9th Class Chemistry Chapter No. 4 Structure of Molecules Notes
This chapter discusses the concept of chemical bonding and the different types of chemical bonds that atoms can form to achieve stability.
Notes on Structure of Atoms
Why do Atoms Form Chemical Bonds?
Atoms tend to become more stable by achieving the electronic configuration of noble gases (He, Ne, Ar, etc.), which have 2 or 8 electrons in their valence shells, indicating stability.
The noble gas electronic configuration is achieved through the duplet rule (2 electrons in valence shell) or octet rule (8 electrons in valence shell).
Atoms combine with one another to form chemical bonds in order to attain the stable electronic configuration of noble gases.
Chemical Bond:
A chemical bond is a force of attraction between atoms that holds them together in a substance.
It is a result of net attractive forces dominating over repulsive forces when atoms come closer to each other.
Types of Chemical Bonds:
a. Ionic Bond:
Formed between atoms of metals (Group 1 and 2) and non-metals (Group 15 to 17).
Involves a complete transfer of electrons from one atom to another, resulting in the formation of oppositely charged ions.
Electrostatic force of attraction between the oppositely charged ions holds the compound together.
Examples: NaCl (Sodium chloride)
b. Covalent Bond:
Formed between atoms of Group 13 to 17 elements when they share electrons.
Involves mutual sharing of valence shell electrons to achieve stability.
Types Covalent Bond
- Single Covalent Bond: One bond pair shared between atoms (e.g., H2, HCl).
- Double Covalent Bond: Two bond pairs shared between atoms (e.g., O2, C2H4).
- Triple Covalent Bond: Three bond pairs shared between atoms (e.g., N2, C2H2).
c. Dative Covalent or Coordinate Covalent Bond
- A type of covalent bond in which one atom donates both electrons of a bond pair to another atom.
- The donating atom is the donor, and the accepting atom is the acceptor.
- Represented by an arrow with the head pointing towards the acceptor.
- Example: NH3 (ammonia) donating a lone pair to H+ to form NH4+ (ammonium).
In summary, chemical bonds are formed by atoms to achieve stability, either through transferring or sharing electrons. Ionic bonds involve complete electron transfer, covalent bonds involve electron sharing, and coordinate covalent bonds involve the donation of a bond pair by one atom to another. Each type of bond plays a crucial role in the formation of different compounds and molecules.
What type of elements have the tendency of sharing electrons?
Non-metals, especially elements from Group 13 to Group 17 in the periodic table, have the tendency to share electrons and form covalent bonds.
If repulsive forces dominate over attractive forces, will a covalent bond form?
No, if repulsive forces dominate, a covalent bond will not form. A chemical bond is a result of net attractive forces overcoming repulsive forces.
Considering the electronic configuration of a nitrogen atom, how many electrons are involved in bond formation, and what type of covalent bond is formed?
The electronic configuration of a nitrogen atom is 1s² 2s² 2p³. Nitrogen has 5 valence electrons, and it can form 3 covalent bonds by sharing its 3 unpaired p-electrons.
Point out the type of covalent bonds in the following molecules:
CH₄: Single covalent bond (Carbon shares four electrons with four hydrogen atoms).
C₂H₄: Double covalent bond (Two carbon atoms share four electrons with each other).
H₂O: Polar covalent bond (Oxygen shares two electrons with two hydrogen atoms).
N₂: Triple covalent bond (Two nitrogen atoms share six electrons with each other).
O₂: Double covalent bond (Two oxygen atoms share four electrons with each other).
What is a lone pair? How many lone pairs of electrons are present on nitrogen in ammonia?
A lone pair is a pair of valence electrons on an atom that is not involved in bonding. In ammonia (NH₃), nitrogen has one lone pair of electrons.
Why is BF₃ electron deficient?
Boron (B) has an atomic number of 5, and its electronic configuration is 1s² 2s² 2p¹. It only has 3 valence electrons, so when it forms three covalent bonds with fluorine (F) atoms in BF₃, it still has an incomplete octet in its valence shell, making it electron deficient.
What types of electron pairs make a molecule a good donor?
Lone pairs of electrons on an atom make it a good donor for a coordinate covalent bond.
What is the difference between a bonded and lone pair of electrons, and how many bonded pairs of electrons are present in the NH₃ molecule?
Bonded pair: A pair of electrons that is shared between two atoms in a covalent bond.
Lone pair: A pair of valence electrons that is not involved in bonding.
In the NH₃ molecule, there are three bonded pairs (one between nitrogen and each hydrogen) and one lone pair on the nitrogen atom.
What do you mean by the delta sign, and why does it develop?
The delta sign (δ) represents a partial charge in a polar covalent bond. It develops when there is an unequal sharing of electrons between two atoms, causing one atom to have a partial negative charge (δ-) and the other a partial positive charge (δ+).
Why does the oxygen molecule not form a polar covalent bond?
Oxygen molecule (O₂) consists of two oxygen atoms with identical electronegativities. They share electrons equally, resulting in a non-polar covalent bond.
Why does water have polar covalent bonds?
In the water molecule (H₂O), oxygen is more electronegative than hydrogen, causing an unequal sharing of electrons. Oxygen pulls the electrons closer to itself, creating partial negative charges around oxygen and partial positive charges around hydrogen, resulting in polar covalent bonds.
What type of elements form metallic bonds?
Metallic bonds are formed between atoms of metals.
Why is the hold of the nucleus over the outermost electrons in metals weak?
In metals, the outermost electrons are loosely held due to the large atomic size and the shielding effect of inner electron shells. The positive charge of the nucleus is not effectively felt by the outermost electrons, resulting in weak attraction or holding force.
Why do electrons move freely in metals?
In metals, the valence electrons become delocalized, meaning they are not strongly attached to any specific nucleus. These delocalized electrons are free to move throughout the metal lattice, creating a sea of mobile electrons. This mobility of electrons allows metals to conduct electricity and heat effectively.
Which types of electrons are responsible for holding the atoms together in metals?
In metals, the valence electrons that become delocalized and form a sea of mobile electrons are responsible for holding the atoms together through metallic bonding.
Why does a dipole develop in a molecule?
A dipole develops in a molecule when there is an uneven distribution of charge due to differences in electronegativity between the atoms. This results in one end of the molecule having a partial positive charge (δ+) and the other end having a partial negative charge (δ-).
What do you mean by an induced dipole?
An induced dipole occurs when an external electric field or a nearby polar molecule causes a temporary separation of charges in a nonpolar molecule. This creates a temporary dipole moment in the nonpolar molecule.
Why are dipole forces of attraction not found in halogen molecules?
Halogen molecules (e.g., Cl2, Br2, I2) are composed of identical atoms with similar electronegativities. As a result, there is no significant difference in electronegativity between the atoms, and no permanent dipole moment is formed. Therefore, dipole forces of attraction are not present in halogen molecules.
What types of attractive forces exist between HCl molecules?
Hydrogen bonding is the attractive force that exists between HCl molecules. The partial positive charge of the hydrogen atom in one HCl molecule is attracted to the partial negative charge of the chlorine atom in another HCl molecule, forming a hydrogen bond.
Define intermolecular forces; show these forces among HCl molecules.
Intermolecular forces are the forces of attraction that exist between molecules and hold them together. In the case of HCl molecules, hydrogen bonding is an intermolecular force. The partial positive charge of the hydrogen atom in one HCl molecule is attracted to the partial negative charge of the chlorine atom in another HCl molecule, forming a hydrogen bond between the two molecules.
Why do ionic compounds have high melting and boiling points?
Ionic compounds have high melting and boiling points due to the strong electrostatic forces of attraction between oppositely charged ions. In an ionic bond, one atom transfers electrons to another, forming positively charged cations and negatively charged anions. These oppositely charged ions are held together by strong electrostatic forces, known as ionic bonds. To melt or boil an ionic compound, significant energy is required to overcome these strong forces and separate the ions from their fixed positions.
What do you mean by malleability?
Malleability refers to the property of a material to be easily deformed or shaped into different forms without breaking or cracking. Malleable materials can be hammered, rolled, or pressed into thin sheets or other desired shapes without losing their cohesion. Metals, in particular, are known for their malleability due to the ability of their atoms to move past each other in a regular pattern.
Why are ionic compounds easily soluble in water?
Ionic compounds are easily soluble in water due to the polar nature of water molecules and the strong electrostatic forces present in ionic bonds. When an ionic compound is added to water, the polar water molecules surround and solvate the ions, breaking the ionic lattice structure. The positively charged hydrogen (H) ends of water molecules surround the negatively charged anions, and the negatively charged oxygen (O) end surrounds the positively charged cations. This process of solvation allows the ionic compound to dissolve in water.
What type of bond exists in sodium chloride?
Sodium chloride (NaCl) exhibits an ionic bond. Sodium (Na) donates one electron to chlorine (Cl), forming a positively charged sodium ion (Na+) and a negatively charged chloride ion (Cl-). The strong electrostatic attraction between these oppositely charged ions forms the ionic bond in NaCl.
Why do covalent compounds of bigger size molecules have high melting points?
Covalent compounds of bigger size molecules tend to have high melting points due to the presence of strong covalent bonds within the molecules. In large covalent molecules, many atoms are bonded together by multiple covalent bonds, creating a network structure. Breaking these strong covalent bonds requires a considerable amount of energy, leading to high melting points in these compounds. Examples of such compounds include diamond (a giant covalent structure of carbon atoms) and silicon dioxide (silica).
Electronegativity difference and nature of bond between the following pairs of elements
(a) H and Cl: Electronegativity difference = 3.0 – 2.1 = 0.9, Nature of bond = Polar covalent bond.
(b) H and Na: Electronegativity difference = 2.1 – 0.9 = 1.2, Nature of bond = Polar covalent bond.
(c) Na and I: Electronegativity difference = 3.0 – 0.9 = 2.1, Nature of bond = Ionic bond.
(d) K and Cl: Electronegativity difference = 3.0 – 0.9 = 2.1, Nature of bond = Ionic bond.
Arranging the compounds in increasing ionic strength based on electronegativity differences
Polar covalent compounds (smallest electronegativity difference) → NaCl, KCl (ionic compounds with similar electronegativity difference) → HCl, HNa (largest electronegativity difference)
Why do atoms react?
Atoms react to achieve a more stable electronic configuration, typically by attaining the electron configuration of noble gases (duplet or octet rule). This stability is achieved through the formation of chemical bonds, where atoms gain, lose, or share electrons to achieve the desired electronic configuration.
Why is the bond between an electropositive and an electronegative atom ionic in nature?
The bond between an electropositive (metal) and an electronegative (non-metal) atom is ionic in nature because there is a significant difference in electronegativity between the two atoms. Electropositive atoms have low electronegativity, while electronegative atoms have high electronegativity. As a result, the electropositive atom tends to lose electrons (forming a positively charged ion), while the electronegative atom tends to gain electrons (forming a negatively charged ion), resulting in the attraction between the oppositely charged ions, thus forming an ionic bond.
Ionic compounds are solids. Justify.
Ionic compounds are solids because of the strong electrostatic forces of attraction between positively and negatively charged ions. These forces hold the ions together in a regular and ordered arrangement, forming a three-dimensional lattice structure. This lattice structure is stable and rigid, giving ionic compounds their solid-state at room temperature.
More electronegative elements can form bonds between themselves. Justify.
More electronegative elements can form bonds between themselves because they have a higher tendency to attract electrons. When two electronegative atoms form a covalent bond by sharing electrons, they can have a relatively equal share of the electrons due to their similar electronegativity values.
Metals are good conductor of electricity. Why?
Metals are good conductors of electricity because they have a large number of free electrons in their valence shells. These valence electrons are loosely held by the positively charged metal ions in the lattice. When an electric field is applied, these free electrons can move throughout the metal structure, creating an electric current.
Ionic compounds conduct electricity in solution or molten form. Why?
Ionic compounds conduct electricity in solution or molten form because the ions are free to move. In a solution or molten state, the ionic lattice breaks down, and the ions become mobile, carrying electric charge. In the solid state, ionic compounds do not conduct electricity because the ions are fixed in the lattice and cannot move.
What type of covalent bond is formed in nitrogen molecule.
A triple covalent bond is formed in a nitrogen molecule (N2). It involves the sharing of three pairs of electrons (6 electrons) between two nitrogen atoms. This results in a strong and stable covalent bond between the two nitrogen atoms.
Differentiate between lone pair and bond pair of electrons.
Lone pair of electrons: It refers to a pair of electrons that are present in the valence shell of an atom but are not involved in bond formation. They are localized on a single atom and are not shared with another atom.
Describe at least two necessary conditions for the formation of a covalent bond.
Bond pair of electrons: It refers to a pair of electrons that are involved in the formation of a chemical bond. These electrons are shared between two atoms to form a covalent bond.
Two necessary conditions for the formation of a covalent bond
a. Both atoms should have incomplete outermost electron shells to allow for electron sharing.
b. The electronegativity of both atoms should be relatively similar to ensure equal sharing of electrons.
Why HCl has dipole-dipole forces of attraction?
HCl has dipole-dipole forces of attraction because it is a polar molecule. The electronegativity difference between hydrogen and chlorine atoms results in an unequal sharing of electrons, creating a partial positive charge on hydrogen and a partial negative charge on chlorine. These partial charges create dipole moments that lead to dipole-dipole interactions between HCl molecules.
What is a triple covalent bond, explain with an example?
A triple covalent bond involves the sharing of three pairs of electrons (6 electrons) between two atoms. An example of a triple covalent bond is the nitrogen molecule (N2), where two nitrogen atoms share three pairs of electrons to form the stable N2 molecule.
What is difference between polar and non-polar covalent bonds, explain with one example of each?
Polar covalent bond: It is formed when there is an unequal sharing of electrons between two atoms due to differences in electronegativity. One atom attracts the shared electrons more strongly, resulting in a partial positive and a partial negative charge on the atoms. Example: HCl.
Non-polar covalent bond: It is formed when there is an equal sharing of electrons between two atoms due to similar electronegativity. There are no partial charges on the atoms. Example: Cl2.
Why a covalent bond becomes polar?
A covalent bond becomes polar when there is an unequal sharing of electrons between two atoms due to differences in electronegativity. The more electronegative atom attracts the shared electrons closer to itself, creating partial charges and resulting in a polar covalent bond.
What is relationship between electronegativity and polarity?
The relationship between electronegativity and polarity is that a higher difference in electronegativity between two atoms leads to a more polar covalent bond. When the electronegativity difference is large, one atom becomes partially positive, and the other becomes partially negative, resulting in a stronger polarity.
Why does ice float on water?
Ice floats on water because it is less dense than liquid water. Water molecules in ice form a stable hexagonal lattice structure due to hydrogen bonding, causing them to be spaced out with larger gaps between the molecules. This arrangement results in lower density, allowing ice to float on water.
Give the characteristic properties of ionic compounds.
Characteristic properties of ionic compounds
a. High melting and boiling points due to strong electrostatic forces between ions.
b. Solubility in polar solvents like water due to the attraction between ions and polar molecules.
c. Conductivity in molten state or in solution due to the mobility of ions.
d. Crystalline structure with a regular arrangement of ions in a lattice.
What characteristic properties do the covalent compound have?
Characteristic properties of covalent compounds:
a. Low melting and boiling points due to weaker intermolecular forces.
b. Generally insoluble in water, except for some polar covalent compounds.
c. Poor conductors of electricity in any state due to the lack of free ions or charged particles.
d. Variable state of matter, as covalent compounds can exist as solids, liquids, or gases at room temperature depending on their molecular forces.