The Octet Rule and Bonding
This topic explains why atoms react: to achieve the stable electron structure of a noble gas. The type of chemical bond formed—ionic or covalent—is determined by whether the reacting atoms are metals or non-metals.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Atoms form chemical bonds to obtain a full outer shell of electrons, mirroring the stable configuration of noble gases (Group 18).
- Bonding between a metal and a non-metal is typically ionic.
- Ionic bonding involves the complete transfer of one or more electrons from metal atoms to non-metal atoms, creating oppositely charged ions.
- Bonding between two or more non-metal atoms is covalent.
- Covalent bonding involves atoms sharing pairs of electrons to complete their outer shells, forming molecules.
- The strong electrostatic attraction between the resulting positive and negative ions holds ionic compounds together in a giant lattice structure.
Definitions
- Ionic Bonding
- The electrostatic force of attraction between positively charged metal ions and negatively charged non-metal ions, formed through the transfer of electrons.
- Covalent Bonding
- A chemical bond formed when two non-metal atoms share one or more pairs of electrons to achieve stable electron configurations.
- Noble Gas Configuration
- A stable electron arrangement where an atom's outermost electron shell is completely full, as seen in the elements of Group 18.
Worked example
An atom of magnesium (atomic number 12) reacts with atoms of chlorine (atomic number 17). Will electrons be shared or transferred, and what type of compound is formed?
- 1
Step 1:
Classify the elements.
Magnesium (Mg) is in Group 2, making it a metal.
Chlorine (Cl) is in Group 17, making it a non-metal.
- 2
Step 2:
Determine their electron configurations.
Mg has the configuration 2,8,2.
Cl has the configuration 2,8,7.
- 3
Step 3:
Determine how they achieve stability.
Mg needs to lose 2 electrons to get the configuration of Neon (2,8).
Each Cl atom needs to gain 1 electron to get the configuration of Argon (2,8,8).
- 4
Step 4:
Conclude the bonding type.
Since a metal is reacting with a non-metal, electrons will be transferred from the metal (Mg) to the non-metal (Cl).
- 5
Step 5:
Identify the compound type.
The transfer of electrons creates ions (Mg2+ and Cl-), so an ionic compound is formed.
Answer: Electrons will be transferred from magnesium to chlorine, forming an ionic compound.
Common mistakes
- ×Confusing electron sharing with electron transfer. Remember: sharing occurs between non-metals (covalent), while a transfer occurs from a metal to a non-metal (ionic).
- ×Incorrectly identifying an element as a metal or non-metal. A quick check of its position in the periodic table (left for metals, top-right for non-metals) prevents this.
- ×Forgetting that atoms react to achieve the *nearest* noble gas structure. For example, Oxygen (2,6) gains 2 electrons to become like Neon (2,8), it does not lose 6.
No-calculator tips
- ✓Mentally place an element in the periodic table to predict its behaviour. Group 1 and 2 elements are metals that lose electrons. Group 16 and 17 elements are non-metals that gain electrons.
- ✓Use the 'octet rule' as a mental shortcut: for many common elements, the goal is to have 8 electrons in the outer shell. Ask yourself 'Does it gain, lose, or share to get to 8?'