Chemistry - Chemical Bonding Study Notes

CHEMISTRY - GRADE 8 (2024-25)

CHAPTER 7 - CHEMICAL BONDING

IONS

An ion is defined as an atom that has a charge resulting from the imbalance between the number of electrons and protons.
The two types of ions are:
- Positive ion (Cation)
- Negative ion (Anion)

POSITIVE IONS OR CATIONS

A positive ion is formed when an atom loses one or more electrons.
Example: Sodium (Na) loses one electron to become sodium ion (Na+).
  - The reaction can be depicted as:
  - $Na ightarrow Na^+ + e^-$
  - Electronic configurations before and after ionization:
    - Sodium: (2, 8, 1)
    - Sodium Ion: (2, 8)
Typical Cations:
  - Alkali Metals (Group 1) typically form +1 ions.
    - Examples: $Li^+$ , $Na^+$ , $K^+$
  - Alkaline Earth Metals (Group 2) typically form +2 ions.
    - Examples: $Be^{2+}$ , $Mg^{2+}$ , $Ca^{2+}$

NEGATIVE IONS OR ANIONS

A negative ion is formed when an atom gains one or more electrons.
Examples of Anions:
  - Halogens gain one electron to form negative ions.
  - Example: Fluorine (F) gains an electron to become a fluoride ion (F−).
    - The reaction can be depicted as:
    - $F + e^- ightarrow F^-$
    - Electronic configurations before and after ionization:
      - Fluorine: (2, 7)
      - Fluoride Ion: (2, 8)
  - Halogens (Group 7) typically form -1 ions.
    - Examples: $F^-$ , $Cl^-$ , $Br^-$
  - Group 6 elements typically form -2 ions.
    - Examples: $O^{2-}$ , $S^{2-}$

ELECTROSTATIC ATTRACTION

Electrostatic attraction refers to the force that holds positively charged ions and negatively charged ions together.

CHEMICAL BOND

Atoms bond to attain a stable electron configuration akin to that of noble gases.
The principle of bonding: Atoms seek to have 8 electrons in their outer shell (or 2 if they have only one shell).
Example: Hydrogen, which has an electronic structure of 1, needs to gain one more electron for stability.
A fully occupied outer shell makes an atom stable. Atoms whose outer shells are already full do not form bonds (unreactive).
Incomplete outer shells lead atoms to combine to achieve full outer shells in order to attain noble gas electronic configuration.
Definition of Chemical Bond: A strong force that holds atoms together in a molecule or crystal, resulting from the transfer or sharing of electrons.
Ways Atoms Bond:
1. Ionic bonding
2. Covalent bonding

IONIC BOND

An ionic bond is a bond formed through strong electrostatic force of attraction between positive metal ions and negative non-metal ions.
Ionic bonds typically form between elements from group I (alkalis) and group VII (halogens), as well as elements from group VI or V when transferring electrons.
The nature of ionic bonding involves electron transfer between metals and non-metals.
All ionic compounds are electrically neutral.

PROPERTIES OF IONIC COMPOUNDS

Solubility: Ionic compounds are generally soluble in water.
Melting and Boiling Points: They possess high melting and boiling points due to the strong forces of attraction between ions, which necessitate a significant amount of energy to break.
Electrical Conductivity: Ionic compounds can conduct electricity when in molten form (liquid) or dissolved in water (aqueous), since ions become free to move. They are not capable of conducting electricity in the solid state due to the immobility of ions.

STRUCTURE OF IONIC COMPOUNDS

Ionic compounds consist of a giant ionic structure formed by the strong attractive forces between positive and negative ions.
The arrangement of ions is regularly structured, known as a lattice.
- Definition of Lattice: A regular three-dimensional arrangement of ions in an ionic compound, where each ion is surrounded by ions of opposite charge.
- All ionic compounds possess a giant lattice structure.
Example: In sodium chloride (NaCl), each sodium ion is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions.

DOT AND CROSS DIAGRAM

Dot and Cross Diagram of Sodium Chloride (NaCl):
  - Sodium (group 1) possesses one electron in its outer shell.
  - Chlorine (group 7) contains seven electrons in its outer shell.
  - Sodium donates one electron to chlorine, achieving a noble gas electronic structure and acquiring a +1 charge.
  - Chlorine gains one electron, also achieving a noble gas structure and acquiring a -1 charge.
  - Positively charged sodium ion (Na+) bonds with negatively charged chloride ion (Cl−), resulting in a sodium chloride molecule.
Dot and Cross Diagram of Potassium Fluoride (KF):
  - Potassium (group 1) has one electron in its outer shell.
  - Fluorine (group 7) has seven electrons in its outer shell.
  - Potassium donates one electron to fluorine.
  - Potassium loses one electron, attaining a +1 charge of potassium ion (K+).
  - Fluorine gains one electron, attaining a -1 charge of fluoride ion (F−).
  - Potassium ion bonds with fluoride ion to form potassium fluoride molecule.
Dot and Cross Diagram of Magnesium Oxide (MgO): (not provided in detail, to be added as per study need)
Dot and Cross Diagram of Calcium Chloride (CaCl₂):
  - Calcium has two electrons in its outer shell.
  - Chlorine (group 7) has seven electrons in its outer shell.
  - Calcium donates two electrons, one to each chlorine atom.
  - Calcium loses two electrons to achieve a +2 charge (Ca²+).
  - Each chlorine atom gains one electron, acquiring a -1 charge (Cl−).
  - The two chlorine atoms form bonds with the calcium ion, resulting in the calcium chloride molecule.
Dot and Cross Diagram of Aluminium Fluoride (AlF₃):
  - Aluminium (group 3) contains three electrons in its outer shell.
  - Fluorine (group 7) has seven electrons in its outer shell.
  - Aluminium loses three electrons, attaining a +3 charge (Al³+).
  - Each fluorine atom gains one electron, acquiring a -1 charge (F−).
  - Three fluorine atoms are involved; all gain one electron.
  - Positively charged aluminium ion bonds with three negatively charged fluoride ions, forming aluminium fluoride molecule.