Which electron arrangements are stable

If there are multiple orbitals of equal energy, they will be filled with one electron in each energy level before a second electron is added. The electrons of the outermost energy level determine the energetic stability of the atom and its tendency to form chemical bonds with other atoms to form molecules.

Under standard conditions, atoms fill the inner shells first, often resulting in a variable number of electrons in the outermost shell. The innermost shell has a maximum of two electrons but the next two electron shells can each have a maximum of eight electrons. This is known as the octet rule , which states, with the exception of the innermost shell, that atoms are more stable energetically when they have eight electrons in their valence shell , the outermost electron shell.

Examples of some neutral atoms and their electron configurations are shown in Figure 2. Notice that in this figure, helium has a complete outer electron shell, with two electrons filling its first and only shell.

Similarly, neon has a complete outer 2n shell containing eight electrons. In contrast, chlorine and sodium have seven and one in their outer shells, respectively, but theoretically they would be more energetically stable if they followed the octet rule and had eight.

Figure 2. Bohr diagrams for hydrogen, helium, lithium, carbon, fluorine, neon, sodium, silicon, chlorine, and argon. Bohr diagrams indicate how many electrons fill each principal shell. Group 18 elements helium, neon, and argon are shown in Figure 2 have a full outer, or valence, shell. A full valence shell is the most stable electron configuration. Elements in other groups have partially filled valence shells and gain or lose electrons to achieve a stable electron configuration.

An atom may give, take, or share electrons with another atom to achieve a full valence shell, the most stable electron configuration. Looking at this figure, how many electrons do elements in group 1 need to lose in order to achieve a stable electron configuration? How many electrons do elements in groups 14 and 17 need to gain to achieve a stable configuration? Understanding that the organization of the periodic table is based on the total number of protons and electrons helps us know how electrons are distributed among the outer shell.

The periodic table is arranged in columns and rows based on the number of electrons and where these electrons are located. Figure 3. The periodic table shows the atomic mass and atomic number of each element. The atomic number appears above the symbol for the element and the approximate atomic mass appears below it.

The group 18 atoms helium He , neon Ne , and argon Ar all have filled outer electron shells, making it unnecessary for them to share electrons with other atoms to attain stability; they are highly stable as single atoms. Their non-reactivity has resulted in their being named the inert gases or noble gases. Compare this to the group 1 elements in the left-hand column. These elements, including hydrogen H , lithium Li , and sodium Na , all have one electron in their outermost shells. That means that they can achieve a stable configuration and a filled outer shell by donating or sharing one electron with another atom or a molecule such as water.

Hydrogen will donate or share its electron to achieve this configuration, while lithium and sodium will donate their electron to become stable. As a result of losing a negatively charged electron, they become positively charged ions. Group 17 elements, including fluorine and chlorine, have seven electrons in their outmost shells, so they tend to fill this shell with an electron from other atoms or molecules, making them negatively charged ions.

Group 14 elements, of which carbon is the most important to living systems, have four electrons in their outer shell allowing them to make several covalent bonds discussed below with other atoms. Although useful to explain the reactivity and chemical bonding of certain elements, the Bohr model of the atom does not accurately reflect how electrons are spatially distributed surrounding the nucleus. They do not circle the nucleus like the earth orbits the sun, but are found in electron orbitals.

The electrons always fill the lowest energy levels available until that level is filled, then electrons fill the next energy level until it is filled. This continues for all of the electrons in an atom. It is willing to lose 2 electrons so that it has the same electron arrangements as the nearest noble gas, which is neon 2, 8.

Atoms will gain or lose electrons to look like the nearest noble gas because the noble gases are unreactive due to the stability of having eight electrons in the highest energy level. This desire of atoms to have eight electrons in their outermost shell is known as the octet rule. How many valence electrons does chlorine have? How many electrons will chlorine gain or lose to form an ion? Chlorine has 7 electrons in its valence shell.

To meet the octet rule, it must either gain one electron or lose seven electrons. Gaining one is easier than losing seven so it will gain one electron to have a total of eight electrons when it forms an ion i. Allison Soult , Ph. Department of Chemistry, University of Kentucky.

Learning Outcomes Determine the energy levels of electrons for the first 20 elements. This is a very unstable arrangement, and the element sodium is a highly reactive, deadly white semi-solid that will burst into flames on exposure to the air or will burn through human flesh on contact.

A reactive substance. Chlorine atoms have 17 electrons. Two in the lowest, eight in the second and 7 in the third energy level. This too is a very unstable arrangement. This element is a gas at room temperature and was used in World War One as a poisonous attack weapon because of its high reactivity with human lungs.

These two atoms were made for one another. Sodium atoms readily give up the single electron in the outermost orbital. This electron is immediately picked up by a chlorine atom and fitted into the last empty space in its outermost orbital. Now both atomic arrangements are much more stable. They both have outermost orbitals which are filled with electrons.

However, there is a price to be paid for this stability. In giving up an electron, the sodium atom has lost a negative electrical charge.

It still has all its positively charged protons, so the remaining structure is no longer electrically neutral. Similarly, the chorine atom has picked up this extra negative charge and no extra protons, so it is now carrying a net negative charge -.