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Thus, in the helium atom, Hund's first rule correctly predicts that the 1s2s triplet state (3 S) is lower than the 1s2s singlet (1 S). Similarly for organic molecules, the same rule predicts that the first triplet state (denoted by T 1 in photochemistry) is lower than the first excited singlet state (S 1), which is generally correct.
The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells .
Atoms can be excited by heat, electricity, or light. The hydrogen atom provides a simple example of this concept.. The ground state of the hydrogen atom has the atom's single electron in the lowest possible orbital (that is, the spherically symmetric "1s" wave function, which, so far, has been demonstrated to have the lowest possible quantum numbers).
Any other configuration is an excited state. As an example, the ground state configuration of the sodium atom is 1s 2 2s 2 2p 6 3s 1, as deduced from the Aufbau principle (see below). The first excited state is obtained by promoting a 3s electron to the 3p subshell, to obtain the 1s 2 2s 2 2p 6 3p 1 configuration, abbreviated as the 3p level ...
[125] [126] Most of the helium in the crust of the Earth (about 99% of the helium from gas wells, as shown by its lower abundance of helium-3) is a product of alpha decay. [127] There are a few trace atoms on Earth that were not present at the beginning (i.e., not "primordial"), nor are results of radioactive decay.
A helium atom is an atom of the chemical element helium. Helium is composed of two electrons bound by the electromagnetic force to a nucleus containing two protons along with two neutrons, depending on the isotope , held together by the strong force .
Energy levels for an electron in an atom: ground state and excited states. After absorbing energy, an electron may jump from the ground state to a higher-energy excited state. The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system.
For example, the table shows that the first pair of vertically adjacent atoms with different ground-state term symbols are V and Nb. The 6 D 1 ⁄ 2 ground state of Nb corresponds to an excited state of V 2112 cm −1 above the 4 F 3 ⁄ 2 ground state of V, which in turn corresponds to an excited state of Nb 1143 cm −1 above the Nb ground ...