sci_chem
Electronic Configuration
Chapter summary, hard words and model exam answers for ICSE Class 10 Hindi.
Free online summary and notes (ICSE Class 10 Hindi). Read it here, no PDF download needed.
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Chemistry · CBSE 10 · ICSE 10 · GCSE (AQA, Edexcel, OCR)
Summary
Electrons orbit the nucleus in energy levels called shells, numbered 1, 2, 3 outward. Shell 1 holds up to 2 electrons, shell 2 up to 8, and shell 3 up to 8 for the first 20 elements. Electrons always fill the lowest shell first.
Take the atomic number (number of electrons), then fill shells from the inside out. Sodium has 11 electrons: 2 in shell 1, 8 in shell 2, 1 left over in shell 3, written 2, 8, 1. Chlorine (17) is 2, 8, 7.
The outermost-shell electrons are the valence electrons, and they decide how an atom reacts. Group 1 metals have 1 valence electron and lose it easily. Group 17 halogens have 7 and gain one. Same group = same valence electrons = similar chemistry.
Noble gases have full outer shells (2 for helium, 8 for the rest) and are stable, so they barely react. Every other atom reacts to reach a full outer shell - by losing, gaining or sharing electrons. This drive to a stable octet is the reason all bonding happens.
Hard words & meanings
| shell | a principal energy level for electrons, numbered 1, 2, 3 outward from the nucleus |
| electronic configuration | the arrangement of an atom's electrons across its shells, e.g. 2, 8, 1 |
| valence electrons | the electrons in the outermost shell; they control bonding and reactivity |
| atomic number | the number of protons (and, in a neutral atom, electrons) in an atom |
| group | a vertical column of the periodic table; members share the same number of valence electrons |
| period | a horizontal row of the periodic table; the number equals the number of occupied shells |
| noble gas | an element with a full outer shell (2 or 8 electrons), making it stable and unreactive |
| octet | a full outer shell of eight electrons, the stable arrangement most atoms aim for |
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