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Helium, atomic structure
Diagram of the nuclear composition, electron configuration, and valence orbitals of an atom of helium-4 (atomic number: 2), the most common isotope of the element helium. The nucleus consists of 2 protons (blue) and 2 neutrons (red). Two electrons (white) fill the first electron shell (ring), a very stable configuration. The stability of an element's outer (valence) electrons determines its chemical and physical properties. Helium is a noble gas in group 18, period 1, and the s-block of the periodic table. At room temperature and pressure it forms an inert gas, liquefying at minus 269 degrees Celsius. Photo by CARLOS CLARIVAN/SCIENCE PHOTO LIBRARY
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