Molecules and Compounds - GRE Subject Test: Chemistry

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.

2-butyne has the following chemical structure.

The end carbons have hybridization (form single bonds only), while the middle two carbons have hybridization (involved in a triple bond). There are no hybridized carbons in this molecule.

The degree of unsaturation is equal to the number of rings plus the number of pi bonds in a molecule. Ephedrine has one ring and three pi bonds, so its degree of unsaturation is four.

To arrive at this answer, one could also use the formula below, where is the number of carbon atoms, is the number of hydrogen atoms, is the number of halogen atoms, and is the number of nitrogen atoms.

For ephedrine, , , , and .

is an ionic compound, while is a nonpolar covalent compound. Remember that polarity results from a difference in the electronegativities of the atoms involved in the bond. Too great of a difference will result in an ionic bond; two of the same atoms will have zero difference, resulting in a nonpolar bond.

, , and contain polar covalent bonds. In the first two, oxygen will carry a slight negative charge, leaving sulfate and phosphorus with slight positive charges. In , chlorine will carry a slight negative charge, leaving the carbon slightly positive. In and , symmetry helps to balance the polar bonds, resulting in an overall nonpolar molecule, even though the individual bonds are nonpolar.

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 () only contains single bonds, which have the least pi-bond character.

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.

A sigma bond is a single covalent bond, involving an electron pair located between the two bonding atoms. A pi bond occurs when the p orbitals above and below the bonding atoms overlap, or when the p orbitals to the left and right overlap. In any covalent bond, the first bond formed is a sigma bond and any additional bonds must be pi bonds. Initial orbital overlap always comes from the sigma, or s, subshell; subsequent overlap comes from the pi, or p, subshells.

Carbon Ais hybridized because this carbon is only bonded to two other atoms. Carbon B is bonded to four atoms, and would therefore be hybridized.

Keep in mind that a carbon involved in a triple bond will always be hybridized, a carbon involved in a double bond will be hybridized, and a carbon involved only in single bonds will be hybridized.

Electronegativity of a chemical bond can be determined by calculating the electronegativity differences of the elements in a chemical bond. For example, using the values in the table given, the electronegativity differences between the bond are given below:

Based on the calculations above, we can see that the bond has the highest difference and therefore has the highest electronegativity.

A molecule is capable of hydrogen bonding when a hydrogen atom is attached to a nitrogen, oxygen, or fluorine atom in the molecule. Methanal consists of a central carbon bound to two hydrogen atoms and a double bond with an oxygen atom. The hydrogen atoms are bound only to the carbon, and thus cannot form hydrogen bonds.

Hydrofluoric acid, ethanol, and ammonia are all capable of hydrogen bonding.

Hydrogen bonding occurs between a hydrogen atom and an atom that has a high electronegativity such as fluorine, oxygen, or nitrogen. In this case, the only compound that exhibits hydrogen bonding is water, which has a hydrogen bonded to an oxygen atom. Note that in most cases hydrogen bonding is intermolecular, but in some cases, hydrogen bonding can occur intramolecularly.

Covalent bonds are formed when two nonmetals are bonded together. This covalent bond means that the electrons are shared by the two atoms in order to satisfy each atom's octet. There is very little difference in the electronegativities of the two atoms involved in the bond, so neither atom pulls the electrons closer to its nucleus.

Ionic bonds are formed between a metal and a nonmetal. Due to the dramatic difference between the electronegativities of metals and nonmetals, the electrons are pulled tightly to the nonmetal, and away from the metal nucleus. This results in each atom having a full octet, even though the electrons are not shared.

Carbon and oxygen are both nonmetals, so we would expect only covalent bonds in carbon dioxide.

In order to determine the charge of the transition metal manganese in the molecule, , we must first determine the net charge of the molecules it is bonded to. The manganese is bonded to three fluoride ions. Fluoride ion carries a negative one charge. since it is a halogen The subscript in front of the fluoride in the molecule tells us that we have three fluoride ions. Each fluoride ion carries a charge and because we have 3 of them, there is a total charge of from these fluoride ions. Molecules like to exist in their most stable state which gives them an overall charge of zero. Therefore, the manganese atom will carry a charge of to counter the charge from the three fluoride ions.

An ionic bond is a bond that occurs between a metal and a nonmetal, which may dissociate into two ions of opposite charges (positive and negative). These bonds are formed by electrostatic forces. We can determine if a compound is ionic from what it is composed of. Ionic compounds can easily be identified because they are generally composed of a metal and non-metal such as . Metals are cations and non-metals are anions. In the options given, the substance that contains an ionic bond is . The ions involved are and .

In order to determine the charge of the barium ion in the molecule, , we must first determine the net charge of the molecules it is bonded to. The barium ion is bonded to two fluoride ions.

Fluoride ion carries a -1 charge. The subscript in front of the fluoride in the molecule tells us that we have two fluoride ions. Each fluoride ion carries a -1 charge and because we have 2 of them, there is a total charge of -2 from these fluoride ions. Molecules like to exist in their most stable state which gives them an overall charge of zero. Therefore, the barium atom will carry a charge of +2 to counter the -2 charge from the two fluoride ions.

In order to determine the charge of the ion in the molecule, , we must first determine the net charge of the molecules it is bonded to. The ion is bonded to a ions.

ion is one a common anion and carries a -2 charge. Molecules like to exist in their most stable state which gives them an overall charge of zero. has an overall charge of zero. Therefore, the atom will carry a charge of +2 to counter the -2 charge from the ion.

Boiling point is highly dependent on the intermolecular forces of a compound. Compounds with stronger intermolecular forces, larger masses, and less branching will have higher boiling points.

Compounds II and III only exhibit intermolecular London dispersion forces, so they would be the two lowest boiling compounds (weakest intermolecular forces). Because compound III has more branching, these London dispersion forces would be weaker, resulting in a lower boiling point than compound II.

III < II

Compounds I and IV would be higher boiling point compounds because of additional hydrogen bonding (strong intermolecular forces). Compound IV would be the highest boiling because the hydroxy group and carboxylic acid group could BOTH participate in intermolecular hydrogen bonding. In addition, compound IV is more polar (more polarized carbon-oxygen bonds), resulting in greater dipole-dipole attraction as well.

III < II < I < IV

Intermolecular forces become relevant when there are partial charge differences between atoms in the molecule, generating polarized bonds. In order for this to happen, the two bonded atoms must have different electronegativities such that one atom pulls the electrons in the bond closer to it.

In liquid bromine , the two bromine atoms have the same electronegativity, so there is no unequal sharing of electrons. As a result, only London dispersion forces are found between bromine molecules.

Ethanol and ammonia are capable of hydrogen bonding, while acetone is capable of dipole-dipole interactions due to the polarized carbon-oxygen bond.

Water is the most polar solvent. Polar molecules contain bonds that have charges that are opposite in charge with high electronegativity differences. The oxygen atom in water draws electrons away from the hydrogen atoms causing the oxygen to be more negatively charged and hydrogen atoms to be more positively charged. Water would not be polar without its bent geometry which allows it to have a non zero dipole moment (1.85D), making it a polar molecule. The polarity of water is essential for life as we know it.

Two solvents are miscible if after mixing them, they form a homogeneous mixture. The saying, like dissolves like, is an explanation of why benzene which is non polar will dissolve in a non polar solvent such as pentane. Polar molecules are soluble in polar solvents. For example, water is miscible in alcohols. Ionic substances such as common table salt which is composed of sodium chloride dissolve in water but do not dissolve to any great extent in most organic solvents which are non-polar. If we were to mix the benzene solution with water, we will find that they are not miscible and form a heterogenous mixture. Methanol , is the only polar solvent in the options given and is miscible with water.

When drawing a Lewis dot structure, we are always trying to reach an electron count where all atoms involved are stable and (usually) have full octets. We are also trying to estabilsh a structure in which we have the smallest formal charge possible. The general rule is first to draw out all of the elements involved and their valence electrons, then start piecing them together trying to reduce the formal charge and get all elements involved to an octet. There are a couple exceptions to the octet rule.

Sodium and chlorine form an ionic bond, meaning that one atom will donate an electron and the other will receive it. This gives each atom a charge. Chlorine has seven valence electrons, while sodium has one valence electron. For each atom to arrive at an octet, sodium will need to lose one electron and chlorine will need to gain one electron. This would give chlorine a negative charge, and sodium a positive charge.

Thus, the answer is a sodium with a positive charge (due to one lost electron) and a chlorine with eight electrons and a negative charge (due to one electron gained).