Helium nuclei were created in the Big Bang and contain two protons and two neutrons each. Helium is the second most abundant element, comprising roughly one quarter of the mass of the Universe. This animation zooms into a standard helium atom, showing its protons (green), neutrons (white), and electrons (blue).
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 184.108.40.206.0