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 Form 4 Physics: Radioactivity online lessons

The nucleus and its components

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Answer Text:
The nucleus
The nucleus is made up of protons and neutrons. They are surrounded by negatively charged ions known as electrons. The number of protons is equal to the number of electrons. Both protons and neutrons have the same mass. The weight of an electron is relatively small compared to neutrons and protons. The number of protons in an atom is referred to as the
proton number (atomic number) and denoted by the symbol Z. the number of neutrons is denoted by the symbol N. Protons and neutrons are called nucleons since they form the nucleus of an atom. The sum of both the protons and neutrons is called the mass number A or nucleon number. Therefore;
A = Z + N and N = A - Z.
The masses of atoms are conveniently given in terms of
atomic mass unit (U) where U is 1/12th the mass of one atom of carbon-12 and has a value of #1.660 × 10^-27# kg. Hence the mass of one proton is equal to #1.67 × 10^-27# and is equal to 1U.