How do covalent compounds form

The puppies represent atoms. The bones represent one of their electrons. Both puppies share both bones Fig. This is how hydrogen and oxygen share electrons; they each have an electron that they can share in a bond. This is a covalent bond , a bond in which atoms share electrons. Covalent bonding generally happens between nonmetals. Covalent bonding is the type of bond that holds together the atoms within a polyatomic ion.

It takes two electrons to make a covalent bond, one from each bonding atom. Lewis dot structures are one way to represent how atoms form covalent bonds. A table of Lewis dot symbols of nonmetal elements that form covalent bonds is shown in Fig.

There can be up to eight dots, for eight valence electrons. The first four electrons are placed as single electrons, then the remaining four are paired. The number of bonds that each element is able to form is usually equal to the number of unpaired electrons.

In order to form a covalent bond, each element has to share one unpaired electron. First, determine how many atoms of each element are needed to satisfy the octet rule for each atom.

In the formation of water, an oxygen atom has two unpaired electrons, and each hydrogen atom has one Fig. To fill its valence shell, oxygen needs two additional electrons, and hydrogen needs one. One oxygen atom can share its unpaired electrons with two hydrogen atoms, each of which need only one additional electron. The single electrons match up to make pairs Fig. The oxygen atom forms two bonds, one with each of two hydrogen atoms; therefore, the formula for water is H 2 O.

When an electron, or dot, from one element is paired with an electron, or dot, from another element, this makes a bond, which is represented by a line Fig. The number of bonds that an element can form is determined by the number of electrons in its valence shell Fig. Similarly, the number of electrons in the valence shell also determines ion formation.

The octet rule applies for covalent bonding, with a total of eight electrons the most desirable number of unshared or shared electrons in the outer valence shell. For example, carbon has an atomic number of six, with two electrons in shell 1 and four electrons in shell 2, its valence shell see Fig.

This means that carbon needs four electrons to achieve an octet. Carbon is represented with four unpaired electrons see Fig. If carbon can share four electrons with other atoms, its valence shell will be full.

Most elements involved in covalent bonding need eight electrons to have a complete valence shell. One notable exception is hydrogen H. Hydrogen can be considered to be in Group 1 or Group 17 because it has properties similar to both groups.

Hydrogen can participate in both ionic and covalent bonding. When participating in covalent bonding, hydrogen only needs two electrons to have a full valence shell. As it has only one electron to start with, it can only make one bond. Hydrogen is shown in Fig 2. In the formation of a covalent hydrogen molecule, therefore, each hydrogen atom forms a single bond, producing a molecule with the formula H 2. A single bond is defined as one covalent bond, or two shared electrons, between two atoms.

Ionic compounds consist of positively and negatively charged ions held together by strong electrostatic forces, whereas covalent compounds generally consist of molecules, which are groups of atoms in which one or more pairs of electrons are shared between bonded atoms. In a covalent bond, atoms are held together by the electrostatic attraction between the positively charged nuclei of the bonded atoms and the negatively charged electrons they share.

This chapter will focus on the properties of covalent compounds. Just as an atom is the simplest unit that has the fundamental chemical properties of an element, a molecule is the simplest unit that has the fundamental chemical properties of a covalent compound. Thus, the term molecular compound is used to describe elements that are covalently bonded and to distinguish the compounds from ionic compounds. Some pure elements exist as covalent molecules. Figure 4. For example, phosphorus exists as P4 tetrahedra—regular polyhedra with four triangular sides—with a phosphorus atom at each vertex.

Elemental sulfur consists of a puckered ring of eight sulfur atoms connected by single bonds. Selenium is not shown due to the complexity of its structure. Each covalent compound is represented by a molecular formula, which gives the atomic symbol for each component element, in a prescribed order, accompanied by a subscript indicating the number of atoms of that element in the molecule.

The subscript is written only if the number of atoms is greater than 1. For example, water, with two hydrogen atoms and one oxygen atom per molecule, is written as H 2 O.

Similarly, carbon dioxide, which contains one carbon atom and two oxygen atoms in each molecule, is written as C O 2. Covalent compounds that predominantly contain carbon and hydrogen are called organic compounds. The convention for representing the formulas of organic compounds is to write carbon first, followed by hydrogen and then any other elements in alphabetical order e. Compounds that consist primarily of elements other than carbon and hydrogen are called inorganic compounds ; they include both covalent and ionic compounds.

The convention for writing inorganic compounds, involves listing the component elements beginning with the one farthest to the left in the periodic table, as in CO 2 or SF 6.

Those in the same group are listed beginning with the lower element and working up, as in ClF. By convention, however, when an inorganic compound contains both hydrogen and an element from groups 13—15, hydrogen is usually listed last in the formula.

Examples are ammonia NH 3 and silane SiH 4. Compounds such as water, whose compositions were established long before this convention was adopted, are always written with hydrogen first: Water is always written as H 2 O, not OH 2. Typically this distinguishes when hydrogen is participating in a covalent bond rather than an ionic interaction, as seen in many of the inorganic acids, such as hydrochloric acid HCl and sulfuric acid H 2 SO 4 , as described in chapter 3.

Covalent molecules, on the otherhand, are typically composed of two nonmetals or a nonmetal and a metalloid. This is an initial screening method that you can use to categorize compounds into the ionic or the covalent cagetogy. Typically compounds that are formed from a combination of a metal with a nonmetal have more ionic bond character whereas compounds formed from two nonmetals or a metalloid and a nonmetal show more covalent character.

Although compounds usually lie on a spectrum somewhere between fully ionic and fully covalent character, for naming purposes, this guideline works well. Chapter 3 described how electrons can be transferred from one atom to another so that both atoms have an energy-stable outer electron shell following the octet rule. However, there is another way an atom can achieve a full valence shell: atoms can share electrons to reach the octet state or the duet state in the case of hydrogen.

This concept can be illustrated by using two hydrogen atoms, each of which has a single electron in its valence shell. For small atoms such as hydrogen atoms, the valence shell will be the first shell, which holds only two electrons. We can represent the two individual hydrogen atoms as follows:. In this situation neither hydrogen can reach the preferred duet state. In contrast, when two hydrogen atoms get close enough together to share their electrons, they can be represented as follows:.

By sharing their valence electrons, both hydrogen atoms now have two electrons in their respective valence shells. Because each valence shell is now filled, this arrangement is more stable than when the two atoms are separate. In this configuration, each hydrogen has an electron configuration equivalent to that of the noble gas, helium.

The sharing of electrons between atoms is called a covalent bond , and the two electrons that join atoms in a covalent bond are called a bonding pair of electrons. A discrete group of atoms connected by covalent bonds is called a molecule —the smallest part of a compound that retains the chemical identity of that compound.

For example, one molecule of water would contain two hydrogen atoms and one oxygen atom H 2 O. Chemists frequently use Lewis electron dot diagrams to represent covalent bonding in molecular substances. For example, the Lewis diagrams of two separate hydrogen atoms are as follows:. The Lewis diagram of two hydrogen atoms sharing electrons looks like this:. This depiction of molecules is simplified further by using a dash to represent a covalent bond.

The hydrogen molecule is then represented as follows:. Remember that the dash, also referred to as a single bond, represents a pair of bonding electrons. The bond in a hydrogen molecule, measured as the distance between the two nuclei, is about 7. This particular bond length represents a balance between several forces: 1 the attractions between oppositely charged electrons and nuclei, 2 the repulsion between two negatively charged electrons, and 3 the repulsion between two positively charged nuclei.

If the nuclei were closer together, they would repel each other more strongly; if the nuclei were farther apart, there would be less attraction between the positive and negative particles. Fluorine is another element whose atoms bond together in pairs to form diatomic two-atom molecules. Two separate fluorine atoms have the following electron dot diagrams:.

Each fluorine atom contributes one valence electron, making a single bond and giving each atom a complete valence shell, which fulfills the octet rule:. The circles show that each fluorine atom has eight electrons around it. As with hydrogen, we can represent the fluorine molecule with a dash in place of the bonding electrons:. Each fluorine atom has six electrons, or three pairs of electrons, that are not participating in the covalent bond.

Rather than being shared, they are considered to belong to a single atom. These are called nonbonding pairs or lone pairs of electrons. Now that we have looked at electron sharing between atoms of the same element, let us look at covalent bond formation between atoms of different elements. Consider a molecule composed of one hydrogen atom and one fluorine atom:.

Each atom needs one additional electron to complete its valence shell. By each contributing one electron, they make the following molecule:. In this molecule, the hydrogen atom does not have nonbonding electrons, while the fluorine atom has six nonbonding electrons three lone electron pairs.

The circles show how the valence electron shells are filled for both atoms recall that hydrogen is filled with two electrons. Larger molecules are constructed in a similar fashion, with some atoms participating in more than one covalent bond. For example, water, with two hydrogen atoms and one oxygen atom, and methane CH 4 , with one carbon atom and four hydrogen atoms, can be represented as follows:.

Atoms typically form a characteristic number of covalent bonds in compounds. Fig 4. Each family shows a representative lewis structure for that group of elements. For the nonmetals Families 4A, 5A, 6A, and 7A they can accept a complementary number of shared bonds to reach the octet state. Exceptions to the octet rule do exist. For example, hydrogen can be considered to be in Group 1 or Group 7A because it has properties similar to both groups.

Hydrogen can participate in either ionic or covalent bonding. When participating in covalent bonding, hydrogen only needs two electrons to have a full valence shell. As it has one electron to start with, it can only make one covalent bond. Similarly, boron has 3 electrons in its outer shell. As you can see from the picture above, Hydrogen gas has a total of 2 Hydrogen atoms. Each Hydrogen atom has 1 valence electron. Since Hydrogen can only fit a max of 2 valence electrons in its orbital, each Hydrogen atom only needs 1 electron.

Each atom has 1 valence electron, so they can just share, giving each atom two electrons each. Write the electron configuration and determine how many electrons are needed to achieve the nearest noble-gas configuration for the following:.

Introduction Only when two atoms of the same element form a covalent bond are the shared electrons actually shared equally between the atoms. Octet Rule The Octet Rule requires all atoms in a molecule to have 8 valence electrons--either by sharing, losing or gaining electrons--to become stable.

More examples can be found here. Single Bonds A single bond is when two electrons--one pair of electrons--are shared between two atoms.

Double Bonds A Double bond is when two atoms share two pairs of electrons with each other. Triple Bond A Triple bond is when three pairs of electrons are shared between two atoms in a molecule. Example 3: Acetylene Below is a Lewis dot structure of Acetylene demonstrating a triple bond. Example: Water, Sulfide, Ozone, etc. References Petrucci, Ralph H. New Jersey: Pearson Education, Inc. Vaczek, Louis. Pickering, H. Kotz, Treichel, Townsend.

Ohio: Cengage Learning, Lagowski, J. Bacskay, George G. Reimers, Jeffrey R. Outside Links Covalent Bond - Wikipedia: en. Problems 1. Write the electron configuration and determine how many electrons are needed to achieve the nearest noble-gas configuration for the following: Arsenic As Silicon Si Tellurium Te Solution: 3.

Which of the following statements are true? There can be more than one true statement. A covalent bond is the same as a ionic bond. The Octet rule only applys to molecules with covalent bonds. A molecule is polar if the shared electrons are equally shared. A molecule is nonpolar if the shared electrons are are equally shared. Methane gas CH 4 has a nonpolar covalent bond because it is a gas. Solution: Only d is true.