The covalent bond is the chemical bond between two atoms where electrons are shared . This causes the atoms to behave as a unit, which we call a molecule. Atoms interact with each other through the outermost electrons forming bonds.
Bonds are the forces that hold atoms together to form molecules and compounds. Unlike the ionic bond in which there is electron transfer between two atoms, in the covalent bond the electrons are shared between the atoms.
Covalent bond characteristics
- Covalent bonds are established between non-metallic elements. For example, hydrogen H, oxygen O, and chlorine Cl are naturally found as covalently bonded diatomic molecules: H 2, O 2, and Cl 2 .
- Covalent bonds include single, double, or triple bonds where 2, 4, or 6 electrons are shared, respectively. For example, in the organic compound ethane H 3C-CH 3 , the bond between carbon-carbon and carbon-hydrogen is simple. In ethene H 2 C = CH 2 the bond between the carbons is double, sharing four electrons.
- Covalent bonds create molecules that can be separated with less energy than ionic compounds.
- The covalent bond is stronger between two atoms with equal electronegativity.
Covalent bond types
Depending on the affinity for the electrons that each atom has, we can have three types of bonds: polar, nonpolar and coordinated.
Nonpolar covalent bond
This union is established between atoms with equal electronegativity. This type of bond can also be maintained between atoms with an electronegativity difference of less than 0.4.
Examples of nonpolar covalent bond
The Cl 2 chlorine molecule is made up of two chlorine atoms with the same electronegativity, which share an electron pair in a non-polar covalent bond. The same happens in the case of the two oxygen atoms to form the oxygen molecule O 2 .
Between the carbon atoms in organic molecules the covalent bond is of the nonpolar type.
Polar covalent bond
The polar covalent bond is formed between two non-metallic atoms that have an electronegativity difference between 0.4 and 1.7. When these interact, the shared electrons stay closer to that more electronegative atom.
Examples of molecules with polar covalent bonds
The water molecule has two polar covalent bonds between oxygen and hydrogens.
In the water molecule H 2 O, the electrons of the hydrogens stay closer and longer around the oxygen, which is more electronegative.
Fluorine F is the most electronegative element (4.0) and has seven valence electrons. When combined with hydrogen, hydrogen fluoride HF is formed, via a polar covalent bond.
The NH 3 ammonia molecule has polar covalent bonds between nitrogen and hydrogens.
Coordinated or dative covalent bond
Nitrogen in ammonia supplies two electrons to boron in the ammonia-boron trifluoride adduct.
This type of bond occurs when one of the atoms in the bond is the one that provides the electrons to share. We achieve this in the reaction between NH 3 ammonia and boron trifluoride BF 3 . Nitrogen has two free electrons and boron is electron deficient. By combining both nitrogen and boron they complete their last shell with eight electrons.
CARACTERISTICS of the covalents compounds
- At room temperature and at normal atmospheric pressure, covalent compounds can present as solids, liquids, or gases.
- Covalent compounds do not show electrical conductivity when dissolved in water. When these compounds dissolve, the molecules separate and remain independent, unlike ionic compounds, which break down into their positive and negative ions.
- Covalent compounds have lower melting and boiling points than ionic compounds. The attractive force between the molecules is less than in the ionic bond, so less energy is required to separate them. For example, the melting point of sodium chloride NaCl (ionic compound) is 801 ºC and the boiling point is 1465 ºC; the melting point of water (covalent compound) is 0 ºC and the boiling point is 100 ºC.
- Covalent compounds tend to be more combustible.
- Many covalent compounds are not easily soluble in water. Those polar covalent compounds, such as ethanol and glucose, dissolve well to some degree. On the other hand, oils and gasoline are not soluble in water.
Byte Rule
Atoms transfer, accept, or share electrons in order to complete their valence level with eight electrons. This is because atoms seek their most stable electronic configuration .
The valence electrons are those found in the external energy last layer. These are the electrons available to interact with other atoms.
Exceptions to the octet rule
Beryl and boron are examples where the octet rule is not followed.
The exception to this rule we have in hydrogen and helium whose last valence only admits a maximum of 2 electrons. Also, the elements beryllium Be and boron B have few electrons to form an octet. The Be has only two valence electrons and the B has three
