Re: How many electrons can fit into the energy levels?

The number of protons in the nucleus of an atom of a certain element 
determines the element’s atomic number. The number of protons in the 
nucleus can be determined by measuring the positive charge on the nucleus. 
For example, an atom with a nuclear charge of +26 has 26 protons in its 
nucleus and must be iron. To be electrically neutral, an atom of iron must 
have 26 electrons surrounding the nucleus.

The total number of protons and neutrons in a nucleus is called the mass 
number, since these particles account for almost the entire mass of an 
atom. Generally, the number of neutrons in the nucleus is equal to the 
number of protons . However, atoms of the same element can have the same 
number of protons but different numbers of neutrons, thus giving rise to 
varieties, or isotopes, of the same chemical element. The word isotope 
(Greek iso and topos) means "same place." Different isotopes of the same 
element occupy the same place in the periodic table of the chemical 
elements and have very nearly identical chemical properties. Thus 
hydrogen, which has a mass number of 1, has an isotope, deuterium, which 
has one proton and one neutron in its nucleus and a mass number of 2. 
Deuterium accounts for 1.5 percent of naturally occurring hydrogen. 
Hydrogen and deuterium undergo the same chemical reactions, although not 
necessarily with equal ease.
The term atomic weight means the average weight (more correctly, the mass) 
of an atom of an element, taking into account the masses of all its 
isotopes and the percentage of their occurrence in nature. The atomic 
weight of an element was originally expressed relative to the weight of an 
atom of the most abundant isotope of carbon, carbon-12 (written 12C) which 
contains six neutrons and six protons. The weight of 12C is arbitrarily 
set equal to its mass number of 12.0000. In 1961 this standard was changed 
by international agreement, and atomic weights are now determined relative 
to oxygen by assigning a value of 16.0000 to the mixture of oxygen 
isotopes found in nature.
Isotopes are generally written as 12C or carbon-12, with the number 
denoting the total number of protons and neutrons in the atom. Four out of 
every five elements occur in nature as mixtures of isotopes (see Atomic 
Weight). For example, chlorine occurs in nature as a combination of two 
isotopes. Samples of chlorine contain 75.77 percent 35Cl (with an atomic 
mass of 34.9689), and 24.23 percent 37Cl (with an atomic mass of 36.9659). 
The average atomic mass of chlorine is (0.7577 × 34.9689) + (0.2423 × 
36.9659) = 35.4527.
The molecular weight of a molecule is the sum of the atomic weights of the 
atoms making up that molecule . Thus the molecular weight of water (H2O) 
is 2 × 1.00794 (for two hydrogen atoms) + 15.9994 (one oxygen atom), or 
18.01528.
	C2	The Electron Cloud  
Most of the physical and chemical properties of atoms, and hence of all 
matter, are determined by the nature of the electron cloud enclosing the 
nucleus.
The nucleus of an atom, with its positive electric charge, attracts 
negatively charged electrons. This attraction is largely responsible for 
holding the atom together. The revolution of electrons about a nucleus is 
determined by the force with which they are attracted to the nucleus. The 
electrons move very rapidly, and determination of exactly where any 
particular one is at a given time is theoretically impossible (see 
Uncertainty Principle). If the atom were visible, the electrons might 
appear as a cloud, or fog, that is dense in some spots, thin in others. 
The shape of this cloud and the probability of finding an electron at any 
point in the cloud can be calculated from the equations of wave mechanics 
(see Quantum Theory). The solutions of these equations are called 
orbitals. Each orbital is associated with a definite energy, and each may 
be occupied by no more than two electrons. If an orbital contains two 
electrons, the electrons must have opposite spins, a property related to 
the angular momentum of the electrons.


 The electrons occupy the orbitals of lowest energy first, then the 
orbitals next in energy, and so on, building out until the atom is 
complete .
The orbitals tend to form groups known as shells (so-called because they 
are analogous to the layers, or shells, around an onion). Each shell is 
associated with a different level of energy. Starting from the nucleus and 
counting outward, the shells, or principal energy levels, are numbered 1, 
2, 3, … , n. The outer shells have more space than the inner ones and can 
accommodate more orbitals and therefore more electrons. The nth shell 
consists of 2n-1 orbitals, and each orbital can hold a maximum of 2 
electrons. For example, the third shell contains five orbitals and holds a 
maximum of 10 electrons; the fourth shell contains seven orbitals and 
holds a maximum of 14 electrons. Among the known elements, only the first 
seven shells of an atom contain electrons, and only the first four shells 
are ever filled.
Each shell (designated as n) contains different types of orbitals, 
numbered from 0 to n-1. The first four types of orbitals are known by 
their letter designations as s, p, d, and f. There is one s-orbital in 
each shell, and this orbital contains the most firmly bound electrons of 
the shell. The s-orbital is followed by the p-orbitals (which always occur 
in groups of three), the d-orbitals (which always occur in groups of 
five), and finally the f-orbitals (which always occur in groups of seven). 
The s-orbitals are always spherically shaped around the nucleus; each p-
orbital has two lobes resembling two balls touching; each d-orbital has 
four lobes; and each f-orbital has eight lobes. The p-, d-, and f-orbitals 
have a directional orientation in space, but the spherical s-orbitals do 
not. The three p-orbitals are oriented perpendicular to one another along 
the axis of an imaginary three-dimensional Cartesian (x, y, z) coordinate 
system. The three p-orbitals are designated px, py, and pz, respectively. 
The d- and f-orbitals are similarly arranged about the nucleus at fixed 
angles to one another.
When elements are listed in order of increasing atomic number, an atom of 
one element contains one more electron than an atom of the preceding 
element . The added electrons fill orbitals in order of the increasing 
energy of the orbitals. The first shell contains the 1s orbital; the 
second shell contains the 2s orbital and the 2p orbitals; the third shell 
contains the 3s orbital, the 3p orbitals, and the 3d orbitals; the fourth 
shell contains the 4s orbital, the 4p orbitals, the 4d orbitals, and the 
4f orbitals.
After the two innermost shells, certain orbitals of outer shells have 
lower energies than the last orbitals of preceding shells. For this 
reason, some orbitals of the outer shells fill before the previous shells 
are complete. For example, the s-orbital of the fourth shell (4s) fills 
before the d-orbitals of the third shell (3d). Orbitals generally fill in 
this order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s.
In a notation frequently used to describe the electron configuration of an 
element, a superscript after the orbital letter gives the number of 
electrons in that orbital. Thus, 1s22s22p5 means that the atom has two 
electrons in the 1s orbital, two electrons in the 2s orbital, and five 
electrons in the 2p orbitals.
Neutral atoms with exactly eight electrons in the outer shell (meaning the 
s- and p-orbitals of the outer shell are filled) are exceptionally stable. 
These neutral atoms are atoms of the noble gases, which are so stable that 
getting them to chemically react with other elements is very difficult. 
The unusual stability of the noble-gas electron structures is of great 
importance in chemical bonding and reactivity. All other elements tend to 
combine with each other in such a way as to imitate this stable structure. 
The structure of helium is 1s2; neon adds another stable shell, 2s22p6, to 
this; argon adds the orbitals 3s23p6; krypton adds the orbitals 
4s23d104p6; and xenon adds the orbitals 5s24d105p6 (the s-orbital fills 
before the d-orbital of the previous shell).

Why the letter k was choosen: It comes from the german word KREIS or KURVE