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In group 17 how many valence electrons does Iodine have? How many valence electrons does chlorine have?

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Hint: There are various shells in the atoms which are denoted by K, L, M, N, etc. According to the number of electrons in the atoms are arranged in the shells and the last shell of the atom is the valence shell and the electrons in that shell are valence electrons. The atomic number of iodine is 53 and the atomic number of chlorine is 17.

Complete Solution :
 - The periodic table is the arrangement of elements according to their increasing number of electrons and these electrons are filled in the orbits according to their increasing energies. There are various shells in the atoms which are denoted by K, L, M, N, etc, which represents the specific energy of the shell. The shell K represents 1, the shell L represents 2, the shell M represents 3, the shell N represents 4. Each shell has specific sub-shells which are s, p, d, f, etc.
- According to the number of electrons in the atoms are arranged in the shells and the last shell of the atom is the valence shell and the electrons in that shell are valence electrons and these valence electrons are responsible for the bond formation. As the valence electron decides the block of the element, if the last electron enters in the s-subshell then the element is of s-block.
-The atomic number of Iodine is 53 so, its electronic configuration is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{2}}3{{d}^{10}}4{{p}^{6}}5{{s}^{2}}4{{d}^{10}}5{{p}^{5}}$. The last shell is 5 and the electrons in the last shell is 7 (2+5).
The atomic number of chlorine is 17 so, its electronic configuration is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{5}}$. The last shell is 3 and the electrons in the last shell is 7 (2+5).

Note: The K shell has one sub-shell (s), the L shell has two sub-shells (s, p), the M shell has three sub-shells (s, p, d), the N shell has four sub-shells (s, p, d, f). The valence electrons are the total number of electrons in the last shell combining all the sub-shell.