mass number is the sum of the protons and neutrons in an atom.
mass number is the sum of the protons and neutrons in an atom.
what is the sum of the protons and neutrons in an atom?
mass number
an atomic nucleus is formed by a number of protons, "z" (the atomic number), and a number of neutrons, "n" (the neutron number), bound together by the nuclear force. the atomic number defines the chemical properties of the atom, and the neutron number determines the isotope or nuclide. the terms isotope and nuclide are often used synonymously, but they refer to chemical and nuclear properties, respectively. strictly speaking, isotopes are two or more nuclides with the same number of protons; nuclides with the same number of neutrons are called isotones. the atomic mass number, symbol "a", equals "z"+"n". nuclides with the same atomic mass number are called isobars. the nucleus of the most common isotope of the hydrogen atom (with the chemical symbol h) is a lone proton. the nuclei of the heavy hydrogen isotopes deuterium (d or h) and tritium (t or h) contain one proton bound to one and two neutrons, respectively. all other types of atomic nuclei are composed of two or more protons and various numbers of neutrons. the most common nuclide of the common chemical element lead, pb, has 82 protons and 126 neutrons, for example. the table of nuclides comprises all the known nuclides. even though it is not a chemical element, the neutron is included in this table.
neutron
every atom is composed of a nucleus and one or more electrons bound to the nucleus. the nucleus is made of one or more protons and a number of neutrons. only the most common variety of hydrogen has no neutrons. protons and neutrons are called nucleons. more than 99.94% of an atom's mass is in the nucleus. the protons have a positive electric charge whereas the electrons have a negative electric charge. the neutrons have no electric charge. if the number of protons and electrons are equal, then the atom is electrically neutral. if an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively. these atoms are called ions.
atom