For Class 8, 9 & 10
Master the basics - and everything after gets easier
Concept-first questions with clear model answers in Physics, Chemistry, Maths and Biology, all NCERT-aligned. Start early, build the habit, and walk into your boards, NEET and JEE already ahead.
Interactive lessons
learn by playingDrag, slide and build - watch each concept come alive, then reveal the answer.
200 interactive lessons
Ohm's law
Class 10 Physics
Slide V & R, watch the bulb glow
Open →pH scale
Class 10 Chemistry
Slide across acids and bases
Open →Atomic number and mass number
Class 9 Chemistry
Add protons & neutrons, build shells
Open →Laws of reflection
Class 8 Physics
Change the angle, watch it bounce
Open →Volume of a sphere
Class 9 Maths
Grow the radius, see the volume
Open →Area of a trapezium
Class 8 Maths
Drag the sides, read the area
Open →Power of a lens
Class 10 Physics
Move the object, trace the rays
Open →Food chain and energy flow
Class 10 Biology
Follow the energy as it flows
Open →Speed
Class 8 Physics
Slide distance & time, watch the speed
Open →Density
Class 9 Physics
Pack mass into volume, float or sink
Open →Work done
Class 9 Physics
Push harder or farther, watch work grow
Open →Kinetic energy
Class 9 Physics
Speed it up - energy grows with the square
Open →Power of a lens
Class 10 Physics
Shorten the focal length, boost the power
Open →Mole concept
Class 9 Chemistry
Weigh out grams, count the moles
Open →Avogadro's number
Class 9 Chemistry
Add moles, count the particles
Open →Microscope magnification
Class 8 Biology
Grow the image, read the magnification
Open →Population density
Class 10 Biology
Add individuals, shrink the land, see crowding
Open →Simple interest
Class 8 Maths
Slide money, rate & time, watch interest
Open →Pythagoras theorem
Class 9 Maths
Stretch the two sides, get the hypotenuse
Open →Probability of an event
Class 10 Maths
Change the outcomes, watch the odds
Open →Newton's second law
Class 9 Physics
Push a mass, pick an acceleration
Open →Momentum
Class 9 Physics
Slide mass & velocity, build momentum
Open →Pressure
Class 8 Physics
Shrink the area, feel the pressure rise
Open →Weight
Class 9 Physics
Change the planet's gravity, watch your weight
Open →Refractive index
Class 10 Physics
Slow light in the medium, raise the index
Open →Resistors in series
Class 10 Physics
Add two resistors in a line
Open →Mass percentage of a solution
Class 9 Chemistry
Dissolve solute, read the strength
Open →Concentration of a solution
Class 9 Chemistry
Pack solute into less liquid
Open →Population change
Class 10 Biology
Balance births against deaths
Open →Compound microscope
Class 8 Biology
Combine eyepiece & objective lenses
Open →Area of a circle
Class 8 Maths
Grow the radius, watch the area square
Open →Volume of a cuboid
Class 8 Maths
Stretch length, breadth & height
Open →Electronic configuration and valency
Class 9 Chemistry
Slide the atomic number, build the atom
Open →Homologous series (alkanes)
Class 10 Chemistry
Add carbons, name the compound
Open →Mass number
Class 9 Chemistry
Add protons & neutrons, get the mass number
Open →Power
Class 9 Physics
More work in less time = more power
Open →Potential energy
Class 9 Physics
Lift a mass higher, store energy
Open →Wave speed
Class 9 Physics
Tune frequency & wavelength, set the speed
Open →Electric current
Class 10 Physics
Push charge per second, get the current
Open →Percentage
Class 8 Maths
Compare part to whole as a %
Open →Electron dot structure
Class 9 Chemistry
Draw valence electrons as dots
Open →Acceleration
Class 9 Physics
Speed up over time, find acceleration
Open →Distance, speed and time
Class 8 Physics
Set speed & time, cover the distance
Open →Frequency and time period
Class 9 Physics
Shorten the period, raise the frequency
Open →Heating effect of current
Class 10 Physics
Raise the current, watch heating soar
Open →Area of a triangle
Class 8 Maths
Set base & height, halve the rectangle
Open →Area of a rectangle
Class 8 Maths
Set length & breadth, fill the area
Open →Mean (average)
Class 9 Maths
Share the total equally across items
Open →Discount
Class 8 Maths
Slide price & % off, see the saving
Open →Heart rate
Class 10 Biology
Set heart rate & time, count the beats
Open →Resistors in parallel
Class 10 Physics
Wire two resistors side by side
Open →Electric charge
Class 10 Physics
Flow current over time, collect charge
Open →Electrical energy and units
Class 10 Physics
Run appliances, add up the units
Open →Kelvin temperature scale
Class 9 Chemistry
Slide Celsius, read the Kelvin
Open →Moles from number of particles
Class 9 Chemistry
Divide particles by Avogadro's number
Open →Ten percent law
Class 10 Biology
See 10% of energy reach the next level
Open →Area of a square
Class 8 Maths
Grow the side, square the area
Open →Volume of a cube
Class 8 Maths
Grow the edge, cube the volume
Open →Circumference of a circle
Class 8 Maths
Grow the radius, roll out the rim
Open →Surface area of a cube
Class 9 Maths
Grow the edge, cover six faces
Open →Potential difference
Class 10 Physics
Share work across charge, get volts
Open →Resistance from Ohm's law
Class 10 Physics
Divide voltage by current, get resistance
Open →Echo and SONAR
Class 9 Physics
Time the echo, find the distance
Open →Mass from moles
Class 9 Chemistry
Multiply moles by molar mass
Open →Breathing rate
Class 10 Biology
Set breathing rate & time
Open →Volume of a cylinder
Class 10 Maths
Set radius & height, fill the can
Open →Compound interest
Class 8 Maths
Compound money over years
Open →Profit and loss percentage
Class 8 Maths
Set cost & selling price, see profit %
Open →Perimeter of a rectangle
Class 8 Maths
Set length & breadth, walk the border
Open →Surface area of a sphere
Class 10 Maths
Grow the radius, wrap the ball
Open →Time period
Class 9 Physics
Raise the frequency, shrink the period
Open →Relative velocity
Class 9 Physics
Two objects approach - add their speeds
Open →Average velocity
Class 9 Physics
Average the start and end speeds
Open →Equations of motion (v = u + at)
Class 9 Physics
Accelerate from u for a time t
Open →Kelvin to Celsius
Class 9 Chemistry
Slide Kelvin, read the Celsius
Open →Population growth rate
Class 10 Biology
Balance births vs deaths per population
Open →Perimeter of a square
Class 8 Maths
Grow the side, walk four edges
Open →Perimeter of a triangle
Class 8 Maths
Add the three sides
Open →Area of a parallelogram
Class 8 Maths
Set base & height, slide the shape
Open →Area of a rhombus
Class 8 Maths
Set the two diagonals
Open →Equations of motion (distance)
Class 9 Physics
Start, accelerate, cover ground
Open →Joule's law of heating
Class 10 Physics
Raise current, resistance or time
Open →Electric power (P = VI)
Class 10 Physics
Multiply voltage by current
Open →Average atomic mass of isotopes
Class 9 Chemistry
Mix two isotopes by abundance
Open →Seed germination percentage
Class 9 Biology
Count sprouted seeds out of the total
Open →Volume of a cone
Class 9 Maths
Set radius & height, fill the cone
Open →Surface area of a cylinder
Class 9 Maths
Wrap the side and both ends
Open →Surface area of a cuboid
Class 9 Maths
Cover all six rectangular faces
Open →nth term of an AP
Class 10 Maths
Step from the first term by d
Open →Sum of an AP
Class 10 Maths
Add up the first n terms
Open →Focal length of a mirror
Class 10 Physics
Halve the radius to find the focus
Open →Speed of light in a medium
Class 10 Physics
Raise the index, slow the light
Open →Percentage purity
Class 9 Chemistry
Weigh the pure part of a sample
Open →Slope of a line
Class 10 Maths
Rise over run gives the steepness
Open →Percentage change
Class 8 Maths
Compare a new value to the old
Open →Volume of a hemisphere
Class 9 Maths
Grow the radius of half a ball
Open →Area of a sector
Class 10 Maths
Cut a slice of angle from a circle
Open →Length of an arc
Class 10 Maths
Measure the curved edge of a slice
Open →Slant height of a cone
Class 9 Maths
Combine radius & height for the slant
Open →Unit conversion (km/h to m/s)
Class 9 Physics
Convert km/h into m/s
Open →Equations of motion (v^2 = u^2 + 2as)
Class 9 Physics
Accelerate over a distance, find v
Open →Impulse
Class 9 Physics
Hit harder or longer, change momentum
Open →Wavelength
Class 9 Physics
Speed over frequency gives wavelength
Open →Oscillations
Class 9 Physics
Vibrate at a frequency for a time
Open →Cost of electricity
Class 10 Physics
Units times rate gives the bill
Open →Number of neutrons
Class 9 Chemistry
Take protons away from the mass number
Open →Curved surface area of a cone
Class 9 Maths
Wrap the slanted side of a cone
Open →Total surface area of a cone
Class 9 Maths
Add the base circle to the cone's side
Open →Curved surface area of a hemisphere
Class 9 Maths
Cover the dome of a hemisphere
Open →Diagonal of a square
Class 9 Maths
Cross a square corner to corner
Open →Unit conversion (m/s to km/h)
Class 9 Physics
Convert m/s into km/h
Open →Distance from velocities
Class 9 Physics
From two speeds, find the distance
Open →Diagonal of a rectangle
Class 9 Maths
Cross a rectangle corner to corner
Open →Diagonal of a cuboid
Class 9 Maths
The longest rod that fits in a box
Open →Area by Heron's formula
Class 9 Maths
Area from just the three sides
Open →Interior angle sum of a polygon
Class 8 Maths
Add up a polygon's inside angles
Open →Exterior angle of a regular polygon
Class 8 Maths
Share 360 among a polygon's corners
Open →Number of diagonals of a polygon
Class 8 Maths
Count the diagonals of a polygon
Open →Discriminant
Class 10 Maths
Test how many roots a quadratic has
Open →Sum of roots
Class 10 Maths
Sum of a quadratic's roots
Open →Punnett square (monohybrid cross)
Class 10 Biology
Cross two parents, predict the offspring
Open →Balancing chemical equations
Class 10 Chemistry
Slide coefficients until atoms balance
Open →Writing chemical formulae (valency)
Class 9 Chemistry
Criss-cross valencies into a formula
Open →Current from power
Class 10 Physics
Divide power by voltage for current
Open →Power (P = V^2 / R)
Class 10 Physics
Voltage squared over resistance
Open →Sine ratio
Class 10 Maths
Opposite over hypotenuse
Open →Cosine ratio
Class 10 Maths
Adjacent over hypotenuse
Open →Tangent ratio
Class 10 Maths
Opposite over adjacent
Open →Area of an equilateral triangle
Class 9 Maths
Area of an equilateral triangle
Open →Curved surface area of a cylinder
Class 9 Maths
Wrap only the curved side
Open →Loss percentage
Class 8 Maths
Sell below cost, find the loss %
Open →Amount with simple interest
Class 8 Maths
Principal plus its simple interest
Open →Distance formula
Class 10 Maths
Straight distance between two points
Open →States of matter
Class 9 Chemistry
Heat particles solid → liquid → gas
Open →Parts of a plant cell
Class 8 Biology
Tap a cell part to see its job
Open →Diagonal of a cube
Class 9 Maths
Longest diagonal through a cube
Open →Total surface area of a hemisphere
Class 9 Maths
Dome plus its flat circle
Open →Sum of first n natural numbers
Class 10 Maths
Add 1 + 2 + ... + n instantly
Open →Range of data
Class 9 Maths
Spread from smallest to largest
Open →Class mark
Class 9 Maths
Midpoint of a class interval
Open →Selling price from profit percent
Class 8 Maths
Mark up cost by a profit %
Open →Perimeter of a sector
Class 10 Maths
Two radii plus the curved arc
Open →Circumference from diameter
Class 8 Maths
Circumference straight from diameter
Open →Power (P = F x v)
Class 9 Physics
Force times velocity gives power
Open →Percentage of a number
Class 8 Maths
Find a percentage of a number
Open →Series and parallel circuits
Class 10 Physics
Break a bulb in series vs parallel
Open →Symbols of elements
Class 9 Chemistry
Match each element to its symbol
Open →Free fall (velocity)
Class 9 Physics
Drop from a height, hit this speed
Open →Free fall (time)
Class 9 Physics
How long a drop takes
Open →Free fall (distance)
Class 9 Physics
Distance fallen in a given time
Open →Complement of an event
Class 10 Maths
Chance an event does NOT happen
Open →Product of roots
Class 10 Maths
Product of a quadratic's roots
Open →Exterior angle theorem
Class 9 Maths
Exterior angle = sum of remote interiors
Open →Complementary angles
Class 10 Maths
What adds to 90 degrees
Open →Supplementary angles
Class 9 Maths
What adds to 180 degrees
Open →Perimeter of a semicircle
Class 10 Maths
Curved half plus the diameter
Open →Area of a semicircle
Class 10 Maths
Half the area of a circle
Open →Turning effect (moments)
Class 9 Physics
Balance the see-saw with moments
Open →Reflex arc
Class 10 Biology
Step through a reflex, stimulus to action
Open →Buoyant force (upthrust)
Class 9 Physics
Displace liquid, feel the upthrust
Open →Relative density
Class 9 Physics
Compare a density to water's
Open →Power in lifting a load
Class 9 Physics
Lift a load, faster needs more power
Open →Cosecant ratio
Class 10 Maths
Hypotenuse over opposite
Open →Secant ratio
Class 10 Maths
Hypotenuse over adjacent
Open →Cotangent ratio
Class 10 Maths
Adjacent over opposite
Open →Height from angle of elevation
Class 10 Maths
Height from an angle of elevation
Open →Area of a quadrant
Class 10 Maths
A quarter of a circle's area
Open →Interior angle of a regular polygon
Class 8 Maths
One inside angle of a regular polygon
Open →Sum of first n odd numbers
Class 10 Maths
Add the first n odd numbers
Open →Sum of first n even numbers
Class 10 Maths
Add the first n even numbers
Open →Quadratic formula (a root)
Class 10 Maths
Larger root of a quadratic
Open →LCM from HCF
Class 10 Maths
LCM from the product and HCF
Open →Depreciation
Class 8 Maths
Value drops by a % each year
Open →Cost price from selling price
Class 8 Maths
Work back to the cost price
Open →Downstream speed
Class 8 Maths
Row with the current
Open →Upstream speed
Class 8 Maths
Row against the current
Open →Average speed for a round trip
Class 8 Maths
Average speed there and back
Open →Sales tax / GST
Class 8 Maths
Tax added on a price
Open →Area of a ring (annulus)
Class 10 Maths
Area of a ring between two circles
Open →Edge of a cube from volume
Class 9 Maths
Edge back from the volume
Open →Radius from area
Class 10 Maths
Radius back from a circle's area
Open →Side from area of a square
Class 8 Maths
Side back from a square's area
Open →Height of a triangle from area
Class 9 Maths
Height back from area and base
Open →Rate from simple interest
Class 8 Maths
Rate back from the interest
Open →Time from simple interest
Class 8 Maths
Time back from the interest
Open →Principal from simple interest
Class 8 Maths
Principal back from the interest
Open →Mean proportional
Class 10 Maths
Geometric mean of two numbers
Open →Fourth proportional
Class 8 Maths
Complete the proportion a : b = c : ?
Open →Marked price from selling price
Class 8 Maths
Marked price back from the sale price
Open →Chambers of the human heart
Class 10 Biology
Tap a heart chamber to see its job
Open →Equation of a line (y = mx + c)
Class 9 Maths
Read y off a straight line
Open →Average term of an AP
Class 10 Maths
Average of first and last term
Open →Number of terms in an AP
Class 10 Maths
How many terms in an AP
Open →Midpoint of two points
Class 10 Maths
x-coordinate of a midpoint
Open →Empirical mode
Class 10 Maths
Estimate the mode from mean & median
Open →Length of a shadow
Class 10 Maths
Shadow from height and sun angle
Open →Train crossing a pole
Class 8 Maths
Speed to cross a pole
Open →Time and work
Class 8 Maths
More workers, fewer days
Open →Dividing in a ratio
Class 8 Maths
Split a total in a ratio
Open →Unitary method
Class 8 Maths
Cost of a single item
Open →Your progress — Foundation - Class 8 to 10
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Showing 14 questions for Class 9. Tap a card to reveal the answer.
ChemistryStructure of the AtomeasyDefine the atomic number and the mass number of an atom.
Reveal answer ↓
What it is
Atomic number = number of protons (the element's identity); mass number = protons + neutrons.
Z = 6 · A = 6 + 6 = 12 · Carbon (C)
Answer
The atomic number (Z) is the number of protons in the nucleus of an atom, which also equals the number of electrons in a neutral atom; it decides the identity of the element. The mass number (A) is the total number of protons and neutrons in the nucleus. So the number of neutrons = A - Z.
A = Z + number of neutrons
- •Atomic number Z = number of protons (= electrons if neutral)
- •Mass number A = protons + neutrons
- •Neutrons = A - Z
- •Z identifies the element
Why learn this
These two numbers decide every element in the periodic table and its isotopes.
💡 Memory trick
Z = protons (Zee identity); A = All heavy bits (protons + neutrons). Neutrons = A - Z.
ChemistryStructure of the AtommediumWhat are isotopes? Give one example.
Reveal answer ↓
What it is
Isotopes are atoms of the same element with the same protons but different numbers of neutrons.
Answer
Isotopes are atoms of the same element that have the same atomic number (the same number of protons) but different mass numbers (different numbers of neutrons). Because they have the same number of protons and electrons, they show the same chemical properties. For example, carbon-12 and carbon-14 are isotopes of carbon.
Same Z, different A
- •Same element, same protons (same atomic number)
- •Different neutrons (different mass number)
- •Same chemical properties
- •Example: carbon-12 and carbon-14
Why learn this
Isotopes are used in cancer treatment, carbon dating and nuclear energy.
💡 Memory trick
Same element (same protons), different mass (different neutrons). ISO = same place in the table.
ChemistryStructure of the AtommediumHow do you write the electronic configuration and find the valency of an element? Slide the atomic number from H to Ca to explore.
Reveal answer ↓
What it is
The atomic number fixes how many electrons an atom has; filling shells by the 2, 8, 8 rule gives its electronic configuration, valence electrons and valency.
C
Carbon
Atomic number Z = 6
6 protons, 6 electrons
Configuration: 2, 4 (K, L)
Valence electrons: 4
Valence electrons are 1–4, so valency = 4.
Valency = 4
Answer
Start with the atomic number Z = number of protons = number of electrons in a neutral atom. Fill the shells K, L, M, N using the 2, 8, 8, 2 pattern (the Bohr-Bury scheme) to get the electronic configuration. The electrons in the outermost shell are the valence electrons. Then: if there are 1-4 valence electrons the valency equals that number; if there are 5-7, valency = 8 - (valence electrons); and if the outer shell is complete (2 for helium, 8 for the other noble gases) the valency is 0. Example: carbon (Z = 6) is 2, 4, so 4 valence electrons and valency 4. Helium (Z = 2) has a full K shell of 2, so its valency is 0.
valency: 1-4 -> n ; 5-7 -> 8 - n ; full shell -> 0
- •Z = protons = electrons (neutral atom)
- •Fill shells by 2, 8, 8, 2
- •Valence electrons = outermost shell
- •1-4 -> valency = that number; 5-7 -> 8 - electrons; full shell -> 0
- •Helium's outer shell (2) is full, so valency = 0
Why learn this
Valency decides how atoms combine, so it is the key to every chemical formula.
💡 Memory trick
Fill shells 2, 8, 8. Valence 1-4 -> valency = that number; 5-7 -> 8 minus it; full shell -> 0.
ChemistryStructure of the AtomeasyAn atom has 6 protons and 6 neutrons. Find its mass number. Slide to explore.
Reveal answer ↓
What it is
The mass number of an atom is the total count of protons and neutrons in its nucleus.
Answer
Mass number A = protons + neutrons = 6 + 6 = 12 (this is carbon-12). The number of protons is the atomic number and fixes which element it is; changing only the neutrons gives a different isotope of the same element.
A = protons + neutrons
- •Mass number A = protons + neutrons
- •Protons = atomic number (Z)
- •Same Z, different neutrons -> isotopes
Why learn this
It distinguishes isotopes and gives the atom almost all of its mass.
💡 Memory trick
A = p + n. Protons name the element; neutrons add mass.
ChemistryStructure of the AtommediumHow do you draw the electron dot structure of an element? Slide the atomic number to explore.
Reveal answer ↓
What it is
An electron dot (Lewis) structure shows only the valence electrons of an atom as dots around its symbol.
Configuration: 2, 4
Valence electrons (dots): 4
Only the outermost electrons are drawn - these are the ones that form bonds.
Answer
First find the number of valence electrons (the electrons in the outermost shell) from the electronic configuration. Then write the element's symbol and place that many dots around it, one for each valence electron. For example, carbon (2, 4) has 4 valence electrons, so 4 dots; oxygen (2, 6) has 6 dots; sodium (2, 8, 1) has just 1. These outer electrons are the ones involved in chemical bonding.
dots = number of valence electrons
- •Show only the valence (outer-shell) electrons
- •One dot per valence electron, around the symbol
- •Carbon -> 4 dots, oxygen -> 6 dots, sodium -> 1 dot
- •These electrons form chemical bonds
Why learn this
Those outer electrons are the ones that form bonds, so dot structures explain how atoms join.
💡 Memory trick
Draw the symbol, then one dot per valence electron around it (up to 8).
ChemistryStructure of the AtommediumChlorine is 75% mass-35 and 25% mass-37. Find its average atomic mass. Slide to explore.
Reveal answer ↓
What it is
The average atomic mass is the weighted mean of the masses of an element's isotopes, using their percentage abundances.
Answer
Average atomic mass = (35 x 75 + 37 x 25) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5 u. It is a weighted average, so the more abundant isotope pulls the value closer to its own mass.
average = (m1 x a1 + m2 x a2) / 100
- •Weighted mean of isotope masses
- •Average = (m1 x a1 + m2 x a2) / 100
- •Explains non-whole atomic masses like 35.5
Why learn this
It's why atomic masses like chlorine's 35.5 are not whole numbers.
💡 Memory trick
Average = (m1 x a1 + m2 x a2) / 100, with a1 + a2 = 100.
ChemistryStructure of the AtomeasyAn atom has mass number 12 and atomic number 6. How many neutrons does it have? Slide to explore.
Reveal answer ↓
What it is
The number of neutrons in an atom is its mass number minus its atomic number.
Answer
Neutrons = mass number - atomic number = 12 - 6 = 6. The atomic number gives the protons, so subtracting it from the mass number (protons + neutrons) leaves the number of neutrons.
neutrons = A - Z
- •Neutrons = A - Z
- •A = mass number, Z = atomic number
- •Carbon-12 has 6 neutrons
Why learn this
It's how we count the neutrons that distinguish one isotope from another.
💡 Memory trick
Neutrons = A - Z (mass number minus protons).
ChemistryStructure of the AtomeasyName the three subatomic particles with their charge, mass and discoverer.
Reveal answer ↓
What it is
An atom is made of protons and neutrons in the nucleus, with electrons in shells around it.
Answer
The three subatomic particles are: the electron, which has a negative charge and negligible mass and was discovered by J. J. Thomson; the proton, which has a positive charge and a mass of about 1 u and was discovered by Goldstein; and the neutron, which has no charge and a mass of about 1 u and was discovered by James Chadwick. Protons and neutrons are in the nucleus, and electrons revolve around it.
- •Electron: negative, negligible mass (Thomson)
- •Proton: positive, 1 u (Goldstein)
- •Neutron: neutral, 1 u (Chadwick)
- •Protons + neutrons in nucleus; electrons outside
Why learn this
It is the basis for understanding chemical behaviour and the periodic table.
💡 Memory trick
Proton (+, discovered by Goldstein), Electron (-, Thomson), Neutron (0, Chadwick).
ChemistryStructure of the AtommediumDescribe Rutherford's model of the atom and how it improved on Thomson's model.
Reveal answer ↓
What it is
Thomson pictured the atom as positive matter with embedded electrons; Rutherford's experiment showed a small, dense, positive nucleus.
Answer
Thomson's model described the atom as a sphere of positive charge with negatively charged electrons embedded in it, like plums in a pudding. Rutherford's alpha-particle scattering experiment showed that most of the atom is empty space, that there is a very small, dense, positively charged centre called the nucleus where the mass is concentrated, and that electrons revolve around this nucleus. This improved on Thomson's model by introducing the nucleus.
- •Thomson: positive sphere with embedded electrons
- •Rutherford: alpha scattering experiment
- •Small, dense, positive nucleus
- •Electrons revolve around the nucleus
Why learn this
Rutherford's model corrected Thomson's and introduced the nucleus.
💡 Memory trick
Thomson = 'plum pudding'. Rutherford = alpha-scattering revealed the tiny nucleus.
ChemistryStructure of the AtommediumState the main postulates of Bohr's model of the atom.
Reveal answer ↓
What it is
Bohr proposed that electrons revolve around the nucleus only in certain fixed energy shells without radiating energy.
Answer
According to Bohr's model, electrons revolve around the nucleus only in certain permitted circular paths called energy levels or shells, which are named K, L, M, N and so on. While revolving in these shells, an electron does not lose or gain energy, so the atom is stable. Energy is absorbed or emitted only when an electron jumps from one shell to another. This model explained the stability of the atom.
Shells K, L, M, N for n = 1, 2, 3, 4
- •Electrons revolve in fixed shells (K, L, M, N)
- •No energy lost while in a shell (stable atom)
- •Energy change only on jumping shells
- •Improved atomic stability over Rutherford
Why learn this
It explained the stability of atoms, which Rutherford's model could not.
💡 Memory trick
Shells are named K, L, M, N (n = 1, 2, 3, 4) - fixed energy 'orbits'.
ChemistryStructure of the AtomeasyAn atom has 17 protons and 18 neutrons. Find its atomic number and mass number.
Reveal answer ↓
What it is
Atomic number is the number of protons; mass number is the sum of protons and neutrons.
Answer
The atomic number (Z) equals the number of protons, so Z = 17. The mass number (A) equals the number of protons plus the number of neutrons, so A = 17 + 18 = 35. This atom is chlorine, written as Cl with mass number 35.
A = Z + number of neutrons
- •Atomic number Z = protons = 17
- •Mass number A = protons + neutrons
- •A = 17 + 18 = 35
- •Element is chlorine (Cl-35)
Why learn this
These numbers identify an element and its isotopes.
💡 Memory trick
Z = protons; A = protons + neutrons; neutrons = A - Z.
ChemistryStructure of the AtommediumDifferentiate between isotopes and isobars with an example each.
Reveal answer ↓
What it is
Isotopes have the same atomic number but different mass numbers; isobars have the same mass number but different atomic numbers.
Answer
Isotopes are atoms of the same element that have the same atomic number but different mass numbers because they have different numbers of neutrons, for example carbon-12 and carbon-14. Isobars are atoms of different elements that have different atomic numbers but the same mass number, for example calcium-40 and argon-40. Isotopes have identical chemical properties, while isobars do not.
- •Isotopes: same Z, different A (C-12, C-14)
- •Isobars: same A, different Z (Ca-40, Ar-40)
- •Isotopes differ in neutrons
- •Isotopes have the same chemical properties
Why learn this
Isotopes explain fractional atomic masses and are used in medicine and energy.
💡 Memory trick
Iso-TOP-es: same TOP (protons/atomic number). Iso-BAR-s: same mass number (A).
ChemistryStructure of the AtommediumWrite the electronic configuration of magnesium (atomic number 12) and state its valency.
Reveal answer ↓
What it is
Electrons fill shells according to the rule that a shell can hold a maximum of 2n^2 electrons, filling inner shells first.
Answer
Magnesium has 12 electrons. Using the rule that each shell holds a maximum of 2n^2 electrons and inner shells fill first: K shell holds 2, L shell holds 8, and the remaining 2 go into the M shell. So the electronic configuration is 2, 8, 2. Since it has 2 electrons in the outermost shell, magnesium loses them easily, so its valency is 2.
Maximum electrons in shell n = 2n^2
- •Maximum electrons in a shell = 2n^2
- •K=2, L=8, then M
- •Magnesium (12): 2, 8, 2
- •Valency = 2
Why learn this
The arrangement of electrons decides an element's chemical behaviour and valency.
💡 Memory trick
Max electrons per shell = 2n^2: K=2, L=8, M=18. Fill from the innermost outward.
ChemistryStructure of the AtommediumHow is valency determined from the electronic configuration? Find the valency of chlorine (2, 8, 7).
Reveal answer ↓
What it is
Valency is found from the number of valence (outermost) electrons: it equals that number if it is 1 to 4, or 8 minus that number if it is 4 to 8.
Answer
Valency is the combining capacity of an atom, determined by the number of electrons in its outermost shell (valence electrons). If the outermost shell has 1 to 4 electrons, the valency equals the number of valence electrons; if it has 5 to 8 electrons, the valency equals 8 minus the number of valence electrons, because the atom tends to complete its octet. Chlorine has the configuration 2, 8, 7, so it has 7 valence electrons and its valency is 8 - 7 = 1.
Valency = valence electrons or (8 - valence electrons)
- •Valency from valence (outermost) electrons
- •1 to 4 electrons: valency = number of electrons
- •5 to 8 electrons: valency = 8 - number
- •Chlorine (2,8,7): valency = 8 - 7 = 1
Why learn this
It lets us predict how atoms combine without memorising every valency.
💡 Memory trick
Outer electrons 1-4 -> valency = that number. Outer electrons 5-8 -> valency = 8 minus it.
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