An atom with a closed shell of valence electrons (corresponding to an electron configuration s 2 p 6) tends to be chemically inert.
https://en.wikipedia.org/wiki/Valence_electron
For a main group element, the number of valence electrons that it may have depends on the electron configuration in a simple way. However, for a transition metal, the relationship is more complex.
https://en.wikipedia.org/wiki/Valence_electron
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and typically a similar number of neutrons (none in hydrogen-1). Protons and neutrons are called nucleons.
https://en.wikipedia.org/wiki/Atom
The value of this number is called the atomic mass. A given atom has an atomic mass approximately equal (within 1%) to its mass number times the atomic mass unit (for example the mass of a nitrogen-14 is roughly 14 u). However, this number will not be exactly an integer except in the case of carbon-12 (see below).
https://en.wikipedia.org/wiki/Atom
Because the number of valence electrons which actually participate in chemical reactions is difficult to predict, the concept of the valence electron is less useful for a transition metal than for a main group element; the d electron count is an alternative tool for understanding the chemistry of a transition metal.
https://en.wikipedia.org/wiki/Valence_electron
The number of protons in the nucleus defines to what chemical element the atom belongs: for example, all copper atoms contain 29 protons. The number of neutrons defines the isotope of the element. The number of electrons influences the magnetic properties of an atom.
https://en.wikipedia.org/wiki/Atom
The presence of valence electrons can determine the element's chemical properties and whether it may bond with other elements: For a main group element, a valence electron can only be in the outermost electron shell. In a transition metal, a valence electron can also be in an inner shell.
https://en.wikipedia.org/wiki/Valence_electron
The essence of these calculations is that when an electron is placed into the 2s orbital the electron is likely to spend more time closer to the nucleus than an electron in a 2p orbital.
http://intro.chem.okstate.edu/1215/lecture/chapter11/fri112098.html
So the second electron could go into the 1s orbital with the opposite spin of the first electron or it could go into the next orbital in the n = 2 level.
http://intro.chem.okstate.edu/1215/lecture/chapter11/fri112098.html
The number of electrons in an atom's outermost valence shell governs its bonding behavior. Therefore, elements whose atoms can have the same number of valence electrons are grouped together in the periodic table of the elements.
https://en.wikipedia.org/wiki/Valence_electron
the number of electrons in an atom's outermost level is called
The number of electrons in an atom's outermost level is called the valence electron. The number of electrons in the outermost level of an atom is called a valence electron.