Chemical Bonding and Chemical Reaction and Its Type – Valency, Ions | Chemical Bonding and Molecular Structure PDF
It is formed by elements to complete eight electrons in their outer shell i.e. to complete their octet. It results in decrease in energy and increase in stability. In other words, the force that holds together different atoms in a molecule is called as chemical bond.
It is the number of electrons taking part in bonding (i.e., bond formation). It generally increases from 1 to 7 in a period with respect to hydrogen but with respect to oxygen, it first increases from 1 to 4 and then decreases to 1.
For alkali metal (i.e., sodium, potassium etc.) it is 1, for alkaline earth metals (i.e., magnesium, calcium etc.) is 2, for aluminium, it is 3 and for nitrogen it varies from -3 to +5.
It is variable for transition elements due to less differences in the energy of (n-1)d and ns-orbitals. e.g. Fe (+2, +3), Cu (+1, +2), Hg (+1, +2) etc.
It is also variable for heavy p-block elements like Pb, due to inert pair effect.
These are of two types : cation and anion. Cations are formed by the loss of electrons and carry positive charge, e.g., Na+, Mg2+. Anions are formed by the gain of electrons, i.e., carry negative charge e.g., CI–, F–, etc.
Electrovalent Bond or Ionic Bond
It is a type of chemical bond formed through an electrostatic attraction between two oppositively charged ions. It is formed between a cation, which is usually a metal and an anion which is usually a non-metal e.g.
Ionic bond containing compounds can conduct electricity in molten state or in solution, but not in solid state. Ionic bond containing compounds i.e., electrovalent or ionic compounds have a high melting point and tend to be soluble in water. Examples of such compounds are limestne (CaCO3), salt (NaCl), lime (CaO), magnesium oxide (MgO) etc.
The energy required to separate the ions of a ionic compound is called lattice energy.
It is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. It may be a single bond (—, formed by sharing of two electrons i.e., one electron from each atom), double bond (=, formed by sharing of four electrons i.e., two electrons from each atom) or triple bond ( ≡, formed by sharing of six electrons i.e., three electrons from each atoms).
Ethanol, amine (except 3° amine) etc., can form H-bond with water, so these are soluble in water although these are covalent compounds.
H2O has angular structure,NH3 (ammonia) has pyramidal structure,methane (CH4) has tetrahedral structure, carbon dioxide (CO2) has linear geometry.
Single bond contains only σ-bond: double bond contains one σ and 1 π-bond; and triple bond contains 1σ and 2 π-bonds.
- are non-conductor of electricity.
- have low thermal conductivity.
- insoluble in water but soluble in organic solvents like benzene, acetone, either etc.
- have low melting and boiling points (diamond and graphite have very high melting point.)
- are directional so have definite geometry.
It is a special type of covalent bonding in which both the electrons for sharing (i.e., shared pair of electrons) are given by only one atom.
Coordinate compounds have properties in between the ionic and covalent compounds. Example of coordinate compounds are
It is the attractive interaction of a hydrogen attached to an electronegative atom with another electronegative atom, such as nitrogen,oxygen or fluorine, that comes from another molecule or chemical group.
It is stronger (has energy 5 to 30 kJ/mol) than a van der Waals’ interaction. It occurs in both inorganic molecules such as water and organic molecules suc as DNA.
Van der Waals’ Interaction/Force
The attractive forces among the non-polar molecules in solid or liquid states. These are relatively weak compared to normal chemical bonds. The ability of geckos- which can hang on a glass surface using only one toe-to climb on sheer surfaces has been attributed to the van der Waals’ forces between these surfaces and spatulae or microscopic projections which cover the hair-like setae found on their footpads.
Types of Chemical Reactions
Chemical reactions are of following types
1. Addition Reaction : In such reactions, two or more substances combine to give a single substance e.g.,
2. Substitution reaction : in such reactions, an atom or a group of atoms of a molecule is replaced by another atom or group of atoms e.g.,
3. Decomposition reaction : These are irreversible reactions in which a molecule decompose into two or more simpler molecules e.g.,
4. Dissociation reaction : These are those reversible reactions in which a molecule dissociates into two or more simple molecules, e.g.,
Reversible reactions are those which occurs in forward as well as in Backward direction but never go to completion. Irreversible reactions occur only in forward direction and go to completion.
5. Double decomposition reaction : These involve exchange of ions between two compounds. e.g.,
Reactions occurring between the ions or ionic compounds, are very fast.
6. Exothermic reactions : These are those reactions in which energy is released, e.g., burning of natural gas, respiration, decomposition of vegetable matter into compost, combustion reactions etc.
7. Endothermic reactions : These are those in which energy is consumed, e.g. digestion.
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