Dear sidharth,

Protons have a positive charge and electrons have a negative charge. Free (unattached) uncharged atoms have the same number of electrons as protons to be electrically neutral. The protons are in the nucleus and do not change or vary except in some nuclear reactions. The electrons are in discrete pathways or shells around the nucleus. There is a ranking or heirarchy of the shells, usually with the shells further from the nucleus having a higher energy. As we consider the electron configuration of atoms, we will be describing the ground state position of the electrons. When electrons have higher energy, they may move up away from the nucleus into higher energy shells. As we consider the electron configuration, we will be describing the ground state positions of the electrons.

A hydrogen atom has only one proton and one electron. The electron of a hydrogen atom travels around the proton nucleus in a shell of a spherical shape. The two electrons of helium, element number two, are in the same spherical shape around the nucleus. The first shell only has one subshell, and that subshell has only one orbital, or pathway for electrons. Each orbital has a place for two electrons. The spherical shape of the lone orbital in the first energy level has given it the name ‘s’ orbital. Helium is the last element in the first period. Being an inert element, it indicates that that shell is full. Shell number one has only one s subshell and all s subshells have only one orbital. Each orbital only has room for two electrons. So the first shell, called the K shell, has only two electrons.

Beginning with lithium, the electrons do not have room in the first shell or energy level. Lithium has two electrons in the first shell and one electron in the next shell. The first shell fills first and the others more or less in order as the element size increases up the Periodic Chart, but the sequence is not immediately obvious. The second energy level has room for eight electrons. The second energy level has not only an s orbital, but also a p subshell with three orbitals. The p subshell can contain six electrons. The p subshell has a shape of three dumbbells at ninety degrees to each other, each dumbbell shape being one orbital. With the s and p subshells the second shell, the L shell, can hold a total of eight electrons. You can see this on the periodic chart. Lithium has one electron in the outside shell, the L shell. Beryllium has two electrons in the outside shell. The s subshell fills first, so all other electrons adding to this shell go into the p subshell. Boron has three outside electrons, carbon has four, nitrogen has five, oxygen has six, and fluorine has seven. Neon has a full shell of eight electrons in the outside shell, the L shell, meaning the neon is an inert element, the end of the period.

Beginning again at sodium with one electron in the outside shell, the M shell fills its s and p subshells with eight electrons. Argon, element eighteen, has two electrons in the K shell, eight in the L shell, and eight in the M shell. The fourth period begins again with potassium and calcium, but there is a difference here. After the addition of the 4s electrons and before the addition of the 4p electrons, the sequence goes back to the third energy level to insert electrons in a d shell.

The shells or energy levels are numbered or lettered, beginning with K. So K is one, L is two, M is three, N is four, O is five, P is six, and Q is seven. As the s shells can only have two electrons and the p shells can only have six electrons, the d shells can have only ten electrons and the f shells can have only fourteen electrons. The sequence of addition of the electrons as the atomic number increases is as follows with the first number being the shell number, the s, p, d, or f being the type of subshell, and the last number being the number of electrons in the subshell.

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6

It is tempting to put an 8s2 at the end of the sequence, but we have no evidence of an R shell. One way to know this sequence is to memorize it. There is a bit of a pattern in it. The next way to know this sequence is to SEE IT ON THE PERIODIC CHART. As you go from hydrogen down the chart, the Groups 1 and 2 represent the filling of an s subshell. The filling of a p subshell is shown in Groups 3 through 8. The filling of a d subshell is represented by the transition elements (ten elements), and the filling of an fsubshell is shown in the lanthanide and actinide series (fourteen elements).