Journey inside the atom - Discovery, models, and subatomic particles
The word "atom" comes from the Greek word "atomos" meaning "indivisible". For a long time, people thought atoms were the smallest particles that couldn't be divided. But scientists discovered that atoms are actually made up of even smaller particles!
Imagine you thought a cherry was the smallest fruit. But then you discovered it has a seed inside, and the seed has smaller parts! Similarly, scientists discovered that atoms have even smaller parts inside them called subatomic particles.
Scientists discovered that atoms are made up of three main particles:
Electrons are negatively charged particles that move around the nucleus of an atom.
β’ Charge: -1 (negative)
β’ Mass: Negligible (1/1840 of proton)
β’ Symbol: eβ»
β’ Location: Outside nucleus, in energy shells
J.J. Thomson used a special tube called cathode ray tube. When he passed electricity through it in vacuum, he saw rays coming from the negative end (cathode) to positive end (anode). These rays were made of tiny negatively charged particles - the electrons!
Key observation: These particles were same from any gas or metal used, showing that electrons are present in all atoms!
Protons are positively charged particles found in the nucleus of an atom.
β’ Charge: +1 (positive)
β’ Mass: 1 atomic mass unit (u)
β’ Symbol: pβΊ
β’ Location: Inside nucleus
Goldstein discovered canal rays (also called anode rays) using a perforated cathode. These rays moved towards the negative plate, showing they were positively charged. Later it was understood that these were protons!
Neutrons are neutral particles (no charge) found in the nucleus of an atom.
β’ Charge: 0 (neutral/no charge)
β’ Mass: 1 atomic mass unit (u), slightly more than proton
β’ Symbol: n
β’ Location: Inside nucleus
Think of an atom like a solar system:
β’ Nucleus (center) = Sun β Contains protons and
neutrons
β’ Electrons = Planets β Revolve around the
nucleus in orbits
β’ Protons = Positive friends in the center
β’ Neutrons = Neutral peacekeepers in the
center
β’ Electrons = Negative particles moving outside
| Particle | Symbol | Charge | Mass (u) | Location | Discovered by |
|---|---|---|---|---|---|
| Electron | eβ» | -1 | ~0 (1/1840) | Outside nucleus | J.J. Thomson |
| Proton | pβΊ | +1 | 1 | Inside nucleus | E. Goldstein |
| Neutron | n | 0 | 1 | Inside nucleus | James Chadwick |
As scientists learned more about atoms, they developed different models to explain their structure. Let's see how these models evolved:
J.J. Thomson proposed that an atom is like a sphere of positive charge with electrons embedded in it like plums in a pudding or seeds in a watermelon.
Main ideas:
β’ Atom is a positive sphere
β’ Electrons are scattered inside like plums in pudding
β’ Overall atom is neutral (positive = negative charges)
Imagine a watermelon! The red part is positive charge, and the black seeds scattered inside are electrons. This was Thomson's idea - also called the "Plum Pudding Model" or "Watermelon Model".
This model couldn't explain certain experiments, especially Rutherford's alpha particle scattering experiment. So scientists had to think of a better model!
Ernest Rutherford performed the famous Gold Foil Experiment and proposed a new model.
Main ideas:
β’ Most of the atom is empty space
β’ All positive charge and mass is concentrated in a tiny center
called the nucleus
β’ Electrons revolve around the nucleus in circular paths
β’ Atom looks like a miniature solar system
What Rutherford did:
He bombarded a very thin gold foil with fast-moving alpha
particles (positively charged particles).
What he expected:
According to Thomson's model, all particles should pass through
the foil easily or be slightly deflected.
What actually happened:
β’ Most alpha particles passed straight through (atom is mostly
empty!)
β’ Some particles deflected at small angles (positive charge
repelled them)
β’ Very few particles bounced back (hit something very dense and
positive)
Conclusion:
There must be a tiny, dense, positively charged center (nucleus)
with lots of empty space around it!
Imagine throwing marbles at a football field. Most marbles will pass through the field (empty space), but if there's a small rock in the center, some marbles will hit it and bounce back. That rock is like the nucleus!
β’ According to physics, a charged particle moving in a circle
should lose energy and spiral into the nucleus. But electrons
don't fall into the nucleus! Why?
β’ This model couldn't explain the stability of atoms
β’ Couldn't explain how atoms emit light
Niels Bohr improved Rutherford's model by explaining why electrons don't fall into the nucleus.
Main ideas:
β’ Electrons revolve around nucleus in fixed circular paths called
orbits or shells
β’ Each shell has a fixed energy level (K, L, M, N...)
β’ Electrons in a shell do NOT lose energy
β’ Electrons can jump from one shell to another by gaining or
losing energy
β’ When electrons jump, they emit or absorb light
Imagine a multi-storey parking lot! Cars (electrons) can park only on fixed floors (shells), not between floors. They need energy to go to a higher floor, and release energy when coming down. Similarly, electrons stay in fixed shells!
The arrangement of electrons in different shells around the nucleus is called electronic configuration or electron distribution.
The maximum number of electrons that can be accommodated in a shell is given by the formula: 2nΒ² (where n is the shell number)
β’ K shell (n=1): 2 Γ 1Β² = 2 electrons maximum
β’ L shell (n=2): 2 Γ 2Β² = 8 electrons maximum
β’ M shell (n=3): 2 Γ 3Β² = 18 electrons maximum
β’ N shell (n=4): 2 Γ 4Β² = 32 electrons maximum
The outermost shell of an atom cannot have more than 8 electrons, even if it can theoretically hold more according to the 2nΒ² formula.
Shells are filled in order - starting from the innermost shell (K). A new shell starts filling only when the previous shell is completely filled or has 8 electrons (for outer shells).
Sodium has 11 electrons to distribute.
Distribution:
β’ K shell: 2 electrons (full)
β’ L shell: 8 electrons (full)
β’ M shell: 1 electron (outermost)
Electronic Configuration: 2, 8, 1
Sodium has 1 electron in its outermost shell, so it easily loses
this electron and becomes NaβΊ ion.
Chlorine has 17 electrons to distribute.
Distribution:
β’ K shell: 2 electrons (full)
β’ L shell: 8 electrons (full)
β’ M shell: 7 electrons (outermost)
Electronic Configuration: 2, 8, 7
Chlorine has 7 electrons in its outermost shell. It needs 1 more
electron to complete 8, so it easily gains 1 electron and becomes
Clβ» ion.
Calcium has 20 electrons to distribute.
Distribution:
β’ K shell: 2 electrons (full)
β’ L shell: 8 electrons (full)
β’ M shell: 8 electrons (can hold 18, but outermost can't exceed
8)
β’ N shell: 2 electrons (outermost)
Electronic Configuration: 2, 8, 8, 2
The total number of protons in the nucleus of an atom is called Atomic Number. It is denoted by Z.
Important: Atomic number = Number of protons = Number of electrons (in neutral atom)
Example:
β’ Hydrogen: Z = 1 (1 proton, 1 electron)
β’ Carbon: Z = 6 (6 protons, 6 electrons)
β’ Oxygen: Z = 8 (8 protons, 8 electrons)
The sum of protons and neutrons in the nucleus of an atom is called Mass Number. It is denoted by A.
Formula: Mass Number (A) = Number of Protons +
Number of Neutrons
OR: A = Z + n
OR: Number of Neutrons = A - Z
Carbon has:
β’ Atomic number (Z) = 6 β means 6 protons
β’ Mass number (A) = 12 β means total 12 particles in nucleus
Number of neutrons = A - Z = 12 - 6 = 6 neutrons
So, Carbon-12 has: 6 protons + 6 neutrons + 6 electrons
An atom is represented as: ᴬπX
Where:
β’ X = Symbol of element
β’ Z = Atomic number (bottom left)
β’ A = Mass number (top left)
Examples:
β’ ΒΉΒ²βC β Carbon with mass 12, atomic number 6
β’ Β²Β³ββNa β Sodium with mass 23, atomic number 11
β’ Β³β΅ββCl β Chlorine with mass 35, atomic number 17
The combining capacity of an atom is called valency. It is determined by the number of electrons in the outermost shell.
If outermost shell has:
β’ 1, 2, or 3 electrons β Valency = Number of electrons
β’ 5, 6, or 7 electrons β Valency = 8 - Number of electrons
β’ 4 electrons β Valency can be 4 (either way)
β’ 8 electrons β Valency = 0 (complete octet, stable)
Examples:
β’ Sodium (2, 8, 1) β 1 electron in outer shell β Valency = 1
β’ Magnesium (2, 8, 2) β 2 electrons β Valency = 2
β’ Oxygen (2, 6) β 6 electrons β Valency = 8-6 = 2
β’ Chlorine (2, 8, 7) β 7 electrons β Valency = 8-7 = 1
β’ Carbon (2, 4) β 4 electrons β Valency = 4
β’ Neon (2, 8) β 8 electrons β Valency = 0 (inert)
Think of valency as "number of hands" an atom has to hold other atoms! If sodium has valency 1, it can hold 1 other atom. If oxygen has valency 2, it can hold 2 atoms. That's why water is HβO - oxygen (valency 2) holds 2 hydrogen atoms (each valency 1)!
The electrons present in the outermost shell of an atom are called valence electrons. These electrons determine the chemical properties of the element.
Examples:
β’ Sodium (2, 8, 1) β 1 valence electron
β’ Oxygen (2, 6) β 6 valence electrons
β’ Chlorine (2, 8, 7) β 7 valence electrons
Atoms of the same element having the same atomic number but different mass numbers are called isotopes.
This means: Same number of protons, different number of neutrons
1. Hydrogen Isotopes:
β’ Protium: ΒΉβH (1 proton, 0 neutrons)
β’ Deuterium: Β²βH (1 proton, 1 neutron)
β’ Tritium: Β³βH (1 proton, 2 neutrons)
2. Carbon Isotopes:
β’ Carbon-12: ΒΉΒ²βC (6 protons, 6 neutrons)
β’ Carbon-14: ΒΉβ΄βC (6 protons, 8 neutrons)
3. Chlorine Isotopes:
β’ Chlorine-35: Β³β΅ββCl (17 protons, 18 neutrons)
β’ Chlorine-37: Β³β·ββCl (17 protons, 20 neutrons)
Isotopes are like twins wearing different weight backpacks! They're the same person (same element) but carrying different loads (different neutrons). So they have different total weight (mass number) but same identity (atomic number).
β’ Have same atomic number (same protons)
β’ Have different mass numbers (different neutrons)
β’ Have same chemical properties (same electrons)
β’ Have different physical properties (different masses)
β’ Occupy same position in periodic table
Atoms of different elements having different atomic numbers but the same mass number are called isobars.
This means: Different elements, but same total nuclear particles
β’ Calcium: β΄β°ββCa (20 protons + 20 neutrons = 40)
β’ Argon: β΄β°ββAr (18 protons + 22 neutrons = 40)
Both have mass number 40 but different atomic numbers!
Atoms of different elements having different atomic numbers and different mass numbers but the same number of neutrons are called isotones.
β’ Carbon-14: ΒΉβ΄βC (6 protons + 8 neutrons)
β’ Nitrogen-15: ΒΉβ΅βN (7 protons + 8 neutrons)
Both have 8 neutrons!
| Property | Isotopes | Isobars | Isotones |
|---|---|---|---|
| Elements | Same element | Different elements | Different elements |
| Atomic Number (Z) | Same | Different | Different |
| Mass Number (A) | Different | Same | Different |
| Number of Neutrons | Different | Different | Same |
| Example | ΒΉΒ²βC and ΒΉβ΄βC | β΄β°ββAr and β΄β°ββCa | ΒΉβ΄βC and ΒΉβ΅βN |
Thomson Model: Plum pudding (positive sphere with
electrons embedded)
Rutherford Model: Nuclear model (dense nucleus
with electrons revolving)
Bohr Model: Shell model (electrons in fixed
energy levels)
Each model improved our understanding of atomic structure!
Q1. If an atom has 12 protons and 12 neutrons,
find its atomic number, mass number, and electronic
configuration.
Answer: Z=12, A=24, Electronic config: 2,8,2
(Magnesium)
Q2. Why do isotopes have similar chemical
properties?
Answer: Because they have the same number of
electrons (same atomic number), and chemical properties depend on
electrons, especially valence electrons.
Q3. Which shell is closest to the nucleus?
Answer: K shell (n=1) is closest to the nucleus.