Chemistry (Elements, Compounds, Acids-Bases)
Syllabus Unit: General Ability Test (GAT)
Why it matters
Chapter overview
The Chemistry component covers atomic structure and periodic trends, the ionic/covalent/metallic bonding that holds compounds together, the acid-base-salt chemistry behind the pH scale and neutralization, and everyday chemical reactions like rusting and combustion ā foundational, fact-based content drawn largely from Class IX-X chemistry.
1Atomic Structure & the Periodic Table
An atom's proton count fixes its identity, its electron arrangement fixes its chemistry, and the periodic table organizes both into one predictive chart.
Learning Objectives
- Describe the atom's structure: protons and neutrons in the nucleus, electrons in shells around it.
- Write electron configurations using the 2,8,8 shell-filling pattern for the first 20 elements.
- State the modern periodic law and describe how atomic radius, ionization energy and electronegativity trend across a period and down a group.
- Distinguish metals, non-metals and metalloids by their position and typical properties.
Key Terms
Every atom is built from the same three particles arranged differently ā protons and neutrons packed into a tiny nucleus, with electrons orbiting in shells around it.
Structure of the atom
- Nucleus ā protons (positive) and neutrons (neutral), together nearly all of the atom's mass.
- Electron shells ā negatively charged electrons orbit in shells (K,L,M,N...), each holding a maximum of 2nĀ² electrons.
The modern periodic table
Elements are arranged by increasing atomic number into 18 groups (columns, same valence electrons) and 7 periods (rows, same number of shells).
Periodic trends
- Atomic radius ā decreases across a period (more nuclear pull), increases down a group (more shells).
- Ionization energy ā increases across a period, decreases down a group.
- Electronegativity ā increases across a period, decreases down a group.
Metals, non-metals, metalloids
Metals occupy the left/centre of the table (lose electrons, form cations, e.g. Na, Fe); non-metals occupy the top-right (gain electrons, form anions, e.g. O, Cl); metalloids like Si and Ge sit along the zig-zag boundary, showing intermediate properties.