Name: valence electrons, lewis symbols, and bonding
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 36 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.1

Lewis Symbols and the Octet Rule

Chemical bonding involves the sharing or transfer of valence electrons between two or more atoms leading to the lowering of potential energy &#8212; a contributing factor for bond formation. 

The process of bond formation using valence electrons is explained by the Lewis model.

In the Lewis model, each valence electron of an atom from the s- or p-block is symbolized as a dot surrounding the abbreviation of the element, called a Lewis symbol. 

Oxygen, for example, has the electron configuration of 1s2, 2s2, 2p4 and thus six valence electrons. The first four dots are placed on each side of the oxygen symbol, one dot in each direction. 

The remaining two valence electrons are placed on two other sides around the atom. Each side is equivalent and can take a maximum of two electrons. 

The number of unpaired dots represents the number of bonds each atom of that element can form.

Elements of the last group, like neon, are noble gases and do not readily participate in chemical bonding. Noble gases have high ionization energies and are very stable given their electron configuration of a full outermost shell. 

The Lewis symbol for neon has eight dots, two dots on each side, representing the filled electron configuration; in other words, an octet. 

The octet rule states that an atom tends to lose, gain, or share electrons in the form of bonds until a stable electron configuration, an octet, is reached.

Consider carbon dioxide. Carbon has four unpaired electrons and oxygen has two. The number of unpaired electrons represents the number of electrons required to reach an octet. Hence, carbon shares two electrons with one oxygen, and the other two electrons with the other oxygen, so that each atom can achieve an octet. 

Hydrogen and helium are an exception to the octet rule since they only have one s-orbital which can hold just 2 electrons; so hydrogen or helium are said to achieve a duet.

lewis symbols and the octet rule

Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a model called the Lewis model that simplified the depiction of chemical bond formation and provided straightforward explanations for the chemical bonds seen in most common compounds. 

<h4>Lewis Model</h4>

The Lewis model depicts chemical bond formation by the sharing or transfer of valence electrons, which helps to attain a stable electron configuration. An ionic bond is formed when electrons are transferred between a metal and a nonmetal, whereas a covalent bond is formed when electrons are shared between two nonmetals.

The Lewis model is solely used to describe bond formations without taking into account the energy changes associated with the attractions and repulsions between electrons and nuclei on neighboring atoms. While these interactions are central to chemical bonding, the exact determination of the values is complex. Instead, Lewis designed special drawings to depict chemical bonds using valence electrons, called Lewis symbols. 

<h4>Lewis Symbols</h4>

Lewis symbols describe valence electron configurations of atoms and monatomic ions. A Lewis symbol consists of an elemental symbol surrounded by one dot for each of its valence electrons. For example, sodium has one valence electron; so one dot is drawn around the symbol Na. 

<img alt="Linear static equilibrium; &#931;Fx=0, MA=0; physics diagram; lever and fulcrum balance concept." src="/files/core/CDNSource/lessons/1055/1055_Image%201.png" style="height: 20px;" data-altupdatedat="1754904545000" data-alt="Static equilibrium diagram with force vectors &#931;Fx=0, &#931;Fy=0, showing forces on a beam." />

For main group elements, the number of valence electrons is indicated by a lettered group number in the periodic table. For example, lithium (Li) belongs to group IA and has one electron; beryllium (Be) is a group IIA element and has two valence electrons. 

There are exceptions to the Lewis model. In helium, the number valence electron is not the same as the group number. The transition metals, lanthanides, and actinides have incompletely filled inner shells; hence they cannot be written in simple Lewis dot symbols. 

<h4>The Octet Rule</h4>

The halogen molecules (F2, Br2, I2, and At2) form bonds like those in the chlorine molecule: one single bond between atoms and three lone pairs of electrons per atom. This allows each halogen atom to have a noble gas electron configuration. The tendency of s- or p-block atoms to form enough bonds to obtain eight valence electrons is known as the octet rule. The octet rule predicts the combinations of atoms that will have lower potential energy when they bond together.

The number of bonds that an atom can form can often be predicted from the number of electrons needed to reach an octet (eight valence electrons); this is especially true of the nonmetals of the second period of the periodic table (C, N, O, and F). 

<ul>
	<li>Group 14 elements have four electrons in their outermost shell and therefore require four more electrons to reach an octet. These four electrons can be gained by forming four covalent bonds, like carbon in CH4 (methane) and silicon in SiH4 (silane). </li>
	<li>Group 15 elements such as nitrogen have five valence electrons in the atomic Lewis symbol: one lone pair and three unpaired electrons. These atoms form three covalent bonds, as in NH3 (ammonia). </li>
	<li>Group 16 elements such as oxygen and other atoms obtain an octet by forming two covalent bonds - like bonding with two hydrogen atoms in H2O (water).</li>
</ul>

There are exceptions to the octet rule. Because hydrogen only needs two electrons to fill its valence shell, it is an exception to the octet rule. In this case, hydrogen is said to have reached a duet. The transition elements and inner transition elements also do not follow the octet rule.

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/7-3-lewis-symbols-and-structures">Openstax, Chemistry 2e, Chapter 7.3: Lewis Symbols and Structures</a>.

Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a ...

Education

JoVE Core

Chemical Bonding: Basic Concepts

KEY TERMS AND CONCEPTS

valence electrons, lewis symbols, and bonding

Chemical bonds form when atoms or ions interact in ways that lower the potential energy of a molecule. Gilbert N. Lewis created a simple model to show bond formation using valence electrons, the electrons in the outer shell. In this model, electrons are shared or transferred to help atoms reach a stable electron configuration.

The Lewis model helps describe two common bond types. An ionic bond forms when electrons move from a metal to a nonmetal. A covalent bond forms when two nonmetals share electrons. The model does not include the detailed energy changes from attractions and repulsions between electrons and nuclei on nearby atoms.

Lewis symbols are special drawings that show valence electrons for atoms and monatomic ions. A Lewis symbol uses the element symbol with one dot for each valence electron. For example, sodium is written with one dot because it has one valence electron. For many main group elements, the group number on the periodic table matches the number of valence electrons, such as lithium in group IA and beryllium in group IIA.

There are exceptions to this pattern. Helium does not match its group number in a simple Lewis symbol. Transition metals, lanthanides, and actinides also do not fit the Lewis symbol pattern well because they have incompletely filled inner shells. They cannot be shown with simple Lewis dot symbols.

The octet rule helps predict how many bonds many s- and p-block atoms will form. These atoms often form enough bonds to reach eight valence electrons, which gives them a noble gas electron configuration. This idea is especially useful for second-period nonmetals such as carbon, nitrogen, oxygen, and fluorine. It can help predict which combinations of atoms are likely to have lower potential energy when they bond.

Group 14 elements have four valence electrons and need four more to reach an octet. They can do this by forming four covalent bonds, as carbon does in methane, CH4, and silicon does in silane, SiH4. Group 15 elements such as nitrogen have five valence electrons, shown as one lone pair and three unpaired electrons, so they usually form three covalent bonds, as in ammonia, NH3. Group 16 elements such as oxygen form two covalent bonds to complete an octet, as in water, H2O.

Not every atom follows the octet rule. Hydrogen is an exception because it only needs two electrons in its valence shell. When hydrogen reaches this state, it has a duet, not an octet. Transition elements and inner transition elements also do not follow the octet rule.

Valence Electrons, Lewis Symbols, and Bonding

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types of chemical bonds

Chemical Bonding and Bond Properties

Chemical bonding explains how atoms and ions join to form molecules and compounds. American chemist Gilbert N. Lewis pioneered chemical bonding theory with the Lewis model. This model helps explain why bonds form, why some are stronger than others, and why compounds have fixed formulas like H2O instead of H4O. Ionic bonding involves ions, which are atoms or molecules with an electrical charge. A cation is a positive ion formed when a neutral atom loses one or more electrons from its valence...

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Predicting Ionic Stability with Lattice Energy

Lattice energy helps explain why ionic compounds are stable. It measures the strength of attraction between positive and negative ions in a crystal. In one common definition, lattice energy is the energy required to separate one mole of an ionic solid into gaseous ions, so it has a positive value. Another definition uses the opposite sign and describes the energy released when gaseous ions form a lattice. When checking data, it is important to confirm which convention is being used. In either...

Chemical bonds form when atoms or ions interact in ways that lower the potential energy of a molecule. Gilbert N. Lewis created a simple model to show bond formation using valence electrons, the electrons in the outer shell. In this model, electrons are shared or transferred to help atoms reach a stable electron configuration. The Lewis model helps describe two common bond types. An ionic bond forms when electrons move from a metal to a nonmetal. A covalent bond forms when two nonmetals share...

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Electronegativity helps explain why some covalent bonds are nonpolar and others are polar covalent. It is a measure of how strongly an atom attracts shared electrons toward itself. That pull controls how electron density is distributed in a bond. Linus Pauling proposed the first electronegativity values by studying bond energies in heteronuclear molecules such as hydrogen fluoride. He expected the bond energy of HF to be about the average of the bond energies of H2 and F2, or 296 kJ/mol. The...

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Ionic Compounds and Crystal Lattices

Ionic compounds form when positive and negative ions attract each other. A cation is a positive ion, and it forms when an atom loses one or more valence electrons. An anion is a negative ion, and it forms when an atom gains one or more valence electrons. Compounds made of these ions are called ionic compounds, or salts, and the force between them is an ionic bond. The properties of ionic solids help show how strong this attraction is. They usually form rigid, brittle crystals and have high...

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Lattice Energy Patterns in Ionic Solids

Lattice energy shows how strongly ions hold an ionic solid together. It is the energy required to separate one mole of a solid into its gaseous ions. For sodium chloride, this process requires 769 kJ per mole. The opposite convention may also be used, where lattice energy is the energy released when gaseous ions form the solid. In either case, a larger magnitude means a more stable ionic compound. For ionic crystals, lattice energy follows a pattern based on charge and ion size. The...

covalent bonding and lewis structures

Shared Electrons in Covalent Molecules

Covalent molecules form when atoms share electrons instead of transferring them. This bonding usually happens between nonmetal atoms with similar tendencies to attract electrons. Because the atoms pull on the electrons in a shared way, the bond is held together by mutual attraction. Covalent compounds often have physical properties that differ from ionic compounds. The attraction between neutral molecules is weaker than the attraction between charged ions. As a result, many covalent compounds...

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Bond polarity depends on electronegativity, the ability of an atom to attract shared electrons. The absolute value of the electronegativity difference, or ΔEN, gives a rough guide to bond type. When the difference is very small or zero, the bond is covalent and nonpolar. When it is larger, the bond may be polar covalent or ionic. Examples show how this works in real compounds. H–H has a ΔEN of 0, so it is nonpolar. H–Cl has a ΔEN of 0.9, so it is polar covalent. Na–Cl has a ΔEN of 2.1, so it...

lewis structures of molecular compounds and polyatomic ions

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Drawing Lewis Structures with Valence Electron Counts

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Resonance Forms in Lewis Structures

Resonance forms in Lewis structures help explain molecules and ions that cannot be shown correctly with just one drawing. In nitrite ion, NO2−, the Lewis structure can be drawn in two ways. The N-O and N=O bonds switch positions in the two drawings, but the atoms stay in the same arrangement. If nitrite really had one single bond and one double bond, the two bond lengths should be different. A double bond is shorter and stronger than a single bond between the same atoms. Experiments show that...

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Lewis Structures with Formal Charge

Lewis structures can sometimes show more than one reasonable arrangement for a molecule or polyatomic ion. Formal charge helps chemists choose the best Lewis structure when that happens. It is a useful way to compare possible structures in chemistry. Formal charge is the hypothetical charge an atom would have if bonding electrons were shared evenly between the atoms. It can also be found by starting with the valence electrons of the neutral atom, subtracting the nonbonding electrons, and then...

exceptions to the octet rule

Octet Rule Exceptions in Lewis Structures

Some covalent molecules do not follow the octet rule in their Lewis structures. Their central atoms can fall into three groups: odd-electron molecules, electron-deficient molecules, and hypervalent molecules. These cases show why a Lewis structure may not always give every atom eight valence electrons. Odd-electron molecules have an odd number of valence electrons, so one electron remains unpaired. Molecules with an odd number of electrons are called radicals. Nitric oxide, NO, is one example,...

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bonding in metals

Electron Sea Model in Metals

Electron sea model explains how metallic bonding holds metals together. In this model, most metal atoms do not have enough valence electrons to form ionic or covalent bonds in the usual way. Their valence electrons are held loosely because metals have low electronegativity, and their ionization energy is low, so electrons are removed easily. When a metal atom loses these outer electrons, it becomes a positively charged metal ion. The freed electrons form a delocalized electron cloud around...

key terms and concepts

Learn how valence electrons, Lewis symbols, and the octet rule help predict ionic and covalent bonding in common high school chemistry examples.

Valence Electrons, Lewis Symbols, and Bonding | JoVE K12

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Valence Electrons, Lewis Symbols, and Bonding

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