency

the minimum frequency of incident light for electrons to be emitted (below this frequency, no electrons are emitted)

Threshold Energy

minimum energy needed for electrons to be emitted from a metal (same as the work function)

Frequency and the Photoelectric Effect

increasing the frequency of the light above the threshold frequency increases the KE of the emitted electrons

Intensity and the Photoelectric Effect

increasing the intensity of the light increases the rate of emission (# of electrons) because it increases the number of photons incident per second

Wave-Particle Duality

light acts as both a particle and as a wave (the photoelectric effect is proof of the particle nature of light and the double-slit interference pattern is proof of the wave nature of light)

The Bohr Model (only for single-electron systems)

an electron orbiting a nucleus would not normally emit radiation, but emit or absorb photons if it moved to a different orbit

Energy/Wavelength/Frequency of Emitted Photon

Ef - Ei= hc/λ

Energy of an Electron in an Orbital

En=– kz²/n²

de Broglie wavelength

similar to the f = v/λ equation, but instead applies to a particle that exhibits the behavior of a wave

Heisenberg Uncertainty Principle

there is a fundamental limit to how accurately one can measure both a particle’s position and its momentum simultaneously (the more accurately we measure the momentum of a particle, the less accurately we can determine its position at that time, and vice versa)

Atomic Orbital

a general region in an atom which an electron is most probable to reside

Atomic Orbital Nodes

areas where you will not find an electron (areas between each orb of the orbital)

Principal Quantum Number (n)

defines the general size and energy of the orbital, exists as an integer

Angular Momentum/Secondary Quantum Number (l)

defines the shape/type of the orbital, exists as an integer only at values from 0 to n-1

l = 0 orbital

s-type orbital

l = 1 orbital

p-type orbital

l = 2 orbital

d-type orbital

l = 3 orbital

f-type orbital

Magnetic Quantum Number (ml)

# of ml values allowed is equal to the number of variants of that orbital, exists as an integer only at values from -l to l

Spin Quantum Number (ms)

describes an intrinsic electron “rotation”, can only be 1/2 or -1/2

Pauli Exclusion

no two electrons in the same atom can have exactly the same set of all the four quantum numbers

Radial Nodes

areas where the probability of finding an electron drops to zero

Calculating Radial Nodes

n - l - 1 (n level - angular momentum quantum number - 1)

• if this value is negative then you are NOT ALLOWED to have an orbital at that n level

Degenerate Orbitals

atomic orbitals within the same subshell, with the same energy but different spatial orientations

Aufbau Principle

each added electron occupies the subshell of lowest energy available

Hund’s Rule

we prefer to fill unoccupied orbitals before pairing electrons in occupied orbitals

Why is the first electron in an orbital upwards spinning?

electrons with an upwards spin have slightly less energy than electrons with a downwards spin, thus the first electron in an orbit will be upwards

Valence Electrons

electrons occupying the outermost shell orbitals (highest value of n)

Core Electrons

the inner electrons belonging to a completed sublevel

Covalent Radius

half the distance between the nuclei of two identical atoms when they are joined by a covalent bond

• decreases as atomic number (Z) increases in a period

• increases as atomic number (Z) increases in a group)

• BUT for transition metals, across a period the radius stays relatively constant

Bond Length/Internucleus Distance

represents the most stable separation of bonded atoms (two times the covalent radius)

Atomic Radius

the distance from the nucleus to the outermost electron shell of a single atom

Ionic Radius

a measure used to describe the size of an ion

• for cations radius decreases

• for anions radius increases

Ionization Energy

the energy required to remove the most loosely bound electron from an atom in its ground state

• as size (atomic radius) increases, ionization energy decreases

• ionization energy increases with successive electron removal

Electron Affinity

the energy change for the process of adding an electron to a gaseous atom to form an anion

• as you move up a group and across a period, the electron affinity generally decreases (larger negative values)

Shielding

the extent to which electrons protect others from the positive charge from the nucleus

• core electrons shield valence electrons very effectively

• valence electrons don’t shield other valence electrons well

Conventions for Creating Cations

• main group elements: remove the electron with the highest energy

• transition metals: remove the electron from the orbital with the highest n-value

Monoatomic Ion

an electrically charged particle composed of a single atom

Polyatomic Ion

a group of two or more atoms held together by covalent bonds that has an electrical charge

Oxyanions

polyatomic ions composed of one or more oxygen atoms covalently bonded to a central atom, which is typically a nonmetal, resulting in a net negative charge