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Chapter 7 Knewton Coursework
Terms in this set (34)
A Lewis symbol consisting of an S and 8 dots must be:
the sulfide anion
Sulfur typically has six electrons, so if there are eight in the Lewis symbol, it must have gained two electrons. Thus the Lewis symbol represents the sulfide anion.
Which of the following is a step in creating a Lewis structure?
Place all remaining electrons on the central atom.
Draw a skeleton structure of the molecule around the central atom.
Rearrange electrons of the outer atoms to make multiple bonds with the central atom.
Which molecule will have a double bond?
In H2CO, carbon and oxygen form a double bond in order to satisfy the octet rule. In CO, carbon and oxygen must form a triple bond to satisfy the octet rule. In OF2 and CCl4, there are only single bonds.
A Lewis symbol with no dots must be a:
Lewis symbols show the valence electrons, so if a Lewis symbol has no dots, the atom must have lost its valence electrons and will therefore be a positively charged cation.
The Lewis symbol for the chloride ion will have how many dots?
If a chlorine atom gains an electron to become the chloride ion, it will have 8 valence electrons and a full valence shell. This means its Lewis symbol will have 8 dots.
Which molecule has the most lone pairs?
The fluorine atom will have three lone pairs, whereas water has two, ammonia has one, and methane has none.
When placing the first electron in a Lewis symbol, it must go:
it doesn't matter
All the coordination sites are identical, so it doesn't matter where the first electron is placed.
What is the lewis structure for H3O+?
We count valence electrons (1(6)+3(1)−1=8) and draw a skeleton with bonds to the central atom (hydrogen is never the central atom). We cannot distribute electrons to the terminal atoms, so place the remaining electrons on the central atom.
Which of the following is an exception to the octet rule?
Hydrogen only needs two electrons in order to fill its valence shell, so it is an exception to the octet rule.
We can predict the number of bonds an atom can form from:
the number of electrons needed to form an octet
We can predict the number of bonds an atom can form from the number of electrons needed to form an octet.
How many lone pairs will be present on the iodine atom in ICl−4?
2 lone pairs
If the negative charge is given to the iodine, it will have eight electrons to work with. Four will go to the four covalent bonds to the chlorine atoms, and the remaining four will exist as two lone pairs on the iodine atom.
When drawing a Lewis structure for a free radical, which type of atom is more likely to receive the unpaired electron?
the less electronegative element
A more electronegative element is more likely to be surrounded by more electrons and fulfill its octet. A less electronegative element is more likely to have the electron deficiency, in this case the unpaired electron. Recall that free radicals are electron deficient and display the same stabiilty trends as electron deficient cations, like carbocations.
Which of the following types of molecule is an exception to the octet rule?
all of the above
all of the above
Electron-deficient molecules will react with:
molecules with lone pairs
An electron-deficient molecule will react with electron excess, namely a lone pair from some other molecule, like ammonia.
How many bonds would carbon need to form to obtain an octet?
Carbon would need 4 bonds to form an octet because it already has 4 electrons.
In a Lewis structure involving iodine and fluorine, if the central iodine atom has exactly one lone pair, how many fluorine atoms must be present?
5 fluorine atoms
Iodine has 7 valence electrons, and since it is a fifth period element, it can exceed the octet rule when it binds to a more electronegative element like fluorine.
Fluorine also has 7 valence electrons, but cannot exceed an octet because it is in the second period. Each fluorine atom can therefore only form a single bond to the central iodine atom, and each single bond between iodine and fluorine accounts for 1 of iodine's valence electrons.
Since there is a lone pair (or non-bonding pair) of electrons on the iodine, that means precisely 2 of the 7 valence electrons are not shared with a fluorine atom, and the remaining 5 are each shared with 1 fluorine atom, so there must be 5 fuorine atoms surrounding central iodine atom.
How many electrons does hydrogen need to fill its outermost shell?
Hydrogen has an outermost shell of n=1, which can only hold two electrons.
In drawing the Lewis structure for ICl−4, we can classify ICl−4 as which type of molecule?
When drawing the lewis structure for ICl−4, we count up the valence electrons (7+4(7)+1=36) and add the octets of electrons to the outer atoms (8 electrons to each of the 4 Cl=32 used electrons), which gives us a remaining 4 electrons (36−32). These 4 electrons are then added to the central atom (I). This is a hypervalent molecule.
The octet rule predicts that carbon will tend to make ____ bonds.
With four valence electrons, carbon will gain an octet by making four covalent bonds.
For which of the following do we make multiple bonds with the central atom in order to obtain octets wherever possible?
all of the above
Odd-electron, hypervalent, and electron-deficient molecules all have us rearrange bonds to make multiple bonds to the central atom if possible.
Which of the following values for the principal quantum number corresponds to elements that can accomodate more than four pairs of electrons with other atoms?
Because elements in the 3rd and higher periods (empty d orbitals) have more than 4 valence orbitals, they can share more than 4 pairs of electrons with other atoms.
How many resonance forms exist for the formate ion, HCO−2?
In this case, the negative charge can be on one of two oxygens, so there will be two resonance forms, one per oxygen.
What is the charge on the carbon atom in carbon monoxide?
There must be a triple bond between the carbon and the oxygen, and the carbon also has a lone pair, meaning the carbon contributes five electrons to the Lewis structure. That's one more than its typical valence, so it will bear a formal negative charge.
What is true about the covalent bonds in the nitrite ion, NO−2?
they are of equal length
Even though each resonance structure shows one single bond and one double bond, these are actually of the same length in the actual molecule.
If a formal negative charge must be present on a molecule, which element would be preferred to accommodate it?
The negative charge will prefer to be on the most electronegative atom possible, and of the above options, that would be oxygen.
Which of the following statements about deciding between resonance structures with formal charges is correct?
The molecular structure where all formal charges are zero is preferred.
The structure that is preferred is the one where all formal charges are zero. This represents the least separation of charge.
The sum of formal charges of all atoms in an ion should always be:
The sum of the formal charges in an ion should equal the charge of that specific ion.
All of the following statements concerning resonance are true EXCEPT:
A molecule described by a resonance hybrid possesses an electronic structure equal to that of the most likely resonance form.
It is false that a molecule described by a resonance hybrid possesses the electronic structure described by the more likely resonance form. Instead, the resonance hybrid is the average of all the resonance forms, it does not represent one form over another.
The distribution of electrons for each bond in a molecule containing one single bond and one double bond is equal to:
The actual distribution of electrons in a molecule containing one single and one double bond is the average of the single and double bond, equal to 1.5 bonds.
If a nitrogen atom has four covalent bonds, what will be its formal charge?
If a nitrogen has four bonds, it will contribute four electrons to the Lewis structure, which is one less than its typical valence of five. That means a formal positive charge.
Which of the following is true?
The overall formal charge in a molecule will always equal 1.
An atom's formal charge is always equal to its actual charge.
In covalent molecules, formal charges are hypothetical.
all of the above
In covalent molecules, formal charges are hypothetical.
In a molecule or ion containing only covalent bonds, the formal charge of an atom is the hypothetical charge that the atom would have if electrons could be redistributed evenly between atoms. In neutral covalent molecules, formal charges are always a formality—each of the atoms contained therein are electrically neutral. Conversely, in covalently bonded ions (which carry an overall charge), the formal charge of a given atom may be the actual charge on the atom, but often is not.
Which molecule will have a formal charge?
Carbon will typically want to make four bonds, so with only three, it must be contributing a different number of electrons than its valence, and will thus have a formal charge.
For a polyatomic ion, the sum of the formal charges present is equal to:
the charge of the ion
The sum of the formal charges on each atom is equal to the charge of the ion.
Which atom will bear the negative charge in the thiocyanate ion, SCN−?
The best Lewis structure for this molecule will leave the negative charge on the nitrogen atom, which will have a double bond to carbon, which has a double bond to sulfur.
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