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.

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.

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.

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.