The covalent bond is the chemical union 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.
Links 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.
Characteristics of covalent bonds
- The covalent bonds are established between non-metallic elements. For example, hydrogen H, oxygen O and chlorine Cl occur naturally as diatomic molecules linked by covalent bonding: 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 3 C-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.
CARACTERISTICS of the covalents compounds
- At room temperature and at normal atmospheric pressure, covalent compounds can be present as solids, liquids or gases.
- Covalent compounds do not show electrical conductivity when dissolved in water. When these compounds are dissolved, the molecules separate and remain independent, unlike the ionic compounds, which decompose in their positive and negative ions.
- Covalent compounds have lower melting and boiling points than ionic compounds. The force of attraction between the molecules is less than in the ionic bond, which is why less energy is required to separate them. For example, the melting point of sodium chloride NaCl (ionic compound) is 801 ° C and that of boiling is 1465 ° C; the melting point of the water (covalent compound) is 0 ° C and the boiling point is 100 ° C.
- 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. In contrast, oils and gasoline are not soluble in water.
Depending on the affinity for electrons in each atom, we can have three types of bonds: polar, non-polar and coordinated.
Non-polar 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 non-polar covalent bond
The chlorine molecule Cl 2 is made up of two chlorine atoms with the same electronegativity, which share a pair of electrons in a non-polar covalent bond. The same happens in the case of the two oxygen atoms to form the O 2 oxygen molecule .
Among the carbon atoms in the organic molecules, the covalent bond is a non-polar 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
In the H 2 O water molecule the electrons of the hydrogens remain closer and for a longer time around 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 through a polar covalent bond.
Coordinated covalent or dative link
This type of bond occurs when one of the atoms in the union is the one that contributes the electrons to share. This is achieved in the reaction between ammonia NH 3 and boron trifluoride BF 3 . Nitrogen has two free electrons and boron is deficient in electrons. When both nitrogen and boron join, they complete their last layer with eight electrons.
Atoms transfer, accept or share electrons in order to complete their valence level with eight electrons. This is because atoms look for their most stable electronic configuration .
The valence electrons are those found in the last external energy layer. These are the electrons available to interact with other atoms.
Exceptions to the octet rule
The exception to this rule is in hydrogen and helium whose last valence only admits a maximum of 2 electrons. Also, the beryllium Be and boron B elements have few electrons to form an octet. The Be has only two valence electrons and the B has three.