Compounds, Molecules, and Bonds
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MCAT Physical › Compounds, Molecules, and Bonds
Which of the following molecules contains the shortest bond between nitrogen and oxygen?
Explanation
This question is testing your ability to draw Lewis dot structures and your knowledge of how resonance effects bond length. The N-O bond with the greatest pi-bond character will be the shortest; thus, we are looking for a double- or triple-bond between nitrogen and oxygen.
Hydroxylamine (
The nitrite and nitrate ions both have a double bond between the nitrogen and oxygen, but also one or more single bonds between these elements. This means that, because of resonance, the N-O bonds in these molecules will be averaged and our average bond order will be somewhere between single and double. Nitrate will have one double bond and two single bonds, for an average bond order of 1.33. Nitrite has one single bond and one double bond, for a bond order of 1.5.
The nitrosyl ion, however, will contain a triple bonds between the nitrogen and oxygen atoms, giving it the greatest pi-bond character. This bond will contain more energy and be shorter than the bonds in the other answer options.
Which of the following molecules contains the shortest bond between nitrogen and oxygen?
Explanation
This question is testing your ability to draw Lewis dot structures and your knowledge of how resonance effects bond length. The N-O bond with the greatest pi-bond character will be the shortest; thus, we are looking for a double- or triple-bond between nitrogen and oxygen.
Hydroxylamine (
The nitrite and nitrate ions both have a double bond between the nitrogen and oxygen, but also one or more single bonds between these elements. This means that, because of resonance, the N-O bonds in these molecules will be averaged and our average bond order will be somewhere between single and double. Nitrate will have one double bond and two single bonds, for an average bond order of 1.33. Nitrite has one single bond and one double bond, for a bond order of 1.5.
The nitrosyl ion, however, will contain a triple bonds between the nitrogen and oxygen atoms, giving it the greatest pi-bond character. This bond will contain more energy and be shorter than the bonds in the other answer options.
Which of the following molecules will have the smallest bond angles?
Explanation
In order to determine which molecule will have the smallest bond angle(s), make sure to factor in both the number of atoms around the central atom as well any lone pairs on the central atom. 
Which statement best describes VSEPR theory?
The repulsion between electrons helps determine the geometry of covalent molecules
The repulsion between atoms helps determine the polarity of molecules
Molecular shapes are determined according to which orbitals are energetically accessible for bonding
Covalent bonds are formed by overlapping valence electron shells
The repulsion between protons in an atom's nucleus determines its size
Explanation
The idea of VSEPR (valence shell electron pair repulsion) theory is that valence electron pairs repel each other, arranging themselves into positions that minimize their repulsions by maximizing the distance between them. The positions of these electron pairs then determine the overall geometry of the molecule. Molecular geometry is thus determined by the arrangement of electrons and nuclei such that the electrons are as far from one another as possible, while remaining as close to the positively charged nucleus as possible.
Which of the following molecules will have the smallest bond angles?
Explanation
In order to determine which molecule will have the smallest bond angle(s), make sure to factor in both the number of atoms around the central atom as well any lone pairs on the central atom.
Which statement best describes VSEPR theory?
The repulsion between electrons helps determine the geometry of covalent molecules
The repulsion between atoms helps determine the polarity of molecules
Molecular shapes are determined according to which orbitals are energetically accessible for bonding
Covalent bonds are formed by overlapping valence electron shells
The repulsion between protons in an atom's nucleus determines its size
Explanation
The idea of VSEPR (valence shell electron pair repulsion) theory is that valence electron pairs repel each other, arranging themselves into positions that minimize their repulsions by maximizing the distance between them. The positions of these electron pairs then determine the overall geometry of the molecule. Molecular geometry is thus determined by the arrangement of electrons and nuclei such that the electrons are as far from one another as possible, while remaining as close to the positively charged nucleus as possible.
Which is not a property of covalent compounds?
The elements involved generally have large differences in electronegativity
They have low melting and boiling points
They are poor electrical conductors
Many are not soluble in water, but are soluble in nonpolar liquids
They contain bonds formed by sharing one or more pairs of valence electrons between atoms
Explanation
The incorrect statement is a property of ionic compounds rather than covalent. Recall that electronegativity is a measure of the ability of an atom to draw electrons to itself. Ionic compounds are formed by elements with very different electronegativities, since elements with different electronegativities will tend to form positive and negative ions (that is, they give away or gain electrons easily). In contrast, covalent bonds are formed by elements which are close in electronegativity and could exist as stable free molecules. All other statements are true of covalent compounds.
Which is not a property of covalent compounds?
The elements involved generally have large differences in electronegativity
They have low melting and boiling points
They are poor electrical conductors
Many are not soluble in water, but are soluble in nonpolar liquids
They contain bonds formed by sharing one or more pairs of valence electrons between atoms
Explanation
The incorrect statement is a property of ionic compounds rather than covalent. Recall that electronegativity is a measure of the ability of an atom to draw electrons to itself. Ionic compounds are formed by elements with very different electronegativities, since elements with different electronegativities will tend to form positive and negative ions (that is, they give away or gain electrons easily). In contrast, covalent bonds are formed by elements which are close in electronegativity and could exist as stable free molecules. All other statements are true of covalent compounds.
The geometry of a certain molecule with the general formula 
Explanation
Octahedral geometry always corresponds to the 
The geometry of a certain molecule with the general formula
Explanation
Octahedral geometry always corresponds to the