The nucleus, a tiny fraction of the volume of an atom, is composed of
protons and neutrons, each almost two thousand times heavier than an electron.
The number of positive protons in the nucleus determines what an atom's
electron configuration can be and so defines the element. In a neutral
atom, the number of electrons equals the number of protons. But an atom
may acquire an unbalanced charge by gaining or losing electrons.
NSES Content Standard B
Physical Science: Structure of atoms Grades 9-12, page 178 The atom's nucleus is composed of protons and neutrons, which are much more massive than electrons. When an element has atoms that differ in the number of neutrons, these atoms are called different isotopes of the element. NSES Content Standard B
|