On this page
Questions on atomic models look like history, but they test one skill: can you link each model to the evidence that supported it and the evidence that broke it? The second half of the chapter is pure calculation: shells, valency, isotopes and average atomic mass. This post covers both, with worked examples and the mistakes that cost marks.
What does the 2026–27 chapter cover?
In the new NCERT book *Exploration*, this is Chapter 8, Journey Inside the Atom. The CBSE Class IX syllabus for 2026–27 asks you to tell apart electrons, protons and neutrons by charge and position, show electrons in K, L, M, N shells, explain valency, atomic number and mass number, and describe how atomic models changed over time. Check the syllabus for your exam year.
What did each model say, and what broke it?
Thomson (plum pudding). In 1897 J. J. Thomson studied cathode rays and concluded they were streams of negatively charged particles much lighter than atoms; his paper is Cathode Rays in the *Philosophical Magazine*. Since atoms are neutral, he pictured a sphere of positive charge with electrons spread through it. NCERT compares it to a watermelon with seeds.
Rutherford (nuclear model). Hans Geiger and Ernest Marsden reported in 1909 that a small fraction of alpha particles striking a metal plate came back out on the side they entered. Thomson's spread-out positive charge could not turn a fast alpha particle back. In 1911 Rutherford published his explanation, The scattering of α and β particles by matter and the structure of the atom: the positive charge and most of the mass sit in a tiny nucleus, and most of the atom is empty space. NCERT gives the sizes: the atom is about 10⁻¹⁰ m across and the nucleus about 10⁻¹⁵ m, about 10⁵ times smaller.
What broke Rutherford's model: an electron moving in a circle is accelerating, and an accelerating charge should lose energy and spiral into the nucleus. Atoms would collapse. They do not.
Bohr (energy levels). In 1913 Niels Bohr proposed in On the constitution of atoms and molecules that electrons occupy only certain allowed orbits, or energy levels, and do not lose energy while in them. They move between levels by absorbing or releasing a fixed amount of energy. NCERT adds that even this model was later replaced by the quantum mechanical picture of electron clouds.
For the full stories, read our posts on Rutherford's gold foil experiment and Niels Bohr's model.
How do you fill electron shells?
NCERT gives the Bohr–Bury rules:
A shell can hold at most 2n² electrons: K (n = 1) holds 2, L holds 8, M holds 18.
The outermost shell holds at most 8 electrons.
Fill shells in order, K, then L, then M.
Examples:
Sulfur (Z = 16): 2, 8, 6.
Magnesium (Z = 12): 2, 8, 2.
Argon (Z = 18): 2, 8, 8.
Potassium (Z = 19): 2, 8, 8, 1. The M shell could hold 18 in principle, but rule 2 stops the outer shell at 8, so the 19th electron starts the N shell.
How do you find valency?
Valency is the number of electrons an atom gains, loses or shares to complete an octet (or a duplet for the K shell).
Magnesium, 2, 8, 2: loses 2, valency 2.
Sulfur, 2, 8, 6: gains 2, valency 2.
Chlorine, 2, 8, 7: gains 1, valency 1.
Carbon, 2, 4: shares 4, valency 4.
Argon, 2, 8, 8: complete octet, valency 0.
How do you handle mass number and isotopes?
Atomic number Z = protons. Mass number A = protons + neutrons. So neutrons = A − Z.
²⁷₁₃Al: 27 − 13 = 14 neutrons.
³⁵₁₇Cl and ³⁷₁₇Cl: 18 and 20 neutrons. Same element, so they are isotopes, with the same chemical properties because they have the same electrons.
Calcium, potassium and argon can each have mass number 40 with different atomic numbers. Those are isobars.
Worked example: average atomic mass. Bromine has two isotopes, ⁷⁹Br (49.7%) and ⁸¹Br (50.3%). This is NCERT's Pause and Ponder question 18.
Weighted average = 79 × 0.497 + 81 × 0.503
= 39.263 + 40.743
= 80.006, so about 80.0 u.
Compare the simple average, (79 + 81)/2 = 80 u. They are nearly equal here because the two isotopes are almost equally common. For chlorine they are not: 35 × 0.75 + 37 × 0.25 = 35.5 u, while the simple average would be 36 u.
Which mistakes come up most?
Mixing up atomic number and mass number when finding neutrons.
Putting 9 or more electrons in the outer shell, as in writing potassium as 2, 8, 9.
Saying isotopes behave differently in reactions. They differ in physical properties, such as mass and boiling point, not in chemical ones.
Writing Rutherford's flaw as "electrons fall in" without the reason: an accelerating charge loses energy.
Taking a simple average when abundances are given.
If shells and valency still feel like rules to memorise, the Fermi Concept Library covers every concept in physics and chemistry, each with its own lesson, so you can revisit atomic structure one idea at a time.
A practice routine
Write the electron distribution and valency for Z = 1 to 20 from memory, then check against NCERT Table 8.4.
For each model, write the three lines from the callout above.
Do Pause and Ponder questions 10 to 13 on protons, neutrons and mass number.
Make up one average-atomic-mass problem with unequal abundances and solve it.
Today, write the shells for potassium and calcium and explain to yourself why the N shell starts before M is full.
