Unit 1 & 2 (Atoms & Compounds)

Question 1

Law of definite proportions

Answer 1

A compound is always composed of the same elements in the same proportion by mass

Question 2

List the general order of accuracy for measuring equipment.

Answer 2

Burets are the most accurate, then graduated cylinders. Volumetric flasks are only accurate for measuring a specific volume

Question 3

Mass spectrometry

Answer 3

The mass spectrum of a sample containing a single element can be used to determine the identity of the isotopes of that element and the relative abundance of each isotope in nature

Question 4

Hund’s Rule

Answer 4

Electrons always enter an empty atomic orbital or subshell before pairing up

Question 5

Aufbau Principle

Answer 5

Electrons fill up atomic orbitals or subshells of the lowest available energy before occupying higher ones (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p…)

Question 6

Pauli Exclusion Principle

Answer 6

No two electrons have the exact same four quantum numbers (their spins will always be in opposite directions)

Question 7

What do the positions and heights of each peak on a Photoelectron Spectroscopy (PES) spectrum represent?

Answer 7

Position of each peak is related to the energy required to remove an electron from the corresponding subshell. Peaks are in order of the subshells. The closer the electrons are to the nucleus, the more ionization energy will be required to remove the electrons.

Height of each peak is proportional to the # of electrons in that subshell

Question 8

How do you determine which element the PES spectrum corresponds to?

Answer 8

Write out the electron configuration based on the relative heights of the peaks

Question 9

Shielding

Answer 9

Shielding occurs when electrons in inner shells decrease the pull/attraction of the nucleus on electrons in outer shells.

The more electron shells there are, the greater the shielding effect experienced by the outermost electrons. EX: electrons in 4s orbital shield electrons in 4p orbital

This can reduce the nuclear charge to the effective nuclear charge

Question 10

Explain the properties of ionic compounds (4)

Answer 10

Relatively higher melting point and boiling point

Brittle because of the repulsion of like charges when one layer slides across another layer –> causes the crystal to shatter

Solid at room temp (atoms held close together)

Do not conduct electricity as a solid, but DO when molten or aqueous. This is because in solid form, the ions in the rigid crystal lattice are unable to move freely. In molten or aqueous form, ions are mobile, meaning they are able to flow and carry an electric current.

Ionic compounds that dissociate into more ions and thus are more soluble (MgCl2 vs NaCl at same concentration) conduct electricity better because more ions are in motion and can thus carry a stronger electrical current

Question 11

Describe Coulomb’s law (with both numbers and words)

Answer 11

F = k [(q1q2)/r^2]

Lattice energy increases with higher charge magnitude and lower ionic radii (or distance between the nuclei)

Question 12

Describe the structure of metallic compounds

Answer 12

A metal consists of a crystal lattice of positive particles (nucleus + core electrons) with a sea of delocalized valence electrons that can move freely throughout the lattice

Question 13

Describe the properties of metallic compounds (3)

Answer 13

Good conductors of electricity as solids (because of the sea of delocalized valence electrons, electrons can move freely throughout the lattice and thus can carry electric currents)

Ductile – able to be drawn into a wire

Malleable – able to be compressed without breaking
When stress is applied, the metal cores can easily rearrange their structure.

Question 14

Two types of alloys

Answer 14

Interstitial alloy: metal atoms with two vastly different radii combine (steel - small C atoms occupy the interstitial spaces between the larger Fe atoms)

Substitutional alloy: metal atoms with two similar radii combine (brass - Zn atoms are substituted for some Cu atoms)

Question 15

How do you make alloys harder or stronger?

Answer 15

Mix metals that have different sized atoms → it will make the atomic layers less regular and the atoms interfere more → more difficult for them to slide past each other

Interstitial alloys > substitution alloys

Question 16

Internuclear distance

Answer 16

Distance between the nuclei of two atoms (bond length)

Question 17

When does a covalent bond form? (in terms of potential energy)

Answer 17

A covalent bond will form at the point of minimum potential energy between the atoms (negative)

Atoms too close → positive max. PE (strongest repulsion/interaction)

Atoms too far → 0 PE (no bond, no attractions, no repulsions)

The more negative the potential energy, the stronger/more stable the covalent bond and the higher the bond energy

Question 18

Describe the axes of a potential energy graph vs internuclear distance graph

Answer 18

Y axis represents the bond energy (PE required to break this bond)

X axis represents the bond length

They are inversely proportional

Question 19

Why can some elements have expanded octets? (hypervalent)

Answer 19

Elements from n = 3 or greater can have up to twelve valence electrons because of empty d orbitals available and larger central atoms

Question 20

Formula for bond order of resonance structures

Answer 20

(# of total bonds in resonance) / (# of total locations for placement)

Question 21

Define and describe the VSEPR model

Answer 21

Electron domains tend to repel each other so they are as far apart from each other in 3D shape as possible

Lone electron pairs have slightly more repulsive strength than bonding pairs, so they will be pressed in more and the bond angle will be smaller

EX: Trigonal pyramidal is in a tetrahedral shape but the bond angle will be less than 109.5° because of the extra lone pair repulsion

Question 22

Formula for formal charge

Answer 22

(normal # of valence electrons) - (actual # of electrons)

Question 23

How can you tell which resonance structure is preferred, based on formal charge?

Answer 23

A formal charge of 0 is the more stable and preferred (while obeying the octet rule)

In a polyatomic ion, the sum of the formal charges on each atom is equal to the overall charge on the ion

If it’s not possible for each of the atoms to have charge of 0, the favored structure has the negative formal charge on the atom with the highest electronegativity value

Question 24

Define hybridization and list the types/products

Answer 24

Hybridization = combining multiple atomic orbitals

4 electron domains → sp3 (tetrahedral)
3 electron domains → sp2 (trigonal planar)
2 electron domains → sp (linear)

Question 25

Define sigma and pi bonds and then compare them.

Answer 25

Sigma bond: head to head overlap of participating atomic orbitals (electron density is concentrated in between the two atoms along the internuclear axis)

Pi bond: side to side overlap of unused p orbitals (electron density is concentrated perpendicular to the internuclear axis)

Sigma bonds are stronger and have greater bond energy than pi bonds; they can also rotate due to cylindrical symmetry about the bonding axis

Question 26

How do you tell if a molecule is polar or nonpolar?

Answer 26

In a nonpolar molecule, all the bond dipoles within the molecule (arrows pointing to the more electronegative atom) are oriented in such a way that they cancel each other out.

In a polar molecule, all the bond dipoles do not cancel each other out. This is present in structures with different atoms, such as CH2Cl2

Question 27

How do you tell if a bond is polar or nonpolar?

Answer 27

In a nonpolar bond, the electrons are shared equally, and the difference in electronegativity values is less than 0.5.

In a polar bond, the electrons are shared unequally, and the difference in electronegativity values is between 0.5 and 1.5.

Question 28

Can a molecule be nonpolar but have polar bonds? How about vice versa?

Answer 28

Yes, a molecule can have polar bonds but if the bond dipoles cancel out and the molecule is symmetric in regards to charge distribution, it will be nonpolar. EX: CO2 – carbon dioxide

Yes, a molecule can have nonpolar bonds and still be polar if the bond dipoles don’t cancel out.
EX: O3 – ozone

Question 29

Describe the properties of network covalent solids (2)

Answer 29

Atoms are covalently bonded in an extensive 3D network

Very hard and higher melting/boiling points (strong because of the great number of covalent bonds)

EX: Diamond, quartz SiO2

Question 30

Describe the properties of covalent molecular solids (3)

Answer 30

Composed of distinct, individual molecules that are attracted to each other through relatively weak intermolecular forces

Low melting points

Do not conduct electricity (because their valence electrons are held tightly within the covalent bonds of each molecule)

EX: Glucose, water

Question 31

What is a common characteristic of all resonance structures?

Answer 31

For all the structures, the bonds in resonance have the same strength/order and length. This is because electrons are shared equally between the bonds.

Question 32

What does it mean for the energy of electrons to be quantized?

Answer 32

Electrons can only exist at specific energy levels

Question 33

Gravimetric analysis

Answer 33

Determine amount by measuring mass (precipitate)