Electrovalent Bonding Atoms of different elements excepting noble gases don?t have complete octet so they combine with other atoms to form chemical bond. The force which holds the atoms or ions together within the molecule is called a chemical bond and the process of their combination is called Chemical Bonding. Chemical bonding depends on the valency of atoms. Valency was termed as the number of chemical bonds formed by an atom in a molecule or number of electrons present in outermost shell i.e., valence electrons. Valence electrons actually involved in bond formation are called bonding electrons. The remaining valence electrons still available for bond formation are referred to as non-bonding electrons.
Cause and Modes of chemical combination. Chemical combination takes place due to following reasons. (1) Chemical bonding takes place to acquire a state of minimum energy and maximum stability. (2) By formation of chemical bond, atoms convert into molecule to acquire stable configuration of the nearest noble gas. Modes: Chemical bonding can occur in the following manner.
Electrovalent bond. When a bond is formed by complete transfer of electrons from one atom to another so as to complete their outermost orbits by acquiring 8 electrons (i.e., octet) or 2 electrons (i.e., duplet) in case of hydrogen, helium etc. and hence acquire the stable nearest noble gas configuration, the bond formed is called ionic bond, electrovalent bond or polar bond. Compounds containing ionic bond are called ionic, electrovalent or polar compounds. Example:
Some other examples are: \[MgC{{l}_{2}},\text{ }CaC{{l}_{2}},\text{ }MgO,\text{ }N{{a}_{2}}S,\text{ }Ca{{H}_{2}},\text{ }Al{{F}_{3}},\text{ }NaH,\text{ }KH,{{K}_{2}}O,\text{ }KI,\text{ }RbCl,\text{ }NaBr,\text{ }Ca{{H}_{2}}\] etc. (1) Conditions for formation of electrovalent bond (i) Number of valency electrons : The atom which changes into cation (+ ive ion) should possess 1, 2 or 3 valency electrons. The other atom which changes into anion (- ive ion) should possess 5, 6 or 7 electrons in the valency shell. (ii) Electronegativity difference: A high difference of electronegativity (about 2) of the two atoms is necessary for the formation of an electrovalent bond. Electrovalent bond is not possible between similar atoms. (iii) Small decrease in energy: There must be overall decrease in energy i.e., energy must be released. For this an atom should have low value of Ionisation potential and the other atom should have high value of electron affinity. (iv) Lattice energy: Higher the lattice energy, greater will be the ease of forming an ionic compound. The amount of energy released when free ions combine together to form one mole of a crystal is called lattice energy (U). Magnitude of lattice energy \[\propto \frac{\text{Charge of ion}}{\text{size}\ \text{of}\ \text{ion}}\] \[{{A}^{+}}(g)+{{B}^{-}}(g)\xrightarrow{{}}AB(s)+U\] Determination of lattice energy (Born Haber cycle) When a chemical bond is formed between two atoms (or ions), the potential energy of the system constituting the two atoms or ions decreases. If there is no fall in potential energy of the system, no bonding
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