Not all facets have enough electrons to fill their outermost shells, however an atom is in ~ its most stable when all of the electron positions in the outermost shell are filled. Due to the fact that of this vacancies in the outermost shells, we view the development of chemistry bonds, or interactions between two or an ext of the same or different elements that an outcome in the development of molecules. To attain greater stability, atoms will have tendency to completely fill their external shells and will bond v other elements to attain this score by sharing electrons, accepting electron from one more atom, or donating electrons to another atom. Due to the fact that the outermost shells that the elements with low atomic numbers (up to calcium, with atomic number 20) have the right to hold eight electrons, this is described as the octet rule. An facet can donate, accept, or re-superstructure electrons through other aspects to to fill its outer shell and satisfy the octet rule.

You are watching: Which element has a full outermost energy level containing only two electrons?

An beforehand model of the atom was arisen in 1913 by the Danish scientist Niels Bohr (1885–1962). The Bohr model shows the atom as a main nucleus include protons and neutrons, v the electron in circular electron shells at certain distances indigenous the nucleus, comparable to planets orbiting around the sun. Every electron shell has actually a different energy level, v those shells closest to the cell nucleus being reduced in power than those farther native the nucleus. By convention, each covering is assigned a number and also the price n—for example, the electron shell closest to the nucleus is dubbed 1n. In order to move in between shells, an electron should absorb or relax an quantity of energy corresponding exactly come the distinction in energy between the shells. Because that instance, if one electron absorbs power from a photon, the may become excited and move to a higher-energy shell; conversely, as soon as an excited electron drops earlier down come a lower-energy shell, it will certainly release energy, frequently in the kind of heat.


Bohr design of one atom, showing energy levels together concentric circles bordering the nucleus. Power must be included to relocate an electron outward to a greater energy level, and energy is released when an electron falls down indigenous a greater energy level come a closer-in one. Image credit: modified indigenous OpenStax Biology

Atoms, like other things administrate by the regulations of physics, often tend to take on the lowest-energy, many stable construction they can. Thus, the electron shells of an atom are lived in from the within out, with electrons filling up the low-energy shells closer to the nucleus prior to they move right into the higher-energy shells more out. The shell closest to the nucleus, 1n, have the right to hold two electrons, when the next shell, 2n, can hold eight, and the 3rd shell, 3n, have the right to hold as much as eighteen.

The number of electrons in the outermost shell of a particular atom determines its reactivity, or propensity to type chemical binding with various other atoms. This outermost covering is well-known as the valence shell, and the electrons discovered in it space called valence electrons. In general, atom are most stable, least reactive, as soon as their outermost electron shell is full. Many of the aspects important in biology require eight electrons in their outermost covering in stimulate to it is in stable, and this ascendancy of ignorance is known as the octet rule. Part atoms have the right to be steady with an octet even though your valence covering is the 3n shell, which have the right to hold as much as 18 electrons. Us will discover the factor for this as soon as we discuss electron orbitals below.

Examples of part neutral atoms and their electron configuration are presented below. In this table, you can see that helium has a full valence shell, through two electron in its an initial and only, 1n, shell. Similarly, neon has actually a complete outer 2n shell containing eight electrons. These electron configurations do helium and neon very stable. Although argon does no technically have actually a full outer shell, due to the fact that the 3n shell have the right to hold up to eighteen electrons, it is stable favor neon and helium due to the fact that it has eight electrons in the 3n shell and also thus satisfies the octet rule. In contrast, chlorine has actually only seven electrons in that outermost shell, when sodium has just one. This patterns execute not to fill the outermost shell or meet the octet rule, making chlorine and sodium reactive, eager to get or shed electrons to reach a more stable configuration.


Bohr diagrams show how numerous electrons to fill each principal shell. Group 18 aspects (helium, neon, and argon) have a full outer, or valence, shell. A complete valence shell is the most stable electron configuration. Elements in other teams have partly filled valence shells and also gain or lose electrons to accomplish a steady electron configuration.

Electron configurations and the periodic table

Elements are inserted in stimulate on the regular table based on their atom number, how many protons they have. In a neutral atom, the variety of electrons will equal the number of protons, so we can conveniently determine electron number from atom number. In addition, the place of an element in the routine table—its column, or group, and also row, or period—provides valuable information around how those electrons room arranged.

If we think about just the very first three rows that the table, which incorporate the significant elements important to life, every row coincides to the pour it until it is full of a different electron shell: helium and hydrogen location their electron in the 1n shell, while second-row elements like Li begin filling the 2n shell, and also third-row aspects like Na continue with the 3n shell. Similarly, one element’s obelisk number offers information about its variety of valence electrons and also reactivity. In general, the number of valence electrons is the very same within a column and increases native left to ideal within a row. Team 1 aspects have simply one valence electron and also group 18 aspects have eight, other than for helium, which has actually only 2 electrons total. Thus, group number is a an excellent predictor of just how reactive each aspect will be:

Helium (He), neon (Ne), and also argon (Ar), as group 18 elements, have outer electron shells the are full or satisfy the octet rule. This provides them very stable as single atoms. Due to the fact that of their non-reactivity, castle are dubbed the inert gases or noble gases.Hydrogen (H), lithium (Li), and also sodium (Na), as team 1 elements, have just one electron in their outermost shells. They are unstable as single atoms, however can come to be stable by shedding or sharing their one valence electron. If these elements fully lose an electron—as Li and Na typically do—they end up being positively fee ions: Li+, Na+.Fluorine (F) and chlorine (Cl), as group 17 elements, have seven electrons in your outermost shells. They tend to attain a stable octet by acquisition an electron from various other atoms, ending up being negatively charged ions: F− and Cl−.Carbon (C), as a team 14 element, has four electrons in its external shell. Carbon commonly shares electron to attain a complete valence shell, forming bonds v multiple other atoms.

See more: A Toaster Dissipates 1500 Watts Of Power And Energy, What Do You

Thus, the columns that the regular table reflect the variety of electrons found in every element’s valence shell, which in turn determines how the aspect will react.