# JEE Main & Advanced Chemistry Structure of Atom Atomic number, Mass number and Atomic species

## Atomic number, Mass number and Atomic species

Category : JEE Main & Advanced

(1) Atomic number or Nuclear charge

(i) The number of protons present in the nucleus of the atom is called atomic number (Z). (ii) It was determined by Moseley as,

$\sqrt{\nu }=a(Z-b)$ or $aZ-ab$

Where, $\nu =X-$ray?s frequency

Z= atomic number of the metal $a\And b$ are constant.

(iii) Atomic number = Number of positive charge on nucleus = Number of protons in nucleus = Number of electrons in nutral atom.

(iv) Two different elements can never have identical atomic number.

(2) Mass number Mass number (A) = Number of protons or Atomic number (Z) + Number of neutrons or Number of neutrons = A ? Z .

(i) Since mass of a proton or a neutron is not a whole number (on atomic weight scale), weight is not necessarily a whole number.

(ii) The atom of an element X having mass number (A) and atomic number (Z) may be represented by a symbol,$_{Z}{{X}^{A}}$. Different types of atomic species

 Atomic species Similarities Differences Examples Isotopes (Soddy) (i) Atomic No. (Z) (ii) No. of protons (iii) No. of electrons (iv) Electronic configuration (v) Chemical properties (vi) Position in the periodic table (i) Mass No. (A) (ii) No. of neutrons (iii) Physical properties (i) $_{1}^{1}H,\,_{1}^{2}H,\,_{1}^{3}H$ (ii) $_{8}^{16}O,\,_{8}^{17}O,\,_{8}^{18}O$ (iii) $_{17}^{35}Cl,\,_{17}^{37}Cl$ Isobars (i) Mass No. (A) (ii) No. of nucleons (i) Atomic No. (Z) (ii) No. of protons, electrons and neutrons (iii)Electronic configuration (iv) Chemical properties (v) Position in the perodic table. (i) $_{18}^{40}Ar,\,_{19}^{40}K,\,_{20}^{40}Ca$ (ii) $_{52}^{130}Te,\,_{54}^{130}Xe,\,_{56}^{130}Ba$. Isotones No. of neutrons (i)  Atomic No. (ii) Mass No., protons and electrons. (iii) Electronic configuration (iv) Physical and chemical properties (v) Position in the periodic table. (i) $_{14}^{30}Si,\,_{15}^{31}P,\,_{16}^{32}S$ (ii) $_{19}^{39}K,\,_{20}^{40}Ca$ (iii) $_{1}^{3}H,\,_{2}^{4}He$ (iv) $_{6}^{13}C,\,_{7}^{14}N$ Isodiaphers Isotopic No. (N ? Z) or (A ? 2Z) (i) At No., mass No., electrons, protons, neutrons. (ii) Physical and chemical properties. (i)$_{92}{{U}^{235}},{{\,}_{90}}T{{h}^{231}}$ (ii) $_{19}{{K}^{39}},{{\,}_{9}}{{F}^{19}}$ (iii) $_{29}C{{u}^{65}},{{\,}_{24}}C{{r}^{55}}$ Isoelectronic species (i) No. of electrons (ii) Electronic configuration At. No., mass No. (i) ${{N}_{2}}O,\,C{{O}_{2}},\,CN{{O}^{-}}(22{{e}^{-}})$ (ii) $CO,\,C{{N}^{-}},\,{{N}_{2}}(14{{e}^{-}})$ (iii) ${{H}^{-}},\,He,\,L{{i}^{+}},\,B{{e}^{2+}}(2{{e}^{-}})$ (iv)${{P}^{3-}},\,{{S}^{2-}},\,C{{l}^{-}},\,Ar,\,{{K}^{+}}and\,C{{a}^{2+}}(18{{e}^{-}})$ Isosters (i) No. of atoms (ii) No. of electrons (iii) Physical and chemical properties. (i) ${{N}_{2}}$ and $CO$ (ii) $C{{O}_{2}}$ and ${{N}_{2}}O$ (iii) $HCl$ and ${{F}_{2}}$ (iv) $CaO$ and $MgS$ (v) ${{C}_{6}}{{H}_{6}}$ and ${{B}_{3}}{{N}_{3}}{{H}_{6}}$

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