Not all aspects have sufficient electrons to to fill their outermost shells, yet an atom is in ~ its many stable when every one of the electron location in the outermost covering are filled. Because of these vacancies in the outermost shells, we check out the formation of chemical bonds, or interactions in between two or more of the exact same or different aspects that result in the formation of molecules. To attain greater stability, atom will often tend to completely fill their outer shells and will bond v other elements to attain this score by share electrons, accepting electron from one more atom, or donating electrons to another atom. Since the outermost shells the the elements with low atomic numbers (up to calcium, v atomic number 20) can hold eight electrons, this is referred to as the octet rule. An facet can donate, accept, or share electrons v other facets to to fill its outer shell and also satisfy the octet rule.

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An at an early stage model the the atom was emerged in 1913 by the Danish scientist Niels Bohr (1885–1962). The Bohr version shows the atom together a central nucleus comprise protons and also neutrons, through the electron in one electron shells at certain distances indigenous the nucleus, comparable to planets orbiting about the sun. Each electron shell has a different energy level, v those shells closest to the cell nucleus being reduced in power than those farther indigenous the nucleus. Through convention, each shell is assigned a number and the prize n—for example, the electron shell closest to the nucleus is referred to as 1n. In order come move between shells, one electron have to absorb or relax an amount of energy equivalent exactly come the difference in energy between the shells. Because that instance, if one electron absorbs energy from a photon, that may come to be excited and move to a higher-energy shell; conversely, when an excited electron drops ago down to a lower-energy shell, it will release energy, frequently in the kind of heat.


Bohr version of one atom, showing energy levels as concentric circles bordering the nucleus. Power must be added to move an electron exterior to a higher energy level, and energy is released when an electron falls down from a greater energy level to a closer-in one. Image credit: modified indigenous OpenStax Biology

Atoms, like various other things administer by the regulations of physics, often tend to take it on the lowest-energy, many stable configuration they can. Thus, the electron shells of an atom are inhabited from the inside out, v electrons pour it until it is full up the low-energy shells closer come the nucleus prior to they move right into the higher-energy shells additional out. The shell closest come the nucleus, 1n, deserve to hold 2 electrons, if the following shell, 2n, deserve to hold eight, and also the 3rd shell, 3n, can hold up to eighteen.

The number of electrons in the outermost shell of a specific atom determines its reactivity, or tendency to type chemical bonds with various other atoms. This outermost shell is well-known as the valence shell, and the electrons discovered in it room called valence electrons. In general, atoms are many stable, the very least reactive, when their outermost electron shell is full. Many of the elements important in biology need eight electrons in your outermost shell in bespeak to be stable, and also this preeminence of thumb is well-known as the octet rule. Part atoms can be secure with one octet also though your valence shell is the 3n shell, which deserve to hold as much as 18 electrons. Us will discover the factor for this as soon as we talk about electron orbitals below.

Examples of some neutral atoms and also their electron configuration are shown below. In this table, you deserve to see the helium has actually a complete valence shell, v two electron in its first and only, 1n, shell. Similarly, neon has actually a finish outer 2n shell containing eight electrons. These electron configurations make helium and also neon really stable. Although argon does not technically have actually a full outer shell, because the 3n shell deserve to hold approximately eighteen electrons, it is stable choose neon and also helium since it has eight electron in the 3n shell and also thus satisfies the octet rule. In contrast, chlorine has actually only 7 electrons in the outermost shell, while sodium has actually just one. These patterns carry out not fill the outermost shell or satisfy the octet rule, make chlorine and sodium reactive, eager to get or lose electrons to reach a an ext stable configuration.


Bohr diagrams suggest how numerous electrons to fill each major shell. Team 18 facets (helium, neon, and argon) have actually a full outer, or valence, shell. A full valence shell is the most stable electron configuration. Facets in other groups have partly filled valence shells and also gain or shed electrons to achieve a secure electron configuration.

Electron configurations and the periodic table

Elements are inserted in bespeak on the routine table based upon their atomic number, how many protons they have. In a neutral atom, the variety of electrons will certainly equal the number of protons, so we can easily determine electron number from atomic number. In addition, the place of an aspect in the routine table—its column, or group, and also row, or period—provides beneficial information about how those electrons space arranged.

If we take into consideration just the very first three rows of the table, which incorporate the significant elements necessary to life, every row synchronizes to the filling of a different electron shell: helium and also hydrogen location their electron in the 1n shell, when second-row elements like Li start filling the 2n shell, and third-row aspects like Na proceed with the 3n shell. Similarly, an element’s shaft number gives information around its number of valence electrons and also reactivity. In general, the variety of valence electrons is the very same within a column and increases native left to ideal within a row. Team 1 facets have just one valence electron and group 18 elements have eight, except for helium, which has actually only 2 electrons total. Thus, group number is a good predictor of just how reactive each facet will be:

Helium (He), neon (Ne), and argon (Ar), as team 18 elements, have actually outer electron shells the are complete or fulfill the octet rule. This provides them highly stable as single atoms. Due to the fact that of your non-reactivity, lock are referred to as the inert gases or noble gases.Hydrogen (H), lithium (Li), and sodium (Na), as group 1 elements, have just one electron in their outermost shells. They room unstable as single atoms, yet can end up being stable by losing or sharing your one valence electron. If these elements totally lose an electron—as Li and also Na typically do—they become positively charged ions: Li+, Na+.Fluorine (F) and also chlorine (Cl), as group 17 elements, have actually seven electrons in their outermost shells. They tend to attain a stable octet by taking an electron from various other atoms, becoming negatively fee ions: F− and Cl−.Carbon (C), together a group 14 element, has 4 electrons in its external shell. Carbon commonly shares electron to attain a complete valence shell, forming bonds v multiple various other atoms.

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Thus, the columns that the routine table reflect the variety of electrons uncovered in each element’s valence shell, which consequently determines how the facet will react.