Question 16
State what the group number in the Modern Periodic Table signifies. Give a reason why elements in the same group, have similar valency and similar chemical properties.
Group number signifies the number of electrons in the outer shell of an atom of the element.
As the number of electrons in the outer shell of all elements in a group is the same. Hence all elements in the same group of the periodic table have same valency and similar chemical properties.
ICSE Class VII Chemistry Chapter 3 15 Marks
Atomic Structure — Quick Study Guide
Atoms are the building blocks of all matter. Understanding atomic structure is essential for all of chemistry.
Key Definitions
- Atom: Smallest particle of an element with its chemical properties
- Molecule: Group of two or more atoms bonded together
- Element: Pure substance with only one type of atom
- Compound: Two or more different elements chemically combined
- Atomic number (Z): Number of protons = Number of electrons
- Mass number (A): Protons + Neutrons
- Valency: Combining capacity of an element
- Isotopes: Same element, different mass numbers
Electronic Configuration Rules
K shell = max 2 electrons, L shell = max 8, M shell = max 8 (for first 20 elements). Fill inner shells first.
Exam Tips
- Memorise symbols of first 20 elements
- Practice calculating protons, neutrons, electrons from Z and A
- Practice drawing Bohr diagrams
- Valency: if outermost ≤ 4, valency = outermost; if > 4, valency = 8 - outermost
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