4.2 Covalent Bonding
Formation of covalent bond
- Covalent bond is the electrostatic force of attraction that two neighbouring nuclei have for a localised pair of electrons shared between them
- Covalent bond is formed without transferring electrons, instead, the atoms share their valence electron(s) to achieve duplet/octet electronic configuration.
- The shared pair of electrons constitutes the covalent bond
- Using dot-and-cross diagram to represent the formation of covalent bond:
Single bond
- Single bond is formed when one pair of electrons is shared between two atoms.
- Examples of compounds with single bonds:
Double bond
- Double bond is formed when two pairs of electrons are shared between two atoms.
- Examples of compounds with double bonds:
Triple bond
- Triple bond is formed when three pairs of electrons are shared between two atoms.
- Examples of compounds with triple bonds:
Lone pair and bond pair of electrons
1) The pair of electrons used in covalent bonding is called the bond pair while the pair of electrons not used in covalent bonding is called the lone pair.
Octet-deficient and expanded octet species
- In general, atoms tend to share their electrons to a achieve a duplet/octet electronic configuration – the octet rule
- In octet-deficient species, the central atom has less than eight electrons.
- Some examples are boron trifluoride, BF3 and nitrogen monoxide, NO.
- In expanded octet species, the central atom has more than eight electrons.
- An example is phosphorus(V) chloride, PCl
- This is possible only for Period 3 elements and beyond, this is because starting from Period 3, the atoms have empty d orbitals in the third energy level to accommodate more than eight electrons
Co-ordinate bond (dative covalent bond)
- A co-ordinate bond is formed when one atom provides both the electrons needed for a covalent bond
- Conditions of forming a co-ordinate bond:
- one atom has a lone pair of electrons
- another atom has an unftlled orbital to accept the lone pair, in other words, an electron-deficient species
- Once the bond is formed, it is identical to the other covalent It does not matter where the electrons come from.
- In a displayed formula, a co-ordinate bond is represented by an arrow, the head of the arrow points away from the lone pair which forms the bond
- An example is the reaction between ammonia and hydrogen In this reaction, ammonium ion is formed by the transfer of hydrogen ion(an octet deficient species) from hydrogen chloride to the lone pair of electrons in the ammonia molecule.
- Another example is aluminium chloride. At high temperature, it exists as AlCl3. At low temperature(around 180-190°C), it exists as Al2Cl6, a dimer(two molecules joined together). This is possible because lone pairs of electrons from the chlorine atom form co-ordinate bonds with the aluminium atom
Tips to draw dot-and-cross diagram for covalent molecules
- Identify the central atom and terminal atom(s). For example, in ammonia, the nitrogen is the central atom while the hydrogens are the terminal atoms
- During the sharing of electrons, the terminal atoms must attain octet configuration(or duplet for hydrogen) but not necessarily for the central atom
- If the central atom is from Period 2 of the Periodic Table, the total number of electrons surrounding it cannot exceed eight(but can less than eight).
- If the central atom is from Period 3 and beyond, the total number of electrons surrounding it can exceed eight.
- For polyatomic anions, the negative charge will be distributed among the most electronegative atom(s). This is to decrease the charge density on a particular atom and to stabilise the ion
- For polyatomic cation, the positive charge will be distributed among the less electronegative atom(s). The reason is same as above.
- If the terminal atom already has octet configuration(for example, Cl⁻), it will contribute two electrons to the central atom to form a co-ordinate bond