Electron Arrangements

Model for The Shell of The Atom

Being able to predict a chemical reaction when an element is mixed with a second substance is a very useful skill for a chemist to have. To help develop this skill, a chemist needs to know how the electrons of a given element are organized. This can be accomplished through learning to predict the arrangements of each element based on its periodic table position.  For the most part, the electrons are arranged in a pattern that is not overly difficult to recognize. Once learned, most chemists can look at the periodic table and predict the most probable ground state electron arrangement of an element within seconds. The atom is considered to be organized into two regions. The first region is the small, dense, positively charged nucleus, which contains the protons and neutrons. This region makes up the greatest proportion of the atom's mass, about 99.99%. However, the nucleus is very dense and tiny; therefore it actually occupies the smallest proportion of the volume of the atom (1/10,000). See figure 1.

The second region, the shell, surrounds the nucleus and is occupied by the electrons. In comparison to the protons and neutrons, the negatively charged electrons are quite small. Since the shell is so large in comparison to the electrons, it is considered to be mostly empty space. This  empty space, and the organization of the electrons that occupy it, is the focus of the quantum model for the shell of the atom.  The quantum model describes the shell of the atom from the standpoint that electrons exhibit an electromagnetic wave and a particle natures simultaneously. This wave nature allows the electron to be described in a fashion that reflects the very shape of the periodic table.  Fortunately, one does NOT need be a physicist and/or mathematician to understand and use the results of the quantum model.

In figure 2 the shell is divided into two parts; the valence shell and the kernel.

The valence shell is defined by the highest s sublevel and its corresponding p sublevel. Any sublevel receiving electrons after the s, and before the p sublevels, is a part of the valence shell. The distribution of the electrons in the valence shell helps determine an element's chemistry. All of the electrons below the valence shell are in the kernel of the atom. The number of electrons in the kernel also helps determine the chemical properties of each element. The valence shell contains the electrons of an element that come into contact with another element during chemical reaction. The electrons can be lost, gained, shared or rearranged during chemical reaction. The number of electrons and the sublevel they occupy is the key to predicting how an element will chemically react

The Principal Energy Level

The principal energy level specifies how far away any given electron is from the nucleus. There are at present seven distances or positions away from the nucleus in which ground state electrons can be found. These distances from the nucleus are assigned an order number very much ike the floors of a tall building. If an electron is at the fourth energy level (n=4), it is further from the nucleus than an electron at the first energy level (n=1). This number is referred to as the principal quantum number or the principal energy level. An alternate way to view this principal quantum number is like saying that someone living on the fourth floor of an apartment building is higher up than a person living on the first floor of the same building. In figure 3, the principal energy levels have values of 1, 2, 3, 4, 5 ,6 and 7.

The Energy Sublevel

The principal energy level is subdivided into energy sublevels. An energy sublevel, figure 4, can be compared to an

apartment on a building floor. When considering the present periodic table containing more than 118 elements, there are four energy sublevels needed to represent all ground state electron arrangements. The first is an s-type energy sublevel. It is able to hold only two electrons. The second energy sublevel, is a p-type energy sublevel and can hold six electrons. Third, is a d-type energy sublevel that is able to hold ten electrons. The fourth energy sublevel type can hold fourteen electrons and is referred to as an f-type energy sublevel. There are additional energy sublevel types; however, they are not needed in the ground state description of the largest elements presently on the periodic table. When elements containing more than 118 electrons are considered, a fifth energy sublevel, g-type, will be needed to describe the additional ground state electron arrangements.

Orbitals

Energy sublevels are divided into orbitals just like an apartment can be divided into rooms. The s-type has one orbital, the p-type has three orbitals, the d-type has five orbitals and the f-type has seven orbitals. Each orbital is able to hold only two electrons. The presently unused g-type sublevel will hold 18 electrons in nine orbitals.

Electron Spin

The principal energy level, energy sublevel, and orbital are three pieces of information that describe where each electron is in an atom's shell. One more piece of information is needed in order to uniquely identify every electron in the shell of an atom. The electron spin identifies whether an electron is spinning clockwise or counter clockwise, figure 5.

Electrons act as little magnets.  Just like magnets, they will repel each other if they have their spins oriented in the same direction and they will attract each other if they are spinning in opposite directions.

Order of Energy Sublevels

The principal energy levels, energy sublevels and orbitals are needed to represent the orbital-filling diagrams with the electrons showing their proper spin. Each element on the periodic table can be represented using the orbitals displayed in figure 6.

The energy sublevels must be shifted vertically to account for energy differences found experimentally. The result shows only the principal energy levels, sublevels and orbitals needed to predict the electronic structure of the 118 neutral, ground state elements represented on the present periodic table. The energy-shifted result is in figure 7.

The energy shifted sublevels allows for quick identification of the proper order of adding electrons to the successive sublevels in the shell of an atom. After the 6s orbital is filled with two electrons, it can be seen that the next sublevel upward to receive an electron is the 4f. Other methods for finding the next lowest energy sublevel used can be found by the internet search phrase, “diagonal rule.”

Three Rules to Remember

When predicting the electronic structure of an element, a few simple rules need to be followed.  First, electrons have spin. The direction of spin is indicated using an arrow up or an arrow down. The spinning electrons act like magnets.  When two electrons spin in the same direction, they repel each other like magnets. Two electrons can occupy the same orbital if, and only if, they have opposite spin. The correct electronic arrangement for helium, atomic number 2, is on the right in figure 8.

The second rule is that the next electron is added to the sublevel with the next lowest energy.  Figure 9  shows for

lithium, atomic number 3, an incorrect result on the left and the correct result on the right. Third, each orbital in an energy sublevel gets one electron before another orbital gets a second electron.  All the unpaired electrons should have the same spin direction. This is referred to as Hund's rule. In figure 10 the correct arrangements are on the right.

© Pat Thayer 2014-2016