In this post, students/learners will be provided with high-quality notes/study material on an important Chapter 4 "Structure of molecules" of their textbook. All answers to short questions are presented in an easy and understandable way. Students can use these notes for their board examinations, while teachers can use these notes for lecture preparation and presentation.
Here, in this post, you will be provided notes on some of the most important topics on the structure of molecules, such as basic terms/definitions widely used in molecular chemistry, why atoms form chemical bonds, concepts and theories related to chemical bonding, Definition of Chemical bonds, Types of Chemical bonds, Coordinate covalent bond or Dative bond, Metallic bond, Intermolecular forces, Hydrogen bonding and its applications, Dipole-Dipole Interaction, Properties of Ionic compounds, properties of covalent compounds, and properties of metals.
These notes/study materials are also beneficial for preparing for competitive and entrance exams, such as NEET, EATA, NTS, NMDCAT, etc.
Define the following terms
- Molecule
- Synthetic molecules
- Natural molecules
- Octet rule
- Doublet rule
I. Molecule
A molecule is composed of two or more atoms that are chemically bonded together. Molecules have a distinct identity and can exist in a variety of forms, such as gases, liquids, and solids.
II. Synthetic molecules
Those molecules that are prepared in the laboratory (i.e. man-made) are called as Synthetic molecules.
For example, Vitamin-C when obtained from artificial sources is said to be a Synthetic molecule.
III. Natural molecules
Those molecules that can be obtained from natural sources are called Natural molecules.
For example, Vitamin-C when obtained from Citrus fruits is said to be a Natural molecule.
IV. Octet rule
The completion of eight electrons in the outermost shell of an atom after the bond formation is called the Octet rule.
For example, In HCl, Chlorine has seven electrons but when it is bonded to Hydrogen it obeys the octet rule i.e. complete eight electrons in its shell.
V. Doublet rule
The completion of two electrons in the outermost shell of an atom after bond formation is called the Doublet rule.
For example, In HCl, Hydrogen has one electron in its shell but when it is bonded to Chlorine it obeys the Doublet rule i.e. complete two electrons in its shell.
Explain "Why do atoms form chemical bonds?"
Why do atoms form chemical bonds?
Atoms form chemical bonds because they have to obey a Universal law, that is
Every object in this universe tends to lower its energy in order to get stability.
Explanation
In our daily life, we see that many objects move from high to low energy levels, for example, water flows from higher to lower region, electric current flows from higher to lower potential and heat flows from hot to a cold body. In the same way, atoms in unbounded form have high energy, to lower their energy they form bonds.
What are the two main concepts that explain chemical bonding?
There are two concepts, which explain the chemical bonding
A. The valence concept (Electronic theory of valance)
In 1916, G.N.Lewis and W.Kossel give the electronic theory of valence. It states that
“In a chemical bond formation, atoms take part by losing, gaining or sharing of electrons, so as to attain the inert or noble gas electronic configuration"
When atoms have two or eight electrons in their outermost shell, they are stable. The electron theory of valence can be named the Octet or Duplet theory of valence.
i. Octet theory of valence or Rule of eight
The tendency of atoms to attain eight electrons in the outer shell in order to attain stability is called the Octet rule.
For example, an Oxygen atom has six valence electrons. It
shares or gains two electrons in its outermost shell and attains stability
by completing its outermost with eight electrons.
ii. Duplet rule or rule of two
The tendency of atoms to attain two electrons in the outer shell in order to attain stability is called the Duplet rule.
For example, Helium has two electrons in its valence shell and is stable. Except for Helium, all other noble gases have their outermost shells filled with eight electrons.
B. The orbital concept
This concept is based on the combination of atomic orbital to produce molecular orbital. The atomic orbitals have one electron. These orbitals when coming close to one another, they overlap each other. This overlapping is either endwise or sidewise. Endwise overlapping produces a sigma bond and sidewise overlapping produces a pi-bond.
Define Chemical bond. What are the types of Chemical bonds?
Chemical bond
The attractive forces which hold the atoms in a compound are called Chemical bonds.
For example, When Sodium (Na) attracts Chlorine (Cl) through electrostatic force of attraction, we say Sodium is chemically bonded to Chlorine.
Types of Chemical bonds
Following are the types of Chemical bonds
- Ionic bond.
- Covalent Bond.
- Coordinate covalent bond Or Dative bond.
- Metallic bond.
Define and explain the Coordinate Covalent bond or Dative bond. Explain your answer by giving examples.
Coordinate Covalent bond or Dative bond
That type of chemical bond in which share pair of electrons is donated by the donor atom and accepted by the acceptor atom is called a Coordinate Covalent bond or Dative bond.
Explanation
In some compounds, for example in Ammonium Ion (NH4)+1 or in Hydronium Ion (H3O)+1, a shared pair of electrons is donated by one atom called the Donor atom and accepted by another atom called the Acceptor atom. An arrow is directed from Donor to the Acceptor atom in the Lewis structure to show a Coordinate Covalent bond or a Dative bond.
Examples of Coordinate Covalent bond formation
1. Formation of Ammonium Ion (NH4)+1
When Ammonia (NH3) reacts with Hydrogen it forms Ammonium Ion (NH4)+1 through a Coordinate Covalent bond. In NH3, Nitrogen is the Donor while Hydrogen is the Acceptor atom.
2. Formation of Hydronium Ion (H3O)+1
When Water (H2O) reacts with Hydrogen it forms Hydronium Ion (H3O)+1 through a Coordinate Covalent bond. In Water Oxygen is a Donor while Hydrogen is the Acceptor atom.
What do you mean by Polar and non-Polar Covalent bonds? Illustrate your answer by giving suitable examples.
Polar Covalent bond
That Covalent bond in which share pair of electrons is attracted more towards a more electronegative atom and form a partial positive (+δ) and partial negative(-δ) charge is called a Polar Covalent bond.
Explanation
Take the example of Hydrochloric Acid (HCl), in HCl, there is a polar Covalent bond. The shared pair of electrons between H and Cl will be attracted more towards Chlorine being more electronegative than Hydrogen. In such case Cl will develop a partial negative charge on it i.e. Cl-δ and Hydrogen will have a partial positive charge i.e. H+δ.Non Polar Covalent bond
That Covalent bond in which share pair of electrons is equally attracted by the bonded atoms and no partial positive and partial negative charge is formed is called a non-polar Covalent bond.
Explanation
Take the example of the H2 molecule, in H2 there is a non-polar Covalent bond. The pair of electrons will be equally attracted by both the Hydrogen atoms having the same electronegativity. In such a case both Hydrogen atoms will have the same partial positive charges on them i.e. H+δ ___ H+δ.
Define and explain the Metallic bond. What do you know about electron sea theory?
Metallic bond
That type of chemical bond in which the atoms of metal attract each other without the formation of molecules is called a Metallic bond.
Explanation
Metals are pure substances that are formed of their own atoms e.g. Gold is made up of its own atoms which are joined through a Metallic bond. The atoms of metals are not joined through Ionic or Covalent bonds.
Electron sea theory
Lorentz, in 1923, presented a theory about the arrangement of metal atoms, this theory is known as the Electron sea theory.
According to this theory, the electrons of the metal atoms are not tightly bonded to the nucleus because of the large size of the metal atoms, these electrons are thus known as free electrons. Free electrons belong to all the atoms in the metal. It is due to the free electrons that metal conducts electricity. In the following diagram, negative signs represent electrons and the circle enclosing the plus signs represent cations of metal atom.
Define and explain Intermolecular forces. What are it's types?
Inter-molecular forces
The forces that exist among the molecules of a substance are called Inter-molecular forces.
Or
The forces of attraction between the polar molecules are called intermolecular forces.
Explanation
As we know that atoms attract each other through forces called Chemical bonds, but molecules attract each other through attractive forces called Inter-molecular forces. For example, one molecule of Water attracts another molecule of Water through an Intermolecular force called Hydrogen bonding.
Types of Inter-molecular forces
The following are the most common types of Intermolecular forces.
A. Hydrogen Bonding B. Dipole-Dipole Interaction C. London dispersion forces
Define and explain the term Hydrogen Bonding. Give some examples of Hydrogen Bonding.
Hydrogen Bonding
That type of Intermolecular force in which the Hydrogen of one molecule is tightly bonded to high electronegative atoms such as Fluorine (F), Nitrogen (N), and Oxygen (O) is called Hydrogen Bonding.
Explanation
Hydrogen bonding is formed only when Hydrogen is bonded to F, N, or O. No other element form bond with Hydrogen in Hydrogen bonding i.e. in H2S there is no Hydrogen bonding. Hydrogen bonding is roughly ten times stronger than Dipole-Dipole Interaction and roughly ten times weaker than Ionic or Covalent bond.
Explanation through example
Consider that four Water molecules are brought close to each other, when this happens the partial positive Hydrogen atom of one water molecule will attract the partial negative Oxygen of another water molecule through Hydrogen Bonding which is shown by dotted lines in the figure.
Examples of Hydrogen Bonding
Hydrogen bonding is found in the molecules of H2O, NH3 (Ammonia) and HF (Hydrogen Fluoride), etc.
Write down the applications of Hydrogen bonding.
Applications of Hydrogen bonding
- Hydrogen bonding explains the weak acidic strength of HF as compared to HCl, HBr, HI.,
- Large protein molecules (e.g. fibers, hair, and muscular proteins) in living bodies are stabilized due to Hydrogen bonding.
- Thousands of Hydrogen bonds are present in DNA and RNA nucleotides.
- Hydrogen bonding is also present in paints, dyes, and glue which increases their adhesive and sticky property.
- Hydrogen bonding also stabilizes food materials like glucose, fructose, sucrose, etc.
- The high boiling point of water is also due to strong hydrogen bonding in water.
Write a short note on Dipole-Dipole interaction.
Dipole-Dipole interaction
The forces of attraction between the weakly polar molecules are called Dipole-Dipole interaction.
Explanation
As we know the covalent bond formed between the atoms of different electronegativities is a polar covalent bond. In some cases, the polarity is weak as in CO2, H2S, NO, etc. there is a weak polar covalent bond, while in other cases the polarity is strong as in H2O, NH3, HF, etc. i.e. in Hydrogen bonding.
The forces of attraction that are present between the weakly polar covalent molecules are Dipole-Dipole interaction, while the forces of attraction present in strong polar molecules are Hydrogen bonding.
Factors affecting Dipole-Dipole interaction
Following are the factors that affect Dipole-Dipole interaction.
1. Intermolecular distance
Closer are the molecules stronger will be the Dipole-Dipole interaction and vice versa. That’s why liquids have greater Dipole-Dipole interaction than Gases.
2. Electronegativity of bonded atoms
Greater is the electronegativity difference stronger will be the Dipole-Dipole interaction and vice versa. Therefore, Dipole-Dipole interaction in HCl is stronger than in HBr.
Write down the properties of ionic compounds.
Properties of an ionic compound
Following are some of the characteristic properties of Ionic compounds like NaCl.
- Ionic compounds are Solid in nature.
- Ionic compounds are hard and brittle.
- Ionic compounds have high melting and boiling points.
- Ionic compounds are soluble in water and in a polar solvent.
- Ionic compounds dissociate into their ions in molten form i.e. in solutions.
- Ionic compounds don’t conduct electricity in solid form but are good conductors in molten form.
- Ionic compounds have fast reactions in solutions.
Write down the properties of covalent compounds.
Properties of covalent compounds
The
properties of covalent compounds depend on the following factors.
- Bond type i.e. whether it is single, double, or triple.
- The shape of the molecule.
- The polarity of the molecule i.e. whether it is polar or non-polar.
- Intermolecular attractive forces i.e. whether it is Dipole-Dipole or Hydrogen bonding.
Following are some of the general characteristic properties of covalent compounds.
- Covalent compounds exist in all three states of matter i.e. in solid like Diamond (the hardest substance in the world), liquid like Water, and Gas like CH4 (Methane).
- Some covalent compounds partially (not fully) dissociate e.g. CH3COOH, while others don’t dissociate e.g. C12H22O11.
- Polar covalent compounds are soluble in polar e.g. Alcohol and Hydrogen Chloride (HCl) dissolve in water. Similarly, non-polar solute dissolves into non-polar solvents e.g. Naphthalene (solute) dissolves in Carbon-tetra-Chloride (CCl4) and Benzene (C6H6).
- Solid covalent compounds form molecular crystals.
- The reactions of covalent compounds are slow.
Write down the properties of metals.
Properties of metals
Following are some of the general characteristic properties of metals.
- All the metals are solid at room temperature and normal atmospheric pressure except Mercury (Liquid).
- Metals are malleable; it means that they can be folded in the form of sheets.
- Metals are ductile means that they are used in wire formation e.g. Copper, used in wires, is a metal.
- Metals are good conductors of heat and electricity because of the free electrons in them.
- Metals are lustrous meaning that they have a shiny surface.
- Metals are sonorous means that they produce a ringing sound when struck.
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