Chemistry

If substance x decays by emitting 100% gas which escapes leaving no residues, then the half-life is half the time elapsed since noon Tuesday

If substance x contains some components which decay by say, K capture (the nucleus captures one of its orbiting electrons) with practically no change in mass, then the picture is complicated because it may have a short half-life, not detectable by weighing.

For this reason half-life is not determined or defined by weight. It is defined by the proportion of the original atomic species which remains after some interval of time, possibly determined by, say, chemical or spectroscopic analysis.

Yes, all isotopes of 91Pa (231, 233 and 234) are radioactive, instable, not naturally occurring.

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