Constituents of the nucleus

Every atomic nucleus is made up of two types of particles, collectively called nucleons. Protons carry positive charge $+e$ and have mass $m_p \approx 1{,}67\cdot 10^{-27}\;\text{kg}$; neutrons have zero charge and a mass $m_n$ slightly greater than that of the proton (by about $1{,}3\;\text{MeV}/c^2$). It is precisely this tiny mass excess of the neutron that makes it unstable when free, and that allows the beta decay we shall discuss later.

Two integers completely identify a nucleus. The atomic number $Z$ counts the protons: it is $Z$ that defines the chemical element, because it fixes how many electrons the neutral atom has and hence its entire chemistry. The mass number $A = Z + N$ counts the total nucleons, where $N$ is the number of neutrons.

Key formula

$Z = \text{protons}, \qquad A = Z + N = \text{total nucleons}$ Notation: ${}^A_Z X$. Examples: ${}^{12}_{6}\text{C}$, ${}^{235}_{92}\text{U}$.

In the notation ${}^A_Z X$ the chemical symbol $X$ and the atomic number $Z$ are redundant (one determines the other), but both are written for ease of reading: carbon is always $Z = 6$, uranium always $Z = 92$. What changes, among the various isotopes of the same element, is only the mass number at the top left.

Links

Topics: Fisica nucleare Objects: Nucleo atomico

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