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Question

State the relation between atomic number and atomic mass for light elements. State which elements are considered radioactive giving reasons.

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Answer

Electronic configuration of lighter elements shows that the elements which have an even number of proton, for example, atomic numbers like 4He, 12C etc., have their mass numbers twice the atomic numbers except for 9Be and 40Ar

Elements which have an odd number of protons like 7Li11B etc., have their mass number twice the atomic numbers + 1 (A = 2Z + 1) except 14N and 

Elements with neutron: proton ratios 1.5 and above are considered as radioactive as the nucleus of such elements becomes unstable causing their radioactive decay. For example, in Uranium (235U92):

  • No. of protons (p) = 92
  • No. of neutrons (n) = 235 – 92 = 143
A neutron : proton ratio greater than 1.5 makes Uranium radioactive.

💡 Some Related Questions

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With reference to group 1 [IA] of the periodic table – fill in the blanks with the correct word:
The elements are …………… [light/heavy] …………… [metals/non metals] since their atomic size is …………… [large/small]. The energy binding the atoms is …………… [high/low] and hence the elements have …………… [high/low] melting points. The melting points of the elements …………… [increases/decreases] down the subgroup. The electropositive character …………… [increases/decreases] down the subgroup and the elements are strong …………… [reducing/oxidizing] agents. The element with electronic configuration 2, 8, 1 will have …………… [higher/lower] electron affinity and …………… [smaller/larger] atomic size than the element with electronic configuration 2, 1.

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In period 2, element ‘A’ is to the right of element ‘B’
(a) The element ‘A’ would probably have a …………… [smaller/larger] atomic size than ‘B’.
(b) The element ‘B’ would probably have …………… [lower/higher] ionization potential than ‘A’.
(c) The element ‘A’ would have …………… [lesser/higher] electron affinity than ‘B’.
(d) Nuclear charge of element ‘B’ would be …………… [less/more] than element ‘A’.
(e) If an element ‘C’ had a low electronegativity and ionization potential it would have more tendency to …………… [gain/lose] electrons.

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