Atomic Structure – NDA Chemistry Notes

Memory Trick

Scientists and discoveries — “Dalton, Thomson, Rutherford, Chadwick, Bohr — DTRČB”:
Dalton = atom theory | Thomson = electron | Rutherford = nucleus | Chadwick = neutron | Bohr = shells/orbits
Chronological order: 1808 → 1897 → 1911 → 1932 → 1913 (Bohr slightly before Chadwick)

Rutherford’s model — what it did NOT have:
Rutherford discovered the nucleus. He did NOT say electrons move in fixed orbits. That was BOHR.
Remember: “Rutherford found the centre. Bohr found the lanes.”

Subatomic particles — “PEN” in the nucleus, electrons outside:
Proton (+, inside) | Electron (−, outside) | Neutron (0, inside)
Proton and Neutron = in NUcleus = NUcleons
Electron = outside in shells

Electron mass trap — “1836, not 2000”:
Electron mass = 1/1836 of proton mass.
The number 2000 is wrong. Always.

Shell capacity — “2, 8, 18, 32”:
K=2 | L=8 | M=18 | N=32
Formula: 2n² where n is the shell number
K: 2×1²=2 | L: 2×2²=8 | M: 2×3²=18 | N: 2×4²=32

Isotopes, Isobars, Isotones — “Z same = isotopes, A same = isobars, neutrons same = isotones”:
Isotopes = same Z (atomic number — same element)
Isobars = same A (mass number — different elements)
Isotones = same neutrons (different elements)

Neutrons formula — “N = A minus Z”:
Neutrons = Mass number (A) − Atomic number (Z)
Example: Al-27 has Z=13, so neutrons = 27−13 = 14

Co-60 trap — “Co for Cancer, Not Goitre”:
Co-60 kills cancer cells with gamma rays.
Goitre = iodine deficiency = treated with iodised salt.
These two facts are never mixed up again after this trick.

Anion vs Cation — “Anion has Extra, Cation is Short”:
Anion (negative) = gained electrons = MORE electrons than protons
Cation (positive) = lost electrons = FEWER electrons than protons
Protons never change when ions form.