Helium is the 2nd element on the periodic table because it has 2 protons in its nucleus. Since the protons carry a positive charge, they will attract 2 electrons in order to balance the charge. Helium is therefore a neutral atom overall, with two electrons surrounding two protons (and 2 neutrons) in the nucleus.
The full-color wireframe represents a pair of electrons — a di-electron. In this case they are forming a sphere-shaped s-orbital. Helium only has 1 electron shell (an s-orbital) containing 2 electrons. We describe it with an electron configuration of 1s2. The “1–s–2” means: shell number 1, the s-orbital, which contains 2 electrons.
The image below provides a different view of the spherical helium atom.
The size of the nucleus in the center is greatly exaggerated. If the electron cloud were the size of a large football stadium, the nucleus would be the size of a penny at the center of the field — barely visible
Helium is the smallest atom on the periodic table. It is smaller than hydrogen because it has twice as many protons in the nucleus attracting twice as many electrons inwards. This higher “effective nuclear charge” shrinks the atom’s size. Since helium’s electrons are the most closely bound to their nucleus, helium has the highest ionization energy (2,370 kJ/mol or 24.5 eV) and is consequently the most unreactive element on the periodic table. Similar to the di-electron that envelops and binds the hydrogen molecule (H2), helium’s two electrons form a very stable and perfectly spherical di-electron state — a boson state — where the two electron wave functions are completely superimposed upon one another. (See Understanding Electrons.)
Helium also contains the most stable nucleus — the alpha particle. Most other nuclei are composed of various combinations of alpha particles (see the Robinson Model of Nuclear Binding), which is why this is the only type of multi-nucleon structure that is ejected during radioactive decay.
With two proton-neutron pairs, the helium nucleus is itself a di-boson, making the helium atom a very stable tri-boson state.
Hydrogen (H2) and helium (He) are the two lightest gases. Since their atoms have the lowest masses, their gases have the lowest densities. This makes them the best gases to use for buoyancy — in balloons and blimps — in the atmosphere. Since hydrogen gas is so highly flammable, however, safety requires that helium be the gas of choice for buoyancy. Helium will never ignite because, due to its di-electron stability, it is not interested in reacting with oxygen (or any other atom).
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