Atomic Structure
Subject: Science (Chemistry) | Topic: Atomic Structure
For TLT, ECT, PST, JEST & All Competitive Exams
We have already studied Elements, Compounds and Mixtures, and the Periodic Table, where elements are arranged by atomic number. But what exactly is inside an atom that gives each element its unique identity? This is explained by Atomic Structure — the building blocks of every element and a fundamental topic tested in TLT, ECT, PST, and JEST exams.
What is an Atom?
An atom is the smallest particle of an element that retains all the chemical properties of that element. Atoms are made up of three main sub-atomic particles: Protons, Neutrons, and Electrons.
Basic Structure of an Atom
Sub-Atomic Particles
| Particle | Charge | Location | Relative Mass |
|---|---|---|---|
| Proton | Positive (+) | Nucleus (center of atom) | 1 |
| Neutron | Neutral (no charge) | Nucleus (center of atom) | 1 |
| Electron | Negative (−) | Orbiting shells around the nucleus | Negligible (very small) |
Atomic Number and Mass Number
Two key numbers describe every atom:
- Atomic Number (Z): The number of protons in an atom's nucleus. It determines which element the atom is, and in a neutral atom, equals the number of electrons.
- Mass Number (A): The total number of protons and neutrons in the nucleus.
Example: An atom of Carbon has 6 protons and 6 neutrons.
Atomic Number = 6, Mass Number = 6 + 6 = 12
Why is an Atom Electrically Neutral?
In a neutral atom, the number of protons (positive charges) is always equal to the number of electrons (negative charges). This balance of charges is why an atom, as a whole, carries no overall charge.
Electron Shells (Energy Levels)
Electrons do not move randomly; they occupy fixed shells or energy levels around the nucleus. Each shell can hold a maximum number of electrons, with the shell closest to the nucleus filling first. Electrons in the outermost shell are called valence electrons, and they determine how an atom bonds with other atoms.
What are Isotopes?
Isotopes are atoms of the same element that have the same number of protons (same atomic number) but a different number of neutrons (different mass number).
Why This Topic Matters
Atomic Structure is the foundation for understanding the Periodic Table, Chemical Bonding, and how elements react with one another. Since questions on protons, neutrons, electrons, atomic number, mass number, and isotopes are commonly tested, this topic is essential for the Chemistry portion of TLT, ECT, PST, and JEST exams.
Practice MCQs
1. Which sub-atomic particle carries a positive charge?
a) Electron b) Neutron c) Proton d) Nucleus
2. Where are protons and neutrons located in an atom?
a) Outer shell b) Nucleus c) Valence shell d) Outside the atom
3. What is the Atomic Number of an element equal to?
a) Number of neutrons b) Number of protons c) Mass Number d) Number of shells
4. What is the Mass Number of an atom with 6 protons and 6 neutrons?
a) 6 b) 12 c) 18 d) 0
5. What are isotopes?
a) Atoms of different elements b) Atoms of the same element with different numbers of neutrons c) Atoms with no electrons d) Compounds formed from two elements
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