Learn Valency and atomic number

Welcome to CBSE Class 9 Science, where we dive into the fascinating "Structure of the Atom." Among the most crucial topics in this chapter are understanding the atomic number and valency of an element. Think of the atomic number as a unique identity card for every element—no two elements share the same number. Valency, on the other hand, acts like the element's "social capacity." It determines how readily an atom will combine, bond, or interact with other atoms to form the molecules that make up the world around us. Mastering these two definitions is the first step to unlocking the rules of chemistry.

At its core, the Atomic Number (Z) is simply the total number of protons present inside the nucleus of an atom. Because atoms are electrically neutral by default, the number of protons equals the total number of electrons. These electrons revolve around the nucleus in specific orbits or shells (K, L, M, N). The electrons located in the very outermost shell are known as valence electrons, and they dictate the atom's Valency (its combining capacity). The rule is logical: atoms want a stable, fully occupied outer shell (an octet). If an atom has 1, 2, 3, or 4 valence electrons, it usually loses or shares them, meaning its valency is directly equal to that number (e.g., 1, 2, 3, 4). However, if the outermost shell holds 5, 6, or 7 electrons, it is easier for the atom to gain electrons to reach 8. In this case, the valency is calculated as 8 minus the number of valence electrons. Elements with a full outer shell (like 8 electrons) are stable and have a valency of 0.

Looking at the illustrative diagram above, you can clearly see the practical application of these rules by comparing Sodium (Na) and Oxygen (O). On the left, Sodium has an atomic number of 11, resulting in an electronic configuration of 2, 8, 1. The outermost M-shell has only one electron (highlighted in blue). To achieve stability, Sodium easily gives up this single electron, which gives it a valency of 1. Conversely, on the right side, Oxygen holds an atomic number of 8, with an electron configuration of 2, 6. Its outermost L-shell contains 6 electrons (highlighted in green). Instead of losing 6, it is much easier for Oxygen to gain the 2 missing electrons (indicated by dashed red circles) to complete its octet of 8. Therefore, applying our formula, Oxygen’s valency is 8 - 6 = 2. Recognizing these patterns is frequently tested in your school exams, especially when you are asked to deduce chemical formulas through the cross-over valency method.

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