Compounds, Molecules, and Bonds - MCAT Chemical and Physical Foundations of Biological Systems

An ionic bond is an electrostatic force between a positively charged cation (formed when electrons are lost) and a negatively charged anion (formed when electrons are gained). While one answer choice mentions positive and negative, it is actually a description of a polar covalent bond. The easiest way to recognize this is the fact that it specifies a bond between two nonmetals. A metal in generally a component of an ionic compound.

Ionic compounds are made of atoms that are held together by ionic bonds. These bonds allow charged particles that have very different electronegativities to stick together, forming 3D crystals. For example, potassium and bromine form potassium bromide, an anti-convulsant. The electrons are shared very disproportionately between the positive and negatively charged particles, with the all the electrons located on the more electronegative atom. Ionic compounds are not non-polar because non-polar is used to describe a molecule whereas ionic compounds are not molecules. Water is an example of a polar molecule but is not an ionic compound since The hydrogens and oxygen share their electrons.

Note how this question has little to do with the passage. Often you will see questions like this on the MCAT. This is a simple, straightforward vocabulary question. The bonds between each compound are covalent. One might get confused and misunderstand the question to read inter-molecular bonds—those between different molecules—as opposed to the bonds within a molecule.

The oxygen atom in water has two lone pairs, though they are not as often drawn into depictions of the molecule as they are in nitgrogenous molecules, such as ammonia. They, along with the two bonds formed with hydrogen in stable water, make up the satisfied octet of oxygen.

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 suffix -yne tells us that this is an aklyne and contains a carbon-carbon triple bond. Ethyne has the stongest carbon-carbon bond because it has the shortest bond length. It is important to remember that as the bond length gets shorter, the bond strength increases, and vice versa.

O2 has a nonpolar covalent bond. Nonpolar covalent bonds are bonds formed between atoms that have the same (or nearly the same) electronegativity. Since both oxygen atoms have the same electronegativity, they will have a nonpolar covalent bond between them.

The intramolecular bonds within a water molecule are polar covalent bonds. O-H bonds are polar covalent since there is more electron density around the oxygen atom than there is around the hydrogen atom, making the oxygen atom slightly negative and the hydrogen atom slighty positive. This polarity allows for intermolecular hydrogen bonding; water does not exhibit intramolecular hydrogen bonding.

This bond is not ionic since the oxygen does not completly steal the electron away from the hydrogen (the electronegativity difference between the two atoms is between 0.4 and 1.7 on the Pauling scale).

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.

Covalency is a form of electron sharing that lets an atom fulfill the octet rule. The sharing may be unequal, in which case the more electronegative species more strongly attracts the electrons than the weaker, less electronegative species, creating a polar covalent bond. In the Lewis dot model, however, we draw the electrons in freeze-frame, equally distributed between the various atoms.

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.

Of the answers, only H2O has a net dipole moment, making water the polar molecule. All the other molecules have balanced structures and no difference in electronegativity between side groups.

The strong polarity of water relative to hydrocarbons means that water will have a more difficult time breaking out of its liquid phase, and into its gas phase to generate a vapor pressure. Substances with a high vapor pressure generally have weaker intermolecular bonds and a lower boiling point.

The question is asking for intramolecular hydrogen bonds, meaning which of the following molecules will contain hydrogen bonds between the atoms within a single molecule. Hydrogen bonds exist only between a hydrogen and a nitrogen, oxygen, or flourine. Although acetone and dimethyl ether contain an oxygen that can make hydrogen bonds, the molecules themselves do not contain hydrogen bonds. These compounds form intermolecular hydrogen bonds only.

Para-Nitrophenol, similarly, will form intermolecular hydrogen bonds. The para positioning of substituents prevents them from interacting within a single molecule. Ortho-nitrophenol allows for such itneractions by having substituents on adjacent carbons. The hydrogen of the phenol and the oxygen of the nitro will form a hydrogen bond within a single molecule, therefore, ortho-nitrophenol is the only molecule present that contains intramolecular hydrogen bonds since it can form hydrogen bonds within itself.

Polarity is determined by differences in electronegativity between the two atoms involved in a bond. A large difference in electronegativity will result in a more polar compound. Symmetry, however, can balance net polarities in bonds, can cancel the differences.

has tetrahedral geometry, and since the four groups attached to the central silicon atom are identical, this molecule has no net dipole moment due to its symmetry. It is the most non-polar compound.

In comparing and , recall that carbon, nitrogen, and oxygen are in the same row of the periodic table, but carbon is farther from oxygen than nitrogen. This means there is a greater electronegativity difference in than , and is going to be more polar.

Finally, is an ionic compound, so it is going to be the most polar of all four compounds.

In this example, methane, , has the lowest molecular weight. Hydrocarbons that have the lowest molecular weight have the least opportunity for van der Waals forces to keep them from moving into the gaseous state; thus, they have the greatest tendency to form vapor and have the greatest vapor pressure. Recognize that the greater the intermolecular forces, the higher the boiling point and lower the vapor pressure.

Sodium chloride is a solid salt. Solids do in fact have vapor pressures, but the ionic structure of this salt makes it very low.

Melting points of hydrocarbons are determined by two main factors: length of the carbon chain and degree of saturation. Longer carbon chains will have higher melting points, and chains with more saturated bonds have higher melting points.

Of the given answers, has the longest carbon chain and is fully saturated. It will thus have the highest melting point.

"Inter-" means between and "intra-" means within. Intermolecular forces are forces that exist between separate molecules and intramolecular forces exist within or inside a single molecule.

Remember that boiling point and melting point depend on the intermolecular forces, not intramolecular forces. Boiling and melting involve separation of molecules from one another. You will need a higher boiling and melting point if the forces between the molecules (intermolecular forces) are strong. The strongest form of intermolecular force is the hydrogen bond. Hydrogen bonds are intermolecular forces that occur between a hydrogen atom in one molecule and a nitrogen, oxygen, or fluorine atom on another molecule. The molecule with the highest boiling point will have the greatest amount of hydrogen bonds, since this would result in the strongest intermolecular interactions.

Covalent bonds are bonds within the molecule (intramolecular forces) and do not have any effect on the boiling point. Hydrogen bonds can form within a molecule, generating intramolecular forces, but will only do so under certain conditions. The formation of intramolecular hydrogen bonds will not affect boiling point unless the ability to form intermolecular hydrogen bonds is inhibited by this process.