Batch 1 · Atomic Structure

1️⃣ Classification of Elements

Chemists group elements according to their properties. All known elements fall into three broad categories:

Category	Physical State (at room temperature)	Key Properties	Examples
Metals	Mostly solid (except mercury, Hg)	Shiny, malleable, ductile, conduct heat and electricity	Na, Mg, Fe, Cu, Au
Non-metals	Solid, liquid, or gas	Dull, brittle (if solid), poor conductors, insulators	C, S, O₂, Cl₂, N₂
Metalloids	Mostly solid	Properties intermediate between metals and non-metals; semiconductors	Si, Ge, B

💡 Quick Memory Trick: "Metals shine and wire — non-metals don't."

2️⃣ Historical Development of Atomic Theory

Our understanding of the atom has evolved through a series of experiments. Each model was refined when new evidence contradicted the old one.

🔹 Dalton — The Indivisible Sphere (1803)

All matter is made of tiny, indivisible particles called atoms.
Atoms of the same element are identical; atoms of different elements are different.
Atoms cannot be created, divided, or destroyed in chemical reactions.

🔹 Thomson — The Plum Pudding Model (1904)

Cathode ray experiments discovered the electron, proving atoms contain smaller particles.
Proposed that atoms are spheres of positive charge with negatively charged electrons embedded inside — like raisins in a plum pudding.

🔹 Rutherford — The Nuclear Model (1911)

The famous gold foil experiment fired alpha particles (He²⁺) at a thin gold sheet.
Most alpha particles passed straight through — but a tiny number bounced back at large angles.
Conclusion: atoms have a tiny, dense, positively charged nucleus at the centre, with electrons orbiting in the mostly empty space around it.

⚡ Rutherford's Three Observations to Remember:

Most passed through → atom is mostly empty space
Few were deflected → there is a concentrated positive charge (the nucleus)
Very few bounced straight back → the nucleus is extremely dense and tiny

🔹 Bohr — Energy Levels / Shells (1913)

Electrons can only occupy certain fixed orbits called energy levels (shells).
Each shell has a fixed energy; electrons cannot exist between shells.
This explained why atoms emit and absorb light at specific wavelengths.

🔹 Schrödinger — The Electron Cloud Model (1926)

Modern quantum mechanical model: electrons do not travel in fixed orbits.
Instead, they occupy regions called orbitals where there is a high probability of finding the electron.
The orbital looks like a fuzzy "cloud" — the denser the cloud, the higher the probability.

🧠 Memory Chain: Thomson's "raisins in a pudding" → Rutherford's "tiny solar system" → Bohr's "fixed orbits" → Schrödinger's "fuzzy probability cloud."

3️⃣ Subatomic Particles

An atom is built from three fundamental particles:

Particle	Symbol	Relative Charge	Relative Mass	Location
Proton	p⁺	+1	1	Nucleus
Neutron	n	0	1	Nucleus
Electron	e⁻	−1	1/1836 (negligible)	Orbitals around the nucleus

💡 Why is an atom neutral?
A neutral atom has equal numbers of protons and electrons — the positive and negative charges cancel out.

4️⃣ Atomic Number, Mass Number & Isotopes

🔹 Atomic Number (Z)

Atomic Number Z = Number of Protons

The atomic number is the defining identity of an element. Every atom of the same element has the same atomic number — change the number of protons and you get a different element.

🔹 Mass Number (A)

Mass Number A = Number of Protons + Number of Neutrons

🔹 Number of Neutrons

Number of Neutrons = Mass Number − Atomic Number = A − Z

🔹 Electrons in a Neutral Atom

Number of Electrons = Number of Protons = Atomic Number Z

🔹 Nuclide Notation

📝 Worked Example: A Chlorine Atom
Shown as ³⁵₁₇Cl
• Atomic Number Z = 17 → 17 protons
• Mass Number A = 35
• Number of neutrons = 35 − 17 = 18
• Number of electrons = 17 (neutral atom)

🔹 Isotopes

Isotopes are atoms of the same element with different numbers of neutrons:

Same number of protons → same element, same chemical behaviour
Different number of neutrons → different mass number, slightly different physical properties (density, diffusion rate)
For example, most hydrogen atoms on Earth have no neutrons (¹₁H), but a small fraction have one neutron (²₁H, called deuterium).

🧠 Isotope Memory Aid: "Same protons, different neutrons" → Iso-topes are top-equal in chemistry.

5️⃣ The First 20 Elements at a Glance

Z	Symbol	Name	p⁺	e⁻	Common n	Mass A
1	H	Hydrogen	1	1	0	1
2	He	Helium	2	2	2	4
3	Li	Lithium	3	3	4	7
4	Be	Beryllium	4	4	5	9
5	B	Boron	5	5	6	11
6	C	Carbon	6	6	6	12
7	N	Nitrogen	7	7	7	14
8	O	Oxygen	8	8	8	16
9	F	Fluorine	9	9	10	19
10	Ne	Neon	10	10	10	20
11	Na	Sodium	11	11	12	23
12	Mg	Magnesium	12	12	12	24
13	Al	Aluminium	13	13	14	27
14	Si	Silicon	14	14	14	28
15	P	Phosphorus	15	15	16	31
16	S	Sulphur	16	16	16	32
17	Cl	Chlorine	17	17	18	35
18	Ar	Argon	18	18	22	40
19	K	Potassium	19	19	20	39
20	Ca	Calcium	20	20	20	40

🧠 First-20 Mnemonic: "Happy Henry Likes Beer But Could Not Obtain Full Nachos. NaMg Al's iPhone SAYS: Classy Karen's Apartment."
(H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca)

6️⃣ Electron Arrangement (Electron Configuration)

Electrons fill the shells closest to the nucleus first. For the first 20 elements:

Shell 1 (K): holds a maximum of 2 electrons
Shell 2 (L): holds a maximum of 8 electrons
Shell 3 (M): holds a maximum of 8 electrons for the first 20 elements

💡 Shell-Filling Rule: Always fill the inner shells first. The outermost shell is the most important for chemical reactions — atoms with 1, 2, 6, or 7 outer electrons are especially reactive.

🔹 Common Electron Arrangements

Element	Electrons	Configuration
H (Hydrogen)	1	1
He (Helium)	2	2
Li (Lithium)	3	2, 1
C (Carbon)	6	2, 4
O (Oxygen)	8	2, 6
Ne (Neon)	10	2, 8
Na (Sodium)	11	2, 8, 1
Cl (Chlorine)	17	2, 8, 7
Ar (Argon)	18	2, 8, 8

7️⃣ Batch 1 Summary

Element Classes: metals, non-metals, metalloids
Atomic Models: Dalton → Thomson → Rutherford → Bohr → electron cloud
Atomic Structure: nucleus (protons + neutrons) + surrounding electrons
Three Key Numbers: atomic number Z, mass number A, neutron number N = A − Z
Neutral Atom: number of electrons = number of protons = Z
Isotopes: same Z, different N
Electron Configuration: fill inner shells first (2, 8, 8, 2…)

🎯 DSE Exam Pitfalls:

When asked for the number of electrons, check whether the atom is an ion — a neutral atom has Z electrons, but an ion does not.
Isotopes share chemical properties but differ in physical properties (density, rate of diffusion, mass).
Atomic number → element identity. Mass number → isotope identity.

⚛️ Batch 1 · Atomic Structure