Benchmark 4D
The Physical Setting: Structure of Matter
Grades 9-12, page 80

Neutrons have a mass that is nearly identical to that of protons, but neutrons have no electric charge. Although neutrons have little effect on how an atom interacts with others, they do affect the mass and stability of the nucleus. Isotopes of the same element have the same number of protons (and therefore of electrons) but differ in the number of neutrons.

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.