Atomic structure, valency and the periodic table for NDA

The NDA syllabus asks for "elementary ideas" of atomic structure, atomic, equivalent and molecular weights, and valency. Elementary does not mean vague. Questions want the scientist behind a discovery, the neutrons in an atom, the valency of an element, a formula built from valencies or a molecular mass added up in your head. The periodic table is not named in the syllabus line, but trend questions come straight from Class 10 NCERT and are worth preparing.

The particles and who found them

How the model of the atom changed:

• Thomson: a sphere of positive charge with electrons embedded in it, like seeds in a watermelon.
• Rutherford (gold foil): alpha particles fired at thin gold foil. Most passed straight through, some were deflected, and a very few bounced back. So the atom is mostly empty space, with a tiny, dense, positive nucleus holding nearly all the mass.
• Bohr: electrons move only in certain permitted orbits (shells) of fixed energy, and do not radiate while in them.

Atomic number, mass number and isotopes

• Atomic number (Z) = number of protons = number of electrons in a neutral atom. It identifies the element.
• Mass number (A) = protons + neutrons. So neutrons = A − Z.

Sodium-23 has Z = 11, so it has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.

Isotopes have the same chemical properties, because chemistry depends on electrons, but different physical properties such as mass and density. Useful isotopes: carbon-14 in dating old organic remains, uranium-235 as nuclear fuel, cobalt-60 in cancer treatment, iodine-131 for thyroid disorders.

Why chlorine's atomic mass is 35.5. Natural chlorine is about 75% chlorine-35 and 25% chlorine-37. Average = 0.75 × 35 + 0.25 × 37 = 26.25 + 9.25 = 35.5.

Shells, electron configuration and valency

A shell can hold at most 2n² electrons: K (n = 1) holds 2, L holds 8, M holds 18. The outermost shell holds no more than 8. For the first 20 elements, fill 2, then 8, then 8, then the rest.

Rule: if the outer shell has 4 or fewer electrons, valency = that number; if more, valency = 8 − that number. Atoms with a full outer shell (8, or 2 for helium) have valency 0 and hardly react.

Writing formulas by cross-over. Write the symbols, put each valency under the other symbol, cross them over as subscripts and simplify.

• Aluminium (3) and oxygen (2) → Al₂O₃.
• Calcium (2) and chlorine (1) → CaCl₂.
• Magnesium (2) and nitrogen (3) → Mg₃N₂.
• Ammonium, NH₄ (1), and sulphate, SO₄ (2) → (NH₄)₂SO₄.
• Aluminium (3) and sulphate (2) → Al₂(SO₄)₃.

Atomic, molecular and equivalent weights

The atomic mass unit (u) is one-twelfth of the mass of a carbon-12 atom. Molecular mass is the sum of the atomic masses in a molecule (H = 1, C = 12, N = 14, O = 16, Na = 23, S = 32, Cl = 35.5, Ca = 40).

A mole is 6.022 × 10²³ particles (Avogadro's number). One mole of a substance has a mass in grams equal to its molecular mass, and one mole of any gas occupies about 22.4 litres at 0 °C and 1 atmosphere.

Equivalent weight, at the elementary level NDA asks, is:

• for an element: atomic mass ÷ valency. Oxygen 16 ÷ 2 = 8; magnesium 24 ÷ 2 = 12; aluminium 27 ÷ 3 = 9.
• for an acid: molecular mass ÷ number of replaceable H atoms. H₂SO₄: 98 ÷ 2 = 49.
• for a base: molecular mass ÷ number of OH groups. Ca(OH)₂: 74 ÷ 2 = 37.

The periodic table

The modern (long-form) table has 18 groups and 7 periods. The period number equals the number of occupied shells. Elements in a group have the same number of valence electrons, so they behave alike.

Fluorine is the most electronegative element.

Worked NDA-style MCQs

Q1. An atom has 17 protons and 18 neutrons. Its mass number and valency are:
(a) 35 and 1 (b) 18 and 7 (c) 35 and 7 (d) 17 and 1

A = 17 + 18 = 35. Configuration 2, 8, 7 needs one electron: valency 8 − 7 = 1. Answer: (a).

Q2. The formula of aluminium sulphate is:
(a) AlSO₄ (b) Al₂SO₄ (c) Al₂(SO₄)₃ (d) Al₃(SO₄)₂

Al is 3 and SO₄ is 2; crossing over gives Al₂(SO₄)₃. Answer: (c).

Q3. Which pair are isobars?
(a) carbon-12 and carbon-14 (b) argon-40 and calcium-40 (c) hydrogen-1 and hydrogen-2 (d) oxygen-16 and oxygen-18

Same mass number, different elements. The other pairs are isotopes. Answer: (b).

Q4. The equivalent weight of aluminium (atomic mass 27) is:
(a) 27 (b) 13.5 (c) 9 (d) 3

27 ÷ valency 3 = 9. Answer: (c).

Q5. The element Mendeleev called eka-silicon was later discovered as:
(a) gallium (b) germanium (c) scandium (d) silicon

Its properties matched his predictions closely. Gallium was eka-aluminium. Answer: (b).

Q6. Moving from sodium to chlorine across period 3, atomic size:
(a) increases (b) decreases (c) stays the same (d) first rises, then falls

The shell number stays 3 while the nuclear charge rises from 11 to 17, pulling electrons closer. Answer: (b).

Common mistakes

• Mass number is not atomic mass. A is a whole-number count; atomic mass (35.5 for chlorine) is an average.
• Valency of 7 for chlorine. It has 7 outer electrons but valency 1.
• Mendeleev and atomic number. He used atomic mass; the modern law uses atomic number.

Practice set

1. The neutron was discovered by: (a) Thomson (b) Rutherford (c) Chadwick (d) Bohr
2. The number of neutrons in sodium-23 (Z = 11) is: (a) 11 (b) 12 (c) 23 (d) 34
3. The maximum number of electrons the M shell can hold is: (a) 2 (b) 8 (c) 18 (d) 32
4. The molecular mass of H₂SO₄ is: (a) 49 (b) 96 (c) 98 (d) 100
5. Group 17 elements are called: (a) alkali metals (b) noble gases (c) halogens (d) metalloids
6. The isotope used to date ancient organic remains is: (a) carbon-12 (b) carbon-14 (c) uranium-235 (d) cobalt-60
7. The valency of nitrogen in ammonia is: (a) 1 (b) 2 (c) 3 (d) 5
8. Rutherford's experiment showed that: (a) electrons have negative charge (b) the atom has a small, dense, positive nucleus (c) neutrons exist (d) electrons move in fixed orbits

Answers:

1. (c). In 1932.
2. (b). 23 − 11.
3. (c). 2n² with n = 3.
4. (c). 2 + 32 + 64.
5. (c).
6. (b). It decays at a known rate after death.
7. (c). 2, 5 needs three more electrons.
8. (b). Fixed orbits were Bohr's idea.

What to do next

• Write configurations and valencies of elements 1–20 from memory.
• Build 20 formulas by cross-over, including ions such as NH₄, SO₄, NO₃ and CO₃.
• Work out the molecular masses in the table without looking.
• See valency at work in chemical reactions and equations and reactivity in metals and non-metals.