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Flashcards in Chemical Bonding Deck (64)
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What are valence electrons, and how do they affect an atom's chemistry?

Valence electrons are the electrons in the atom's highest (outermost) energy subshells. They are chemically relevant because they are the electrons that form chemical bonds.


How would you quickly find the number of valence electrons by using the periodic table?

Valence electron number can quickly be read off of the periodic table as it will be the column number (counting from the left) that the element is in.

Ex: Nitrogen is in the 5th column from the left, so it must have 5 valence shell electrons. Nitrogen is 1s22s22p3n=2 is the outermost energy level. Add all of those electrons (2 from the s, 3 from the p) to get valence = 5.


What is the difference in valence electrons for: 
oxygen (O) vs silicon (Si)?

Oxygen has 6 electrons in the outermost energy level (2 in the 2s and 4 in the 2p).

Silicon has more electrons total, but it only has 4 valence electrons (2 in the 3s and 2 in the 3p). 


the octet rule

The octet rule states that atoms desire eight electrons in their valence shells, as this gives them the electron configuration of a noble gas.  

An atom will usually have to bond with other atoms to acquire this electron structure.


According to energy rules, what will cause two atoms to form a bond between them?

Chemical bonds form because they lower the potential energy of the electron clouds.  Electrons are shared between atoms across the bond, allowing the final state to be more stable than the two were alone.

In general: bonding proceeds so that as many atoms as possible gain a full octet of valence shell electrons.


What are the three possible types of interatomic bond?

  1. single bond: one pair of electrons is shared between two atoms
    Ex: O-H bond in H2O, or C-H bond in CH4
  2. double bond: two pairs of electrons are shared between atoms
    Ex: O=O bonds in O2, or C=O bonds in CO2
  3. triple bond: three pairs of electrons are shared between atoms
    Ex: N≡N bonds in N2, or C≡N bonds in CNOH


What are the three notable exceptions to the octet rule?

  1. odd-electron species. Molecules which have an odd total number of valence electrons to distribute (hence some atom will come up lacking)
  2. incomplete octets. Atoms where attaining a full octet would require too many bonds. H, He, Li, Be, B are all considered incomplete octet species due to the high number of electrons they would need.
  3. expanded octets. Atoms in period 3 or higher that can hold electrons in their d orbitals. The term is a misnomer, as this is no longer an octet, but will instead have 10, 12, or 14 electrons depending on the central atom being bonded.


Why does nitrogen not succeed in getting a full octet in nitric oxide (NO)?

The most electronegative atom will always gain a full octet first.

Nitric oxide has 11 valence electrons. Oxygen is more electronegative, so will end up with a full octet (two sets of lone e-pairs and a double bond with N). Nitrogen will be left with 3 lone electrons (and two bonds), as it is less electronegative, for a total of 7 electrons. 


Why will it be unlikely for Li to ever attain a full octet?

Lithium has only one valence electron. It would need to acquire 7 additional electrons in order to have a full octet. This is statistically (and structurally) unlikely.

H, He, Li, Be, B are all elements that will from incomplete octets due to the improbability of them acquiring enough electrons.


Why can phosphorous expand its octet to form 5 bonds to chlorine in PCl5?

Phosphorus can expand its five valence electrons into the 3d block, allowing it to bond to five chlorine atoms, completing those chlorine atoms' octets.

Elements in the third row of the periodic table and beyond (such as P, Cl, S, Xe, and Ar) often exhibit expanded octets of 10, 12, or 14 electrons.  This is due to them expanding into the d-block for greater bonding stability. 


What are the three major types of chemical bonds?

  1. ionic bonds: electrons are transferred from a metal to nonmetal
  2. covalent bonds: electrons are shared between nonmetals
  3. metallic bonds: electrons '"float" between metals in a lattice

Define and give an example of:

an ionic bond

An ionic bond is formed when a metal transfers one or more electrons to a nonmetal. Often in chemistry exams this will be metal from column I or II transferring electrons to a halogen. 

Ex: The classic example is an "ionic salt" like NaCl.  Sodium transfers its electron to chlorine.


How does Coulomb's law apply to ionic bonds?

Coulomb's law states that the magnitude of the electrostatic force between two charged particles is directly proportional to the product of the magnitude of each of the charges, and inversely proportional to the square of the distance between the two particles.

This holds for all charged particles, and can be applied to calculate force between the atoms in an ionic bond.

F ∝ q1*q2 / r2

  • q1 and q2: charge magnitude of the ions
  • r = distance between the ions 


Knowing that Na and Cl have a larger difference in electronegativity than K and Br, what can be predicted about the strength of the ionic bond in NaCl bond vs. that of KBr?

NaCl will have stronger bonds between adjacent atoms than KBr, due to the stronger electrostatic force between ions.


What would be the change in the force between ions in an ionic compound if the distance between adjacent ions is decreased by one-half?

The electrostatic force would be increased by a factor of 4, quadruple the original value. 

Forig∝ q1*q2 / r2
since new R = r/2 :
Fnew ∝ q1*q2 / R2
= q1*q2 / (r/2)2
= q1*q2 / (r2/4)
= Forig/(1/4) = Forig*4


lattice energy

The lattice energy of an ionic compound is the energy associated with forming a crystalline lattice of the compound from the gaseous ions.  This may also be referred to as "heat of formation".

Note: The value of lattice energy is negative, showing that the formation of an ionic compound is exothermic. 


What is the electrostatic energy of an ionic bond?

E ∝ q1*q2 / r
This holds for all charged particles, hence can be applied to calculate energy between the atoms in an ionic bond.

  • q1 and q2: charge magnitude of the atom
  • r = distance 

Note: the value of E will always be negative, due to the charges always being opposite in an ionic compound.


How does the energy of an ionic bond change if the distance between the two atoms in the bond doubles?

The energy will be decreased by a factor of 2, or half the initial value.  
Eorig ∝ q1*q2 / r
since new R = 2r :
Enew ∝ q1*q2 / R
= q1*q2 / 2r = Eorig/2


What type of bond will exist between atoms in molecules like KBr, CaF2, and LiCl?

These are all ionic compounds, commonly referred to as salts. Ionic bonds will generally form between metals and nonmetals.

Ex: Classic examples of ionic compounds will usually include a cation from column I or II bonded to an anion from the halogen family.


covalent bond

A covalent bond forms when a nonmetal bonds with another nonmetal by sharing electrons between them, resulting in an overlap of their electron orbitals.

The image below of methane shows the polar covalent C-H bonds. 


What are the three types of covalent bonds?

  1. polar covalent (electrons are held more closely by the higher electronegative species)
  2. non-polar covalent bond (electrons are perfectly shared between atoms of the same element)
  3. coordinate covalent bond (molecules share electrons for greater net stability)


What is the difference in bond strength between molecules in an ionic compound vs a covalent compound?

How will the compounds' melting points and boiling points compare?

Ionic bonds create stronger intermolecular forces than pure covalent bonds. Since ionic bonds are extremely polar (more so than ANY covalent bond, by definition), there is a strong electrostatic interaction between the ions.

The melting point and boiling point will both be higher for ionic compounds, as more energy is required to pull the molecules apart. 

Define and give examples of:

a nonpolar (or pure) covalent bond

In a nonpolar covalent bond, both atoms are the same element hence the electron pair is shared equally. On many exams like the AP Chem exam, pure covalent can only be the following diatomics: Br2, I2, N2, Cl2, H2, O2, F2.

Note: though some chemistry texts will also include bonds like the C-H bond (since it only measures .4 on the Pauling scale), this is clearly still polar since electrons favor carbon over hydrogen. 

Define and give examples of:

a polar covalent bond

In a polar covalent bond, the electron pair is pulled closer to the more electronegative atom. The result of this is a bond dipole (one end positive, one end negative), hence the term "polar". The atom that is more electronegative will carry a partial negative charge and the atom that is less electronegative will carry a partial positive charge. 

Examples of polar covalent bonds include O-H bonds, N-H bonds, and C-N bonds. 

Define and give an example of:

a coordinate covalent bond

In a coordinate covalent bond between molecules, one atom from one molecule will contribute both electrons to the bond pair. This creates better stability between both molecules.

Ex: Lewis Acid/Base pairs: NH3 (N donates the electron pair) and H+ (H+ accepts the electron pair). Nitrogen had a full octet to start with, but was very polar until donating the electrons to the bond with hydrogen; H+ didn't have any electrons, but now will have a full 1s subshell; both are now more stable. 
The first equation below is showing all of the actual valence electrons, the second equation below is the Lewis diagram representation.


Specifically, why do Lewis acids and Lewis bases generally combine to form coordinate-covalent bonded molecules, instead of common ionic salts?

By definition, a coordinate-covalent bond is sharing electrons from one molecule to another. Lewis acid/base pairs combine in exactly that fashion. 

A Lewis acid is a species that will accept an electron pair (ex: Boron in BF3). 
A Lewis base is a species that will donate an electron pair (ex: Nitrogen in NH3).


Describe the bond formed between two nonmetallic elements.

Nonmetallic elements form covalent bonds.

Nonmetals tend to have similar values of electronegativity, and thus share electron density fairly evenly when bound together.


Describe the bond formed between a metal and a nonmetal in a compound.

Metals-nonmetal bonds will be ionic.

The electronegative nonmetal withdraws one or more electrons from the electropositive metal, leaving each species ionically charged.


Describe the bonding for metals in their standard states.

A lattice of positively charged nuclei in a background "sea" of free-flowing negatively charged electrons.

The valence electrons of metals are loosely bound to the atomic cores, and can be considered to be effectively unattached. 


Explain why metals, or metallic bonded elements, are good conductors of electricity.

The "sea" of electrons are able to drift through the electron structure, moving from ion to ion gives these molecules the ability to conduct electricity.

In metallic bonding each metal atom in the crystal structure contributes valence electrons to form a "sea" of delocalized electrons.