My teachers taugh me that alpha particle is a helium nucleus carrying net +2 charge, hence if an radioactive subastance emits alpha particle it looses two of its protons and two neutrons but no electrons. Hence its atomic weight will decrease by 4 units. But books says its atomic number also decreases by 2 units and the atom gets transfromed into another atom e.g

Bi(z=213/A=83) > Tl(Z=209/A=81)

Since electrons are not lost how can the atoms change, only its nucleus must change , and if the atom change then the daughter atom must have -2 charge since the electrons are not lost . Please help me out.


In nuclear decay we are normally interested in the behavior of atomic nuclei.

Electrons from the atom's electron shell are considered to be spectators only and are ignored.

Since the number of protons in the atomic nucleus decreases by two in alpha emission, the atomic number also decreases by two. However, the principle of electroneutrality tells us that the atom will also lose two electrons from its electron shell.

  • $\begingroup$ So electrons will also be emiited correct. Hence we should get spots for negative and positve charge on electric plates placed to trace particles. $\endgroup$
    – Yogi Joshi
    Sep 2 '17 at 14:57

While chemical properties are determined by the electronic structure of an element, that structure is, in turn, caused by the number of protons in the nucleus of the element (which is the atomic number). And it isn't the number of electrons that characterises and element (strip electrons from an iron atom to give Fe2+ or Fe3+ and we still recognise the element as iron.)

What matters in radioactive decay is what changes in the nucleus. If a nucleus loses two electrons it becomes a different element and the electronic structure will be different. Usually the orbital electrons are ignored as what happens during the decay involves the nucleus only. After the nuclear decay the orbital electrons may rearrange in chemical processes.

We don't normally account for the orbital electrons in nuclear reactions as the orbital electrons are engaging in chemical not nuclear processes and. Besides, there are nuclear processes that involve electrons (neutrons in some nuclei can emit electrons and protons in others can capture them: these processes do involve electrons but it would be confusing to mix accounting for them with accounting for the chemical changes that result from things happening to orbital electrons).

Nuclear chemists only count what happens to the nucleus and ignore the ensuing chemical changes.


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