CDS Chemistry · Formula sheet
Atomic Structure and Periodic Classification formulas
2 formulas, 7 reference tables and 12 common traps for CDS Chemistry Atomic Structure and Periodic Classification, grouped by subtopic.
Atomic Models: Dalton to Bohr
Learn this subtopic in the notesDalton's atomic theory and its limits
| Statement | Explained by Dalton's theory? |
|---|---|
| Law of conservation of mass | Yes |
| Law of constant composition | Yes |
| Law of multiple proportions | Yes |
| Atoms contain electrons, protons and neutrons | No — the atom is indivisible in this theoryQ CDS 2025 (I): the presence of sub-atomic particles is what Dalton's theory does not explain. |
Rutherford's gold-foil experiment and its limit
| Scientist | Key result |
|---|---|
| J. J. Thomson | Discovered the electron; plum-pudding model |
| Ernest Rutherford | Gold-foil experiment: discovered the atomic nucleusQ Asked three times: CDS 2019 (II), 2020 (I) and 2025 (I). The answer is always the nucleus, never the proton or neutron. |
| Niels Bohr | Fixed orbits; explains why the atom is stableQ CDS 2025 (I): Rutherford's model could not explain the stability of the atom. |
| James Chadwick | Discovered the neutron |
Common traps
Dalton's symbols versus today's symbols
A question asking who introduced the symbol of an element usually wants Dalton, who first used symbols. The one- and two-letter symbols in use today came later, from Berzelius. Read the stem for which one it means.
Gold foil found the nucleus, not the proton
The large deflections showed a positively charged nucleus. Options naming the electron, proton, neutron or 'isotopes of gold' are distractors.
Rutherford's weakness is stability
Rutherford's model does explain the nucleus, its positive charge and its small size compared with the atom. It does not explain why the circling electron does not fall in: the stability of the atom.
Atomic Number, Isotopes and Isobars
Learn this subtopic in the notesCounting protons, neutrons and electrons
Mass number and electron count
- mass number (protons + neutrons)
- atomic number (protons)
- number of neutrons
- charge on the ion (+ or −)
Isotopes and isobars
| Term | Same | Different | Example |
|---|---|---|---|
| Isotopes | Atomic number Z | Mass number A | ³⁵Cl and ³⁷Cl (both Z = 17)Q |
| Isobars | Mass number A | Atomic number Z | ⁴⁰Ar (Z = 18) and ⁴⁰Ca (Z = 20)Q CDS 2019 (II): 'isobars have the same atomic number' was the incorrect statement. |
| Hydrogen isotopes | Z = 1 | A = 1, 2, 3 | Protium, deuterium, tritium; tritium is radioactiveQ |
Isoelectronic species
| Electrons | Species |
|---|---|
| 10 | N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺, Al³⁺ |
| 18 | P³⁻, S²⁻, Cl⁻, Ar, K⁺, Ca²⁺Q CDS 2022 (II): Cl⁻ is not isoelectronic with Mg²⁺, which has only 10 electrons. |
Common traps
A positive ion has FEWER electrons
A charge of +q means q electrons have been lost. O₂⁺ has 16 − 1 = 15 electrons, not 17.
Isobars share A, not Z
Isobars have the same mass number and different atomic numbers. 'Same atomic number, different mass number' describes isotopes. CDS has used this swap as the wrong statement more than once.
Hydronium is not an isotope
H₃O⁺ is an ion. The isotopes of hydrogen are protium, deuterium and tritium.
Count the ion's electrons, not the atom's
Magnesium has 12 electrons, but Mg²⁺ has 10. So Mg²⁺ belongs with neon, not with the 18-electron ions such as Cl⁻ and K⁺.
Electron Configuration and the Periodic Table
Learn this subtopic in the notesElectron configuration and valency
Valency from valence electrons
- number of electrons in the outermost shell
How the modern periodic table is built
Trends in atomic size and metallic character
Noble gases, alkali metals and other element facts
Common traps
Chemistry comes from electrons, not the nucleus
Protons fix which element it is and neutrons add mass, but chemical properties come from the valence electrons.
Sn is tin, Sb is antimony
Tin is Sn (stannum). Sb is antimony, and Ga is gallium, not gold. Gold is Au.
Lanthanoids fill 4f, actinoids 5f
Samarium (Sm) is a lanthanoid; neptunium (Np) is an actinoid. Osmium (Os) is a d-block transition metal and caesium (Cs) an alkali metal. Match-the-list options swap the two f-series.
Radius falls across a period
'Atomic radius increases from left to right' is false. More protons in the same shell pull the electrons closer, so size decreases across a period.
Noble gases are single atoms
He, Ne and Ar exist as single atoms. A pair that says 'helium : diatomic gas' is the wrongly matched one.