Characteristics of Covalently Bonded Substances

Covalently bonded substances, or molecules, have significantly lower melting and boiling points than ionic compounds. If ionic compounds can be described as hard and brittle, covalent compounds can be described as soft and squishy.  Covalent compunds exist as gases, volatile liquids, or soft solids.  The state the substance is in depends on the bond energy of the substance.  If the bond energy is low, the substance is a gas.  If the bond energy is moderate, then the substance is a volatile liquid.  If the bond energy of the substance is very high, then it is a soft solid. This is because the atoms in a molecule aren't fixed in one place. They are able to move around and remain bonded. A good example of this is a playground ball pit. While the balls themselves are held together very tightly (like the molecules), the balls aren't stuck to each other.  Covalent compounds do not conduct electricity in water, electricity is conducted in water through the movement of ions from place to place. Covalent compounds also generally don't dissolve well in water. This happens because water is polar, and most covalent compounds are mostly non-polar. There are exceptions, of course. Molecular shape can affect a substance's properties. The shape of a molecule can affect its polarity. A molecule's (that has more than two atoms) polarity is determined by the polarity of each bond and the way each bond is arranged. In turn, the polarity of a molecule affects the properties of the molecule.

Characteristics of Ionically Bonded Substances

A substance that is ionically bonded is called an ionic compound. All ionic compounds share certain properties, such as high melting and boiling points. This is because the strong bonds between ions in an ionic compound prevent the ions from moving much without a large amount of energy. Because of the aforementioned high boiling point, ionic compounds are almost never found as a gas at room temperature. Ionic compounds contain charged particles, so they are able to conduct electricity. However, because ionic solids are in fixed locations, they are usually not good conductors of electricity. When ions can move, as in a liquid, salts are good conductors of electricity. If a solid salt is dissolved in water, it would then be a good conductor of electricity because its ions are no longer tightly packed together and are now able to move. There are some exceptions, though. There are a few ionic compounds who have an unusually open lattice structure, so ions can move past each other. These solids are good conductors of electricity. Most ionic compounds are hard and brittle. These qualities can be associated with the layered pattern of the cations and anions in salts. When a force is applied to the salt and a layer shifts, the cations of one layer are lined up with the cations of the next layer (the same happens with the anions). This causes them to repel each other and the layers to split apart. That is why all salts break along what is called a cleavage plane, or a line extending throughout the crystal. The ions in the salt form reoccurring patterns because each ion is held in place by attractive forces, which are stronger than the repulsive ones. These repeating units form the crystal lattice, and are the reason for the crystal shape found in most salts. The smallest repeating unit found in the crystal is called a unit cell. The shape of the crystal structure depends on the ratio of each element in the compound.

Multiple Bonds

In a covalent bond, atoms can share more than one pair of electrons. For example, in an oxygen molecule, each oxygen atom has six valence electrons. If only one pair of electrons was shared, then each atom would only have seven electrons, one short of the eight needed for a full octet. To make an octet, each oxygen atom needs two additional electrons added to its current six. In order to gain two more electrons, each atom must share two electrons with the other atom, so that there are four electrons being shared. The covalent bond formed by sharing two pairs of electrons is known as a double bond. Single or multiple bonds will for, depending on how many electrons the atom needs to complete an octet. Triple bonds may also be formed, a triple bond of course being a covalent bond in which two atoms share three pairs of electrons.

Salt Formation, Explained

I will now explain the earlier posted process. Before the process can occur, two materials are needed, these being sodium metal and chlorine gas. Energy must be contributed to sublime the sodium. Energy is also required to take away an electron from an atom of the sodium gas. As chlorine is already a gas, no phase transition is needed. Energy is needed to separate the two Cl atoms bonded together in the gas, so that each individual atom can react with the sodium. All of the steps so far have been endothermic, they require energy. The next steps will all release energy, so they are called exothermic. After the Cl2 is separated, an electron is added to a Cl atom to form an anion. In the last step, the Cl anions and the Na cations come together due to the attractive forces of each. In the case of sodium chloride, a large amount of energy is released from this last step. This is the lattice energy.

Salt Formation, The Process

Salt formation is an important part of ionic bonding. I will now explain the steps of salt formation using sodium chloride as an example.

(Process courtesy of Holt Chemistry textbook)

A "^" followed by either a "+" or an "-" signifies a superscripted "+" or "-", while a stand-alone "+" means "and".

0. Starting: Na(s) and Cl2(g)
1. Energy must be added to the sodium to make it a gas: Na(s) + energy ---> Na(g)
2. More energy must be added to remove an electron from each sodium atom:
Na(g) + energy ---> Na^+(g) + e^-
3. Energy must be added to break up Cl2 molecules to produce Cl atoms: 1/2 Cl2(g) + energy ---> Cl(g)
4. Some energy is released as an electron is added to each Cl atom to form a Cl^- ion:
CL(g) + e^- ---> Cl^-(g) + energy
5. Much more energy is released as Na^+ and Cl^- ions come together to form an ionic crystal:
Na^+(g) + Cl^-(g) ---> NaCl(s) + energy = Lattice Forms

NaCl Crystal Structure

here is a webpage showing the NaCl lattice from different viewpoints

Ionic Bonding, In-Depth

Ionic Bonding requires the removal of an electron from one atom and the subsequent transfer of that electron to another atom. When an electron is removed from an atom, that atom is now positively charged. When another atom receives that electron, that atom is negatively charged. The attraction between these opposing forces is what makes the ionic bond. For example, Na has one valence electron. It gives up that one valence electron to form a stable Na+ cation. Cl has seven valence electrons. It is easier for it to gain one electron than lose seven, so it gains the one valence electron that Na lost. The Cl is now a Cl- anion. This particular combination of elements is called Sodium Chloride, or table salt. Salt is the term used to describe many ionic compounds. All salts that are ionic compounds are neutrally charged. The cations and anions in salts are present in a simple, whole-number ratio. (For example, NaCl is 1:1, sodium to chloride) The attractions between anions and cations do not stop with a single one of each in a salt, each attracts several of the opposite. As a result of this, many ions are packed tightly together in a crystalline structure. Moving an electron from an atom requires energy (ionization energy) and energy is also needed to transfer an electron to an atom (electron affinity). However, some elements readily accept extra electrons. For these elements, when an electron is added, energy is released. Be that as it may, the energy released is less than the energy required to remove an electron from the other atom. The rest of the energy needed to form an ionic bond comes from the process of salt formation. Actually, the salt formation process will produce more than enough energy so that the overall process actually releases energy.

Polarity And Covalent Bonding

Non-polar covalent bonds form when electrons in the molecular orbital are shared equally among the atoms in the bond, this usually occurs when the atoms being bonded are the same. If the atoms have significantly different electronegativity values, the electrons are shared unequally between atoms in the molecular orbital. When this happens, the bond formed is a polar covalent bond. In this bond, the shared electrons in the molecular orbital are more likely to be found close to the atom with the higher electonegativity. However, if the elctronegativity values of the atoms vary greatly, an electron will be removed from the electron with lower electronegativity and the atoms will be bonded ionically. Polar covalent bonds are called polar because the ends of them are opposites. In these polar molecules, the atom that attracts the most electrons has a partial negative charge (symbolized by δ-) while the other atom has a partial positive charge (symbolized by δ+). When a molecule has one end partially positively charges and the other end partially negatively charged, it is called a dipole. Even though positive and negative charges are present, the bond is not ionic, as an electron is not removed from one atom and transferred to the other. The electrons are still shared, but the shared pair is more likely to be found near the atom with the highest electronegativity. This makes the charge unequal and the bond is therefore polar covalent. The greater the polarity of a bond, the greater the electronegativity difference will be. The greater electronegativity differs between atoms, the stronger the bond will be.

Covalent Bonding, In-Depth

Covalent Bonding requires the sharing of electrons to fill outermost orbitals. For this to happen, the repulsive and attractive forces of each atom must be in a proportion that allows electrons to move around between both atoms. The space that the shared electrons move in is called a molecular orbital, as a molecule is what is formed when the two atoms bond. After bonding covalently, the atoms are stable, and have a low potential energy. This decrease in energy causes the extra energy to be released. As atoms are moving closer to each other the potential energy of these atoms is decreasing, until the atoms are at a point where the attractive and repulsive forces between the two atoms balance, and the atoms are bonded. At this point the atoms are no longer moving closer together, and do not give off any more energy. When two atoms are bonded covalently, they are at their minimum potential energy (as stated before). The distance between these atoms at their minimum potential energy is called their bond length. Covalent bonds are flexible, the nuclei of the bonded atoms move back and forth. Because of this, bond length is actually the average distance between the two nuclei. In order to break a bond, energy is required. This is called bond energy.

Another Helpful Link

I encourage you to use this link. It contains a very helpful and informative activity/quiz.  When doing the activity, be sure to click on the narrated version.  It does a good job explaining different types of bonding.
Post a comment with your score on the quiz.

Some Slideshows About All Aspects Of Chemical Bonding

The old video wasn't working, so here are a few slideshows.

Chemical Bonding - 1

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Basics of Chemical Bonding - 2

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Ionic Bonding

Ionic Bonding is another concept that can be challenging. Ionic bonds tend to form between metals and non-metals. Unlike covalent bonding, ionic bonding involves the removal of electrons from one atom and the consequent attachment of those electrons to another atom, resulting in the formation of attracting positive and negative ions. The element that loses the electrons is the atom that started with the least amount of electrons. This is because it is easier to lose a small number of electrons than it is to gain a large number of electrons. The atom that then gains the electrons is the one that has the most starting electrons, because it is easier to gain a small number of electrons than it is to lose a large number of them. The atom that lost electrons now has a positive charge, and is called a cation. The atom that gained electrons now has a negative charge and is called an anion.

Covalent Bonding

Covalent Bonding can be complicated. It involves atoms combining to share electrons and attempting to attain an octet, or set of eight electrons. Nonmetals usually form covalent bonds, and the electron affinities of the different atoms need to be very close in order for the atoms not to make ions.  For example, to make a hydrogen molecule, two hydrogen atoms must come together. (Hydrogen of course being an exception to the octet rule, as it has only one energy level, but its the same idea.) When these atoms come together, they share their respective electrons to form a molecule, H2. For the bond to be the strongest, the two atoms need to be at a distance that allows the forces of attraction and the force of repulsion between the two atoms to be in a proportion that will keep the atoms together.