Valency of Elements & How to Find It

What is Valency?

Valency is the combining capacity of an element, which determines how many electrons an atom
can lose, gain, or share to form a chemical bond. It is based on the number of electrons in the
outermost shell (valence electrons).

How to Find Valency?

The valency of an element can be determined using these methods:

1. By the Octet Rule:

o Elements lose, gain, or share electrons to achieve a stable electronic
configuration (8 electrons in the outer shell).
o Metals (left side of the periodic table) tend to lose electrons → positive valency.
o Non-metals (right side) tend to gain electrons → negative valency.
2. By the Periodic Table:
o Group 1 elements (H, Li, Na, K, etc.) → Valency = 1
o Group 2 elements (Be, Mg, Ca, etc.) → Valency = 2
o Group 13 elements (B, Al, etc.) → Valency = 3
o Group 14 elements (C, Si, etc.) → Valency = 4
o Group 15 elements (N, P, etc.) → Valency = 3
o Group 16 elements (O, S, etc.) → Valency = 2
o Group 17 elements (F, Cl, etc.) → Valency = 1
o Group 18 elements (He, Ne, Ar, etc.) → Valency = 0 (Noble gases, already
stable)

Valency of Some Common Elements

Element Atomic Number Electronic Configuration Valency

Hydrogen (H) 1 1 1
Helium (He) 2 2 0
Lithium (Li) 3 2,1 1
Beryllium (Be) 4 2,2 2
Boron (B) 5 2,3 3
Carbon (C) 6 2,4 4
Nitrogen (N) 7 2,5 3
Oxygen (O) 8 2,6 2
Fluorine (F) 9 2,7 1
 Element Atomic Number Electronic Configuration Valency
Neon (Ne) 10 2,8 0
Sodium (Na) 11 2,8,1 1
Magnesium (Mg) 12 2,8,2 2
Aluminium (Al) 13 2,8,3 3
Silicon (Si) 14 2,8,4 4
Phosphorus (P) 15 2,8,5 3
Sulfur (S) 16 2,8,6 2
Chlorine (Cl) 17 2,8,7 1
Argon (Ar) 18 2,8,8 0

Let me know if you need valency for any other element! 😊

Bohr’s Model of Atom

Niels Bohr proposed a model of the atom in 1913 to explain the structure of atoms, especially the
hydrogen atom. His model is based on quantum theory and explains how electrons are arranged
around the nucleus.

Postulates of Bohr’s Model:

1. Electrons revolve in fixed orbits (energy levels): Electrons move around the nucleus in
fixed circular paths called orbits or shells, without radiating energy.
2. Quantization of energy levels: Each orbit has a specific energy, and electrons in these
orbits do not lose or gain energy unless they jump from one orbit to another.
3. Energy absorption or emission: When an electron moves from a lower energy level to a
higher one, it absorbs energy. When it moves from a higher level to a lower one, it emits
energy in the form of electromagnetic radiation.
4. Angular momentum is quantized: The angular momentum of an electron in an orbit is
given by the formula:

mvr=nh2πmvr = \frac{nh}{2\pi}

where m = mass of electron, v = velocity, r = radius of orbit, n = quantum number, and h

= Planck’s constant.

Rutherford’s Model of Atom

Ernest Rutherford conducted the gold foil experiment in 1911, which led to his nuclear model of
the atom.

Postulates of Rutherford’s Model:

 1. Most of the atom’s mass is concentrated in the nucleus: The nucleus is positively
charged and contains protons (and later discovered neutrons).
2. Electrons revolve around the nucleus: Electrons move in circular orbits around the
nucleus, similar to planets around the sun.
3. An atom is mostly empty space: Most of the alpha particles in the gold foil experiment
passed through the atom without deflection, proving that atoms have a lot of empty space.
4. The nucleus is very small and dense: A few alpha particles bounced back, indicating
that the nucleus is small but carries a high mass.

🔴 Limitation of Rutherford’s Model:

It could not explain why electrons do not spiral into the nucleus due to attraction between
opposite charges.

Difference between Isotopes and Isobars

Property Isotopes Isobars

Atoms of the same element with Atoms of different elements that have
Definition different mass numbers but the same the same mass number but different
atomic number. atomic numbers.
Hydrogen isotopes: Protium (¹H),
Example Argon-40 (⁴⁰Ar) and Calcium-40 (⁴⁰Ca)
Deuterium (²H), Tritium (³H)
Same number of protons but different Different number of protons and
Proton Count
neutrons. neutrons.
Chemical Same chemical properties due to the Different chemical properties because
Properties same number of electrons. they are different elements.
Physical Different physical properties due to Different physical properties due to
Properties different mass numbers. different atomic structures.

Let me know if you need more details! 😊

Difference between Eukaryotic and Prokaryotic Cells

Feature Prokaryotic Cells Eukaryotic Cells

Simple, primitive cells without a
Definition Complex cells with a well-defined nucleus
nucleus
Nucleus Absent (DNA is in nucleoid) Present, surrounded by a nuclear membrane
Size Small (1-10 µm) Large (10-100 µm)
Many membrane-bound organelles (e.g.,
Organelles Few organelles (e.g., ribosomes)
mitochondria, ER)
Cell
Binary fission Mitosis or meiosis
division
 Feature Prokaryotic Cells Eukaryotic Cells
Examples Bacteria, Archaea Plants, Animals, Fungi, Protists

Difference between Animal Cell and Plant Cell

Feature Animal Cell Plant Cell

Cell wall Absent Present (made of cellulose)
Chloroplasts Absent Present (for photosynthesis)
Vacuole Small and temporary Large central vacuole
Shape Round or irregular Rectangular or fixed
Centrioles Present Absent
Energy Storage Glycogen Starch

Diagram:
📌 Animal Cell & Plant Cell Diagram:
(You can refer to a textbook or search for a labeled diagram for a clear illustration.)

Why are Lysosomes Called Suicide Bags?

 Lysosomes contain digestive enzymes that break down waste, damaged organelles, and
harmful substances.
 If a lysosome bursts, its enzymes can digest the entire cell, leading to self-destruction
(autolysis).
 That’s why they are called suicide bags of the cell.

Why is Mitochondria Called the Powerhouse of the Cell?

 Mitochondria produce ATP (adenosine triphosphate), the energy currency of the cell.
 They carry out cellular respiration, converting glucose and oxygen into energy.
 All cellular activities require ATP, making mitochondria essential for energy production.

Structure and Function of the Nucleus

Structure:

 Nuclear Membrane: Double-layered membrane with pores for exchange of materials.

  Nucleoplasm: Jelly-like substance inside the nucleus.
 Nucleolus: Produces ribosomes.
 Chromatin: Contains DNA, which controls cell functions.

Functions:

1. Controls cell activities by regulating gene expression.

2. Stores genetic material (DNA).
3. Coordinates cell division (mitosis & meiosis).
4. Produces ribosomes in the nucleolus.

Types of Solutions: Hypertonic, Hypotonic, and Isotonic

Type of
Definition Effect on Cell
Solution
Hypertonic Higher solute concentration Water moves out, cell shrinks
Solution outside the cell (Plasmolysis in plants)
Hypotonic Lower solute concentration outside Water moves in, cell swells (may burst in
Solution the cell animals, turgid in plants)
Isotonic Equal solute concentration inside No net movement of water, cell remains
Solution and outside the cell normal

Let me know if you need diagrams or more explanations! 😊

Newton’s Laws of Motion with Examples

Sir Isaac Newton formulated three laws of motion, which describe the relationship between
force, motion, and inertia.

1st Law: Law of Inertia

📌 Statement:
"An object at rest stays at rest, and an object in motion stays in motion with the same speed and
in the same direction unless acted upon by an external unbalanced force."

📌 Explanation:

 If no external force is applied, an object will either remain stationary or continue moving
at a constant velocity.
 This property of objects to resist changes in motion is called inertia.
📌 Examples:

1. A passenger jerks forward when a moving bus stops suddenly → Due to inertia, the
body tends to remain in motion.
2. A book on a table remains there unless pushed.
3. A coin placed on a card over a glass falls straight into the glass when the card is
flicked away.

2nd Law: Law of Acceleration (F = ma)

📌 Statement:
"The rate of change of momentum of an object is directly proportional to the force applied and
occurs in the direction of the applied force."

📌 Mathematical Form:

F=maF = ma

where,

 F = Force (N)
 m = Mass of the object (kg)
 a = Acceleration (m/s²)

📌 Explanation:

 A greater force produces a greater acceleration.

 If mass increases, more force is required to accelerate the object.

📌 Examples:

1. Pushing a heavier object requires more force than pushing a lighter one.
2. A fast-moving cricket ball hurts more than a slow-moving one because of its higher
momentum.
3. A football moves faster when kicked with more force.

3rd Law: Action-Reaction Law

📌 Statement:
"For every action, there is an equal and opposite reaction."
📌 Explanation:

 When an object applies a force on another object, the second object applies an equal and
opposite force back.
 These forces act on different objects and do not cancel out.

📌 Examples:

1. A swimmer pushes water backward, and the water pushes the swimmer forward.
2. When a gun is fired, the bullet moves forward, and the gun recoils backward.
3. A rocket moves upward by pushing gases downward with high force.

Summary of Newton’s Laws

Law Definition Formula Example

Objects stay at rest or in uniform
A person falls forward when
1st Law (Inertia) motion unless acted upon by an -
a bus stops suddenly.
external force.
A heavier ball requires more
2nd Law Force is the product of mass and F=maF =
force to push than a lighter
(Acceleration) acceleration. ma
one.
3rd Law (Action- Every action has an equal and A rocket launches by
-
Reaction) opposite reaction. pushing gases downward.

Let me know if you need further explanations! 😊

Difference Between Mass and Weight

Property Mass Weight

The force exerted on an object due to
Definition The amount of matter in an object.
gravity.
Symbol m W
Formula - W=mgW = mg
Unit Kilogram (kg) Newton (N)
Nature Scalar (has only magnitude) Vector (has magnitude & direction)
Changes with Yes, depends on gravity (varies on
No, remains constant everywhere.
Location? Earth, Moon, etc.).
A person has the same mass on A person's weight on the Moon is 1/6th
Example
Earth and the Moon. of their weight on Earth.
Universal Law of Gravitation
📌 Statement:
"Every object in the universe attracts every other object with a force that is directly proportional
to the product of their masses and inversely proportional to the square of the distance between
them."

📌 Mathematical Formula:

F=Gm1m2r2F = G \frac{m_1 m_2}{r^2}

where,

 F = Gravitational Force
 G = Universal Gravitational Constant (6.674×10−11Nm2/kg2)(6.674 \times 10^{-11}
Nm^2/kg^2)
 m₁ & m₂ = Masses of two objects
 r = Distance between them

📌 Importance:

1. Explains why planets revolve around the Sun.

2. Helps in calculating the weight of objects on different planets.
3. Used in satellite motion and space exploration.

Archimedes' Principle
📌 Statement:
"A body completely or partially submerged in a fluid experiences an upward buoyant force equal
to the weight of the fluid displaced by it."

📌 Formula:

Fb=Weight of displaced fluidF_b = \text{Weight of displaced fluid}

where FbF_b = Buoyant force

📌 Applications:

1. Ships and Submarines: Ships float because their shape ensures they displace enough
water to balance their weight.
2. Hot Air Balloons: Buoyancy helps balloons rise in the air.
 3. Hydrometers: Used to measure the density of liquids based on buoyancy.

Let me know if you need further explanations! 😊

Situations Where Work is Done or Not

📌 Definition of Work (Physics):

Work is said to be done when a force is applied on an object and the object moves in the
direction of the applied force.

📌 Formula for Work:

W=F×d×cos⁡θW = F \times d \times \cos\theta

where:

 W = Work done
 F = Force applied
 d = Displacement of the object
 θ = Angle between force and displacement

Situations Where Work is Done ✅

1. Lifting a Book Upward → You apply an upward force, and the book moves upward.
2. Pushing a Cart Forward → Force is applied in the forward direction, and the cart
moves forward.
3. A Ball Falling from a Height → Gravity applies force, and the ball moves downward.
4. A Car Moving Due to Engine Force → The engine applies force, and the car moves.
5. Cycling on a Road → You apply force on the pedals, and the cycle moves forward.

Situations Where Work is Not Done ❌

1. Holding a Bag Without Moving → Force is applied, but there is no displacement.

2. Pushing a Wall → You apply force, but the wall does not move.
3. Standing Still While Carrying a Load → Force is applied, but no movement happens.
4. A Satellite Orbiting the Earth → The direction of force (gravity) is perpendicular to
displacement, so work done is zero.
5. Trying to Move a Heavy Object Without Success → Force is applied, but there is no
displacement.

📌 Key Condition for Work:

  If displacement = 0, then work = 0, even if a force is applied!

Let me know if you need more explanations! 😊

Derivation of Kinetic Energy Equation

📌 Kinetic energy (KE) is the energy possessed by a body due to its motion. It depends on mass
(m) and velocity (v) of the object.

Derivation:

1. Work-Energy Theorem:
o Work done on an object is equal to the change in its kinetic energy.
o Work (W) = Force × Displacement

W=F×dW = F \times d

2. Using Newton’s Second Law (F = ma):

o Force F=maF = ma (mass × acceleration)
o Displacement using third equation of motion:

v2−u2=2asv^2 - u^2 = 2as

o If the object starts from rest (u=0u = 0):

v2=2asv^2 = 2as s=v22as = \frac{v^2}{2a}

3. Substituting in Work Equation:

W=ma×v22aW = ma \times \frac{v^2}{2a} W=12mv2W = \frac{1}{2} mv^2

4. Conclusion:
o The work done on the object is converted into kinetic energy:

KE=12mv2KE = \frac{1}{2} mv^2

o This is the kinetic energy equation.

Examples of Kinetic and Potential Energy

Kinetic Energy (Energy of Motion)

1. A moving car
 2. A running athlete
3. A rolling ball
4. Flowing water in a river
5. A flying bird

Potential Energy (Stored Energy due to Position)

1. Water stored in a dam (Gravitational PE)

2. A stretched rubber band (Elastic PE)
3. A compressed spring
4. A book placed on a table
5. A hanging fruit on a tree

Let me know if you need further clarifications! 😊

Difference Between Kinetic Energy and Potential Energy

Feature Kinetic Energy (KE) Potential Energy (PE)

Energy possessed by a body due to its Energy stored in a body due to its position
Definition
motion. or state.
Formula KE=1/2mv2 PE=mgh
Depends on Mass and velocity of the object. Mass, height, and gravity.
Type of
Energy of motion. Stored energy.
Energy
- A moving car 🚗💨 - A rolling ball - Water stored in a dam 💦 - A stretched
Examples ⚽ - A running person 🏃♂️ - A flying rubber band 🏹 - A book on a table 📚 - A
airplane ✈️ compressed spring 🔧

📌 Key Difference:

 KE exists when an object is in motion.

 PE exists due to position or stored energy, even when the object is at rest.

Let me know if you need more clarifications! 😊