Atomic Structure – NDA Chemistry Notes

Everything around you (air, water, food, metals) is made of atoms. An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms are so small that millions of them can fit on the tip of a pin.

For a long time, scientists thought the atom was the smallest thing that existed. Then they discovered that atoms are themselves made of even smaller particles: protons, neutrons, and electrons. Understanding how these particles are arranged inside an atom is what this chapter is about.

NDA tests this chapter every year. The most important topics are: who discovered which particle, what Rutherford’s experiment showed, what Rutherford’s model could NOT explain, the difference between isotopes and isobars, how many electrons fit in each shell, and the electron mass confusion. NDA 2025-I asked four questions from this chapter in a single paper.

1. The Scientists and Their Discoveries

Different scientists discovered different parts of the atom at different times.

Scientist	Discovery	Year
John Dalton	Atomic theory: proposed that matter is made of atoms	1808
J. J. Thomson	Electron: discovered using cathode ray tube experiment	1897
Ernest Rutherford	Nucleus: discovered using gold foil experiment	1911
James Chadwick	Neutron: discovered by bombarding beryllium with alpha particles	1932
Niels Bohr	Electrons move in fixed circular orbits (shells) around the nucleus	1913

Chadwick discovered the neutron. [NDA 2025-I]

Rutherford’s alpha-particle scattering experiment was responsible for the discovery of the atomic nucleus. [NDA 2017-I, NDA 2021-II]

The atomic theory of matter was first proposed by John Dalton.

2. Dalton’s Atomic Theory

John Dalton proposed the first scientific atomic theory in 1808. His main ideas were:

• All matter is made of tiny, indivisible particles called atoms.
• Atoms of the same element are identical: same size, mass, and properties.
• Atoms of different elements are different from each other.
• Atoms combine in simple whole number ratios to form compounds.
• Atoms cannot be created, destroyed, or broken down in a chemical reaction.

What Dalton got wrong: He said atoms are indivisible. We now know atoms are made of protons, neutrons, and electrons. They can be broken down.

Dalton used symbols to represent elements. He drew a circle with a cross pattern inside to represent phosphorus. [NDA 2023-II]

3. Thomson’s Model: The Plum Pudding Model

J. J. Thomson discovered the electron in 1897 using a cathode ray tube.

He proposed that an atom is a sphere of positive charge with electrons embedded in it, like raisins in a pudding or plums in a Christmas cake. That is why this model is called the plum pudding model.

Key point: Thomson thought the positive charge was spread throughout the atom. He did not know about the nucleus.

4. Rutherford’s Gold Foil Experiment

This is the most tested experiment in this chapter. You must know what Rutherford did, what he observed, and what he concluded.

What Rutherford did: He fired a beam of alpha particles (positively charged, heavy particles) at a very thin gold foil. He put a detector screen around the foil to see where the alpha particles went.

What he expected: Since Thomson’s model said positive charge was spread throughout the atom, Rutherford expected the alpha particles to pass straight through with very slight deflections.

What he actually observed:

• Most alpha particles passed straight through the gold foil without deflection.
• A small number were deflected at large angles.
• A very small number bounced straight back.

What he concluded:

• Most of the atom is empty space. That is why most particles passed through.
• The positive charge and almost all the mass of the atom is concentrated in a very small, dense region at the centre. He called this the nucleus.
• The size of the nucleus is about 100,000 times smaller than the size of the atom.

What Rutherford’s model looked like: A tiny, dense, positively charged nucleus at the centre. Electrons moving around the nucleus at a distance, like planets around the Sun.

NDA directly tests what Rutherford’s model does NOT include. Electrons moving in circular paths of fixed energy called orbits. That was Bohr’s contribution, NOT Rutherford’s. [NDA 2021-I, NDA 2025-I]

The nucleus of an atom is positively charged, NOT neutral. A neutron is neutral, but the nucleus contains both protons (positive) and neutrons (neutral), making the nucleus overall positive. [NDA 2025-I]

Rutherford’s model (what it could NOT explain):

• Why electrons moving in circular orbits do not lose energy and spiral into the nucleus.
• The arrangement of electrons in the atom.
• The stability of atoms.

These limitations led to the development of Bohr’s model.

5. Bohr’s Model of the Atom

Niels Bohr improved Rutherford’s model in 1913. His key ideas:

• Electrons move around the nucleus in fixed circular paths called orbits or shells.
• Each orbit has a fixed energy. As long as an electron stays in its orbit, it does not gain or lose energy.
• Electrons can jump from one orbit to another. When an electron jumps to a lower orbit, it releases energy. When it absorbs energy, it jumps to a higher orbit.
• Each shell can hold a maximum number of electrons.

Shell	Name	Maximum electrons	Formula
1st shell	K shell	2	2n² where n=1
2nd shell	L shell	8	2n² where n=2
3rd shell	M shell	18	2n² where n=3
4th shell	N shell	32	2n² where n=4

The maximum number of electrons in the M shell is 18. [NDA 2021-II]

6. Subatomic Particles

An atom is made of three types of particles: protons, neutrons, and electrons.

Particle	Symbol	Charge	Mass	Location	Discovered by
Proton	p	+1	1 u	Inside nucleus	Rutherford (1919)
Neutron	n	0 (neutral)	1 u	Inside nucleus	Chadwick (1932)
Electron	e⁻	−1	1/1836 u	Outside nucleus (in shells)	J. J. Thomson (1897)

Common Confusion for NDA: The mass of an electron is approximately 1/1836 of the mass of a proton. NOT 1/2000. NDA 2025-I Q156 directly tests this. Option (b) says “1/2000” and it is the FALSE statement. [NDA 2025-I]

Another Confusion (how a neutron forms): A common wrong statement is that a neutron is formed by the combination of an electron and a proton. This is FALSE. A neutron is a separate fundamental particle. It was discovered by Chadwick and it is NOT a proton-electron combination. [NDA 2025-I]

Protons and neutrons are inside the nucleus. Together they are called nucleons. Electrons are outside the nucleus. They move around the nucleus in shells.

7. Atomic Number and Mass Number

These two numbers are the most fundamental facts about any atom.

Atomic number (Z): The number of protons in the nucleus of an atom. Every element has a unique atomic number. No two elements have the same atomic number.

Atomic number = Number of protons

In a neutral atom, number of protons = number of electrons. So atomic number also tells you the number of electrons in a neutral atom.

Mass number (A): The total number of protons and neutrons in the nucleus.

Mass number = Protons + Neutrons

Finding number of neutrons: Neutrons = Mass number − Atomic number. Neutrons = A − Z

Example: Aluminium is written as ²⁷₁₃Al

• Atomic number (Z) = 13 → 13 protons, 13 electrons
• Mass number (A) = 27
• Neutrons = 27 − 13 = 14

Number of neutrons in ₁₃Al²⁷ is 14. [NDA 2010-I]

Atomic mass: The atomic mass of an element is equal to the sum of protons and neutrons. It is approximately equal to the mass number. [NDA 2020 I & II]

Atomic mass is expressed in atomic mass units (u). The standard used is Carbon-12 (¹²C). [NDA 2007-I]

8. Ions: Cations and Anions

A neutral atom has equal numbers of protons and electrons.

When an atom loses electrons, it becomes positively charged. This is called a cation. When an atom gains electrons, it becomes negatively charged. This is called an anion.

Important rule: The number of protons NEVER changes when an atom forms an ion. Only electrons change.

A negatively charged atom (anion) has MORE electrons than protons. [NDA 2011-II]

A positively charged body has LOST some electrons. It has acquired a deficiency of electrons.

The number of protons in an anion is the same as the atomic number of the element. Protons do not change when ions form. [NDA 2011-II]

9. Isotopes, Isobars and Isotones

This is one of the most directly tested topics in this chapter. NDA asks definition questions and “which pair is correctly matched” questions.

Isotopes

Atoms of the SAME element that have the same atomic number (Z) but different mass numbers (A). They have the same number of protons but different numbers of neutrons.

Isotopes of an element have the same chemical properties but different physical properties (like density and melting point).

Examples of isotopes:

• Hydrogen has three isotopes: H-1 (protium, 0 neutrons), H-2 (deuterium, 1 neutron), H-3 (tritium, 2 neutrons), all have 1 proton
• Carbon: C-12 (6 neutrons), C-13 (7 neutrons), C-14 (8 neutrons), all have 6 protons
• Chlorine: Cl-35 (18 neutrons) and Cl-37 (20 neutrons), both have 17 protons

Chlorine occurs in two isotopic forms of masses 35 u and 37 u in the ratio 3:1. This is why the average atomic mass of chlorine is 35.5 u.

Isotopes used in carbon dating are C-12 and C-14.

Isobars

Atoms of DIFFERENT elements that have the same mass number (A) but different atomic numbers (Z).

Example: Carbon-14 (Z=6, A=14) and Nitrogen-14 (Z=7, A=14) are isobars: same mass number 14, different elements.

Isotones

Atoms of DIFFERENT elements that have the same number of neutrons but different atomic numbers and mass numbers.

Example: Carbon-13 (Z=6, A=13, neutrons=7) and Nitrogen-14 (Z=7, A=14, neutrons=7) are isotones. Both have 7 neutrons.

Term	Same	Different	Example
Isotopes	Atomic number (Z)	Mass number (A)	C-12 and C-14 (both Z=6)
Isobars	Mass number (A)	Atomic number (Z)	C-14 and N-14 (both A=14)
Isotones	Number of neutrons	Both Z and A	C-13 and N-14 (both 7 neutrons)

NDA directly tested: Isotopes have the same atomic number but different mass number. Isobars have the same mass number but different atomic number. Isotones have the same number of neutrons. [NDA 2008-II, NDA 2012-I]

Elements in the same group of the periodic table have similar chemical properties. Elements in the same group are NOT isotopes or isobars of each other.

10. Electronic Configuration

Electronic configuration tells us how electrons are arranged in the shells of an atom.

Rules:

• Fill shells from the innermost outward.
• K shell first (maximum 2), then L shell (maximum 8), then M shell (maximum 18).
• The outermost shell (valence shell) cannot have more than 8 electrons.

Element	Symbol	Atomic number	K	L	M	Valence electrons
Hydrogen	H	1	1	—	—	1
Helium	He	2	2	—	—	2
Lithium	Li	3	2	1	—	1
Carbon	C	6	2	4	—	4
Nitrogen	N	7	2	5	—	5
Oxygen	O	8	2	6	—	6
Sodium	Na	11	2	8	1	1
Magnesium	Mg	12	2	8	2	2
Aluminium	Al	13	2	8	3	3
Chlorine	Cl	17	2	8	7	7
Argon	Ar	18	2	8	8	8
Calcium	Ca	20	2	8	8	2

The valence electronic configuration ns²np³ means 2 electrons in s and 3 electrons in p of the outer shell: total 5 valence electrons. This is found in Nitrogen (N). [NDA 2008-I]

An element with electronic configuration 2, 8, 2 has 2 valence electrons. It will most likely be in the same group as Magnesium (Mg) which also has 2 valence electrons. [NDA 2025-I]

11. Valency

Valency is the combining capacity of an atom. It tells us how many bonds an atom can form.

How to find valency:

• If valence electrons ≤ 4: valency = number of valence electrons (electrons are lost)
• If valence electrons > 4: valency = 8 − valence electrons (electrons are gained to complete the shell)

Element	Valence electrons	Valency
Hydrogen (H)	1	1
Carbon (C)	4	4
Nitrogen (N)	5	3
Oxygen (O)	6	2
Sodium (Na)	1	1
Magnesium (Mg)	2	2
Aluminium (Al)	3	3
Chlorine (Cl)	7	1

Some elements show variable valency. For example, phosphorus can show valency 3 and valency 5. Iron shows valency 2 and 3. Copper shows valency 1 and 2.

An element A with valency 3 combines with element B with valency 2 to form A₂B₃. An element A with valency 5 combines with element B with valency 2 to form A₂B₅. [NDA 2007-I]

The property that changes with valency is equivalent weight. Atomic weight and molecular weight do not change with valency.

12. Important Radioisotopes

Radioisotopes are radioactive isotopes. They are used in medicine, industry, and research.

Radioisotope	Use
Carbon-14 (C-14)	Carbon dating: finding the age of ancient objects and fossils
Cobalt-60 (Co-60)	Cancer treatment: gamma rays kill cancer cells
Iodine-131 (I-131)	Thyroid treatment: thyroid gland absorbs iodine
Uranium-235 (U-235)	Nuclear reactor fuel

Critical NDA Confusion (Co-60 and goitre): Co-60 is used for treating CANCER, not goitre. Goitre is caused by iodine deficiency in the diet. It is treated by eating iodised salt, NOT by radiation. Iodine-131 is used for thyroid treatment, but even that is for hyperthyroidism and thyroid cancer, not simple goitre. [NDA 2025-I]

This confusion appeared in NDA 2025-I Q156. Option (c) says “an isotope of cobalt is used in the treatment of goiter.” This is the FALSE statement.

Heavy water (D₂O) is water where hydrogen is replaced by deuterium (H-2). It is used as a moderator in nuclear reactors. Its molecular weight is 20 u, not 18 u like ordinary water.

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