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Confusion about decays

Many heavy elements decay into simpler things. But a close observation of these decays reveals several confusing problems.

Consider uranium-238 decay.      

A lump of uranium-238 will decay at a constant rate such that in 4,460,000,000 years -- give or take a few days -- half the uranium will be gone. But there is no way to tell when a specific uranium atom will decay; it could decay five minutes from now, or in ten billion years. Why will an atom decay only according to some probability?

Uranium-238 has a mass of 238.0508 atomic mass units (u). It can decay into thorium (234.0436 u) and an alpha particle (4.0026 u). But uranium's mass minus the mass of its decay products is 0.0046 u. Why is there missing mass?