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Learning Objectives
- Determine the molecular mass of a molecule.
- Predict the general shape of a simple covalent molecule.
- Predict the polarity of the molecule.
- Compare the properties of ionic and molecular compounds.
Unlike the ions in ionic compounds, which are arranged in lattices called crystals, molecules of covalent compounds exist as discrete units with a characteristic mass and a certain three-dimensional shape.
Molecular Mass
The mass of a molecule—the molecular mass (sometimes called the molecular weight)—is simply the sum of the masses of its atoms. As with formula masses, it is important that you keep track of the number of atoms of each element in the molecular formula to obtain the correct molecular mass.
Example \(\PageIndex{1}\)
What is the molecular mass of each covalent compound?
- H2O
- C6H6
- NO2
- N2O5
Solution
Use the atomic masses from the Periodic Table in Section 2.7.
The molecular formula H2O indicates that there are two hydrogen atoms and one oxygen atom in each molecule. Summing the masses of these atoms,
| 2 H: | 2 × 1.01 = | 2.02 amu |
|---|---|---|
| 1 O: | + 16.00 amu | |
| Total: | 18.02 amu |
The molecular mass of H2O is 18.02 amu.
| 6 C: | 6 × 12.01 = | 72.06 amu |
|---|---|---|
| 6 H: | 6 × 1.01 = | + 6.06 amu |
| Total: | 78.12 amu |
The molecular mass of C6H6 is 78.12 amu.
| 1 N: | 14.01 amu | |
|---|---|---|
| 2 O: | 2 × 16.00 = | + 32.00 amu |
| Total: | 46.01 amu |
The molecular mass of NO2 is 46.01 amu.
| 2 N: | 2 × 14.01 = | 28.02 amu |
|---|---|---|
| 5 O: | 5 × 16.00 = | + 80.00 amu |
| Total: | 108.02 amu |
The molecular mass of N2O5 is 108.02 amu.
Note that the two different nitrogen and oxygen compounds in these examples have different molecular masses.
Exercise \(\PageIndex{1}\)
What is the molecular mass of each covalent compound?
- C2H2
- CO
- CO2
- BF3
- Answer a:
-
26.04 amu
- Answer b:
-
28.01 amu
- Answer c:
-
44.01 amu
- Answer d:
-
67.81 amu
Molecular Shape: VSEPR Theory
Unlike ionic compounds, with their extended crystal lattices, covalent molecules are discrete units with specific three-dimensional shapes. The shape of a molecule is determined by the fact that covalent bonds, which are composed of negatively charged electrons, tend to repel one another. This concept is called the valence shell electron pair repulsion (VSEPR) theory. For example, the two covalent bonds in \(\ce{BeCl2}\) stay as far from each other as possible, ending up 180° apart from each other. The result is a linear molecule:
The three covalent bonds in BF3 repel each other to form 120° angles in a plane, in a shape called trigonal planar:
The molecules \(\ce{BeCl2}\) and \(\ce{BF3}\) actually violate the octet rule; however, such exceptions are rare and will not be discussed in this text.
Try sticking three toothpicks into a marshmallow or a gumdrop and see if you can find different positions where your “bonds” are farther apart than the planar 120° orientation.
The four covalent bonds in CCl4 arrange themselves three dimensionally, pointing toward the corner of a tetrahedron and making bond angles of 109.5°. CCl4 is said to have a tetrahedral shape:
| Atoms Around Central Atom | Geometry | Example |
|---|---|---|
| 2 \(\ce{AB_2}\) | Linear | \(\ce{BeCl_2}\) |
| 3 \(\ce{AB_3}\) | Trigonal Planar | \(\ce{BF_3}\) |
| 4 \(\ce{AB_4}\) | Tetrahedral | \(\ce{CCl_4}\) |
In determining the shapes of molecules, it is useful to first determine the Lewis diagram for a molecule. The shapes of molecules with multiple bonds are determined by treating the multiple bonds as one bond. Thus, formaldehyde (CH2O) has a shape similar to that of BF3. It is trigonal planar.
Molecules With Lone Pairs Around Central Atom
Molecules with lone electron pairs around the central atom have a shape based on the position of the atoms, not the electron pairs. For example, NH3 has one lone electron pair and three bonded electron pairs. These four electron pairs repel each other and adopt a tetrahedral arrangement. However, the shape of the molecule is described in terms of the positions of the atoms, not the lone electron pairs. Thus, NH3 is said to have a trigonal pyramidal shape, not a tetrahedral one.
Similarly, H2O has two lone pairs of electrons around the central oxygen atom and two bonded electron pairs. Although the four electron pairs adopt a tetrahedral arrangement, the shape of the molecule is described by the positions of the atoms only. The shape of H2O is bent with an approximate 109.5° angle.
In summary, to determine the molecular geometry:
Step 1: Draw the Lewis structure.
Step 2: Count the number of bonds (a double/triple bond counts as one) and lone pairs around the central atom.
Step 3: Use Table 4.5.1 to determine the molecular geometry.
Table 4.5.1: The molecular geometry depends on the number of bonds and lone pairs around the central atom, A.
Example \(\PageIndex{1}\)
What is the geometry of the ammonium ion, NH4+? Its Lewis structure is shown below. How is this different from ammonia, NH3?
Solution
In ammonium ion, the central atom N has 4 bonds and no lone pair. It is equivalent to the below in Table 4.5.1. Hence, this is tetrahedral.
In ammonia (NH3), shown below, N has 3 bonds and one lone pair.
It is equivalent to the below in Table 4.5.1. Hence, the shape of this molecule is trigonal pyramid.
Exercise \(\PageIndex{1}\)
What is the molecular shape of nitrosyl chloride, a highly corrosive, reddish-orange gas? Its Lewis structure is shown below.
- Answer
-
Focus on the central atom, N. It has a double bond to O, count this as one bond. It also has a single bond to Cl. Thus, N has 2 bonds and one lone pair. These 3 electron pairs will spread out 120 degrees from each other. But, since the shape is defined by the arrangement of the atoms only, the shape is bent or angular. If you consult Table 4.5.1, this molecule is equivalent to the below. Hence, two bonds and one lone pair has a bent or angular shape.
Molecular Polarity
In general, a molecule is nonpolar if all its bonds are nonpolar. Examples are I2, O2, H2, CH4, C2H6 and C3H8.
In general, a molecule is polar if it contains polar bonds EXCEPT when the bond polarities cancel each other. As mentioned in Section 4.4, the shape of the CO2 molecule (linear) orients the two C=O polar bonds directly opposite each other, thus cancelling each other's effect. Carbon dioxide (CO2) is a nonpolar molecule.
On the other hand, water (as discussed above) is a bent molecule because of the two lone pairs on the central oxygen atom. Because of the bent shape, the dipoles do not cancel each other out and the water molecule is polar. In the figure below, the individual H-O polar bonds represented by the two red arrows are not directly opposite each other. These two dipoles don't cancel each other out. In fact, the net dipole (blue arrow) points upward. There is a resultant partial positive charge at one end (between the two H atoms) and a partial negative charge on the other end (where O is located). The uneven distribution of charge or the overall dipole is shown by the blue arrow below (Figure 4.5.1). Hence, water is polar (has + and - poles) while carbon dioxide is nonpolar.
Similarly, in BF3 (trigonal planar), the effect of a B-F bond is cancelled by the sum of the other two B-F bonds (see video). Hence, a trigonal planar molecule (BF3) is nonpolar because the bond polarities cancel each other, but a trigonal pyramidal molecule (NH3) is polar.
Some other molecules are shown in the figure below. Notice that a tetrahedral molecule such as CCl4 is nonpolar. However, if the peripheral atoms are not of the same electronegativity, the bond polarities don't cancel and the molecule becomes polar, as in CH3Cl.
Physical Properties of Molecular Compounds
The physical state and properties of a particular compound depend in large part on the type of chemical bonding it displays. Molecular compounds, sometimes called covalent compounds, display a wide range of physical properties due to the different types of intermolecular attractions such as different kinds of polar interactions. The melting and boiling points of molecular compounds are generally quite low compared to those of ionic compounds. This is because the energy required to disrupt the intermolecular forces (discussed in Section 8.1) between molecules is far less than the energy required to break the ionic bonds in a crystalline ionic compound. Since molecular compounds are composed of neutral molecules, their electrical conductivity is generally quite poor, whether in the solid or liquid state. Ionic compounds do not conduct electricity in the solid state because of their rigid structure, but conduct well when either molten or dissolved into a solution. The water solubility of molecular compounds is variable and depends primarily on the type of intermolecular forces involved. Substances that exhibit hydrogen bonding or dipole-dipole forces are generally water soluble, whereas those that exhibit only London dispersion forces are generally insoluble. Most, but not all, ionic compounds are quite soluble in water. The table below summarizes some of the differences between ionic and molecular compounds.
| Property | Ionic Compounds | Molecular Compounds |
|---|---|---|
| Type of elements | Metal and nonmetal | Nonmetals only |
| Bonding | Ionic - transfer of electron(s) between atoms | Covalent - sharing of pair(s) of electrons between atoms |
| Representative unit | Formula unit | Molecule |
| Physical state at room temperature | Solid | Gas, liquid, or solid |
| Water solubility | Usually high | Variable |
| Melting and boiling temperatures | Generally high | Generally low |
| Electrical conductivity | Good when molten or in solution | Poor |
In summary, covalent compounds are softer, have lower boiling and melting points, are more flammable, are less soluble in water and do not conduct electricity compared to ionic compounds. The individual melting and boiling points, solubility and other physical properties of molecular compounds depend on molecular polarity.
Example \(\PageIndex{2}\)
Describe the shape of each molecule. Is it polar or nonpolar?
- PCl3
- CO2
Solution
- The Lewis diagram for PCl3 is as follows:
Focus on the central atom, P that has 3 bonds and one lone pair. The four electron pairs arrange themselves tetrahedrally, but the lone electron pair is not considered in describing the molecular shape. Like NH3, this molecule is pyramidal. The 3 P-Cl bonds don't cancel each other. This is polar.
- The Lewis diagram for CO2 is as follows:
Focus on the central atom, C. The multiple bonds are treated as one group, hence C has 2 bonds and zero lone pair. CO2 has only two groups of electrons that repel each other. They will direct themselves 180° apart from each other, so CO2 molecules are linear. This is highly symmetrical, with the two opposite dipoles cancelling each other. The CO2 molecule is nonpolar.
Exercise \(\PageIndex{2}\)
Describe the shape of each molecule. Is it polar or nonpolar?
- CBr4
- BCl3
- Answer a:
- or
The Lewis structure shows 4 groups attached to the central atom, hence tetrahedral. All the 4 groups are identical and the shape is symmetrical. Hence, it is nonpolar.
- Answer b:
-
The Lewis diagram shows 3 groups attached to the central atom, hence trigonal planar. All the 3 groups are identical and shape is symmetrical, hence, it is nonpolar.
(Video) 4.5 Dendritic spines
or 
Key Takeaways
- A molecule has a certain mass, called the molecular mass.
- Simple molecules have geometries that can be determined from VSEPR theory.
- Polar molecules result from differences in electronegativity of the atoms in the molecule.
- Dipoles that are directly opposite one another cancel each other out.
FAQs
What are 4 characteristics of molecules? ›
- Molecules are extremely small in size. They cannot be observed with naked eyes.
- Molecules have space between them. The space between the molecules of a matter is called intermolecular space.
- The molecule particles are in a state of continuous motion.
Three important characteristics of a molecule are: Molecules have definite mass and size. A molecule is formed when two or more atoms of the same or different elements combine in a fixed ratio. When a molecule is broken then the properties of that substance are lost.
What are 5 examples of molecules? ›- H2O (water)
- N2 (nitrogen)
- O3 (ozone)
- CaO (calcium oxide)
- C6H12O6 (glucose, a type of sugar)
- NaCl (table salt)
General physical properties that can be explained by the covalent bonding model include boiling and melting points, electrical conductivity, bond strength, and bond length.
What are 4 characteristic properties? ›Examples of characteristic properties include melting points, boiling points, density, viscosity, solubility, crystal shape, and color.
What are the 4 types of molecules? ›There are four major classes of biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids), and each is an important component of the cell and performs a wide array of functions. Combined, these molecules make up the majority of a cell's mass.
What are 3 characteristics of molecules? ›- They are very small in size.
- They have space between them.
- They are in constant random motion.
What are the properties of molecular compounds? Molecular compounds tend to be gases and liquids at room temperature; and those that are solids are soft and pliable. They have low melting and boiling points, low solubility, and are not good conductors of electricity.
What is the characteristic definition of molecules? ›molecule, a group of two or more atoms that form the smallest identifiable unit into which a pure substance can be divided and still retain the composition and chemical properties of that substance.
What are the 3 main types of molecules? ›Molecules are of three types: Molecule of an atom, Molecule of an element and Molecule of a compound.
What are the 3 main molecules? ›
Proteins, carbohydrates, nucleic acids, and lipids are the four major classes of biological macromolecules—large molecules necessary for life that are built from smaller organic molecules.
What are 4 types of bonding? ›There are four types of chemical bonds essential for life to exist: Ionic Bonds, Covalent Bonds, Hydrogen Bonds, and van der Waals interactions. We need all of these different kinds of bonds to play various roles in biochemical interactions.
What are the 4 main properties of ionic bonds? ›- They are crystalline solids, and are brittle in nature.
- They have high melting and boiling points.
- They are soluble in water.
- They conduct electricity in their solution and molten states.
- The covalent compounds exist as gases or liquids or soft solids.
- The melting and boiling points of covalent compounds are generally low.
- Covalent compound are insoluble in water but dissolve in organic solvents.
- They are non-conductors of electricity in solid, molten or aqueous state.
The five broad personality traits described by the theory are extraversion (also often spelled extroversion), agreeableness, openness, conscientiousness, and neuroticism. The five basic personality traits is a theory developed in 1949 by D. W.
What are characteristics examples? ›Characteristics are the distinguishing features or quality of something. You might like to think of characteristics as those qualities that make a person or a thing different from others. For example, the ability to camouflage is a characteristic of the chameleon.
What are the 7 characteristics? ›The seven characteristics what makes an organism living are: Environmental responses, cells, change and growth, reproduction, having complex chemistry, and homeostasis and energy processing.
What are the 4 main molecules of life? ›- Carbohydrates.
- Lipids.
- Nucleic acids.
- Proteins.
- Each of these exists as a polymer, composed of the monomers shown in the table. ...
- monosaccharide, disaccharides, and polysaccharides; quick energy for the cell. ...
- and a little O. ...
- sometimes S.
- Proteins: Molecular Machines. ...
- Nucleic Acids: Information Repositories. ...
- Lipids: Waterproof Membranes. ...
- Carbohydrates: Stored Energy.
What are two characteristics of molecules of matter? ›
The properties of molecules of a matter are: Particles of matter have space between them. that is they have inter-particle space between them. Particles of matter are continuously moving.
What are 3 examples of simple molecules? ›Hydrogen, ammonia, methane and pure water are also simple molecules. All have strong covalent bonds between their atoms, but much weaker intermolecular forces between molecules.
What are characteristics of simple molecules? ›Simple molecules have no overall charge , or charged particles that can separate, so simple molecular substances cannot conduct electricity, even when liquid or dissolved in water. When simple molecular substances melt or boil, their weak intermolecular forces are overcome, not the strong covalent bonds.
What are the important properties of molecules? ›- They are very small in size.
- They have spaces between them.
- They are in constant motion and possess kinetic energy.
The three main properties of a molecular substance include: low melting points, or a temperature that indicates when a solid substance changes to a liquid; and boiling points, or a temperature that indicates the point at which a liquid changes to a gas, or vapor; poor conductivity; and low solubility, which is a ...
What is a molecule answer? ›Molecules are made up of one or more atoms. If they contain more than one atom, the atoms can be the same (an oxygen molecule has two oxygen atoms) or different (a water molecule has two hydrogen atoms and one oxygen atom). Biological molecules, such as proteins and DNA, can be made up of many thousands of atoms.
What is the general characteristics of each molecule? ›All molecules have a definite mass and size that are dependent on the atoms from which the molecule is made. The mass is equal to the sum of the masses of all the individual atoms in the molecular structure.
What characteristics of a molecule determine its function? ›The molecule's shape and chemical properties facilitate interactions with other molecules and determine its role in the cell.
What are the 4 characteristics of atoms? ›1) they are natural (not artificial) substances. 2) they are solid. 3) they form by inorganic processes. 4) they have a specific chemical composition.
What are the 4 simple molecules of life? ›- Carbohydrates.
- Lipids.
- Nucleic acids.
- Proteins.
What are the 4 molecules essential to life? ›
These four essential molecules are classified as proteins, nucleic acids, carbohydrates and lipids. They provide structure and energy to the cells of all living organisms. In addition, these molecules are necessary for the various functions in cells, therefore, they are referred to as fundamental molecules of life.
What are molecules with 4 atoms called? ›The molecule of an element which have more than three atoms is called polyatomic molecule. The example of polyatomic molecule is phosphorus (P4) which have four atoms are called tetra-atomic. Therefore, the molecules of an element, which have more than three atoms, are called polyatomic molecules.
What are the 5 properties of an atom? ›- Electronic configuration: ...
- Atomic Radii: ...
- Ionization Enthalpy: ...
- Electronegativity: ...
- Density: Initially there is gradual increase Cgraphite to Si followed by rapid increase for higher members.
In atoms, there are a total of four quantum numbers: the principal quantum number (n), the orbital angular momentum quantum number (l), the magnetic quantum number (ml), and the electron spin quantum number (ms).
What are the 4 types of molecules important for cell processes? ›It is, however, the organic molecules that are the unique constituents of cells. Most of these organic compounds belong to one of four classes of molecules: carbohydrates, lipids, proteins, and nucleic acids.
What are the types of molecules? ›Molecules are of three types: Molecule of an atom, Molecule of an element and Molecule of a compound.
What are 3 facts about molecules? ›Atoms are the basic or fundamental units of matter that rarely exist independently but combine to form different substances. Whenever two or more atoms combine, they form a molecule. It is the smallest unit of a chemical substance having all the properties of that substance. Molecules are neutral and carry no charge.
What are molecules question and answer? ›Answer: Solution : The smallest unit of matter which can exist independently is called a molecule.
What are the 4 most important elements in living things? ›Scientists believe that about 25 of the known elements are essential to life. Just four of these – carbon (C), oxygen (O), hydrogen (H) and nitrogen (N) – make up about 96% of the human body.
What 3 elements are present in all 4 molecules of life? ›The four main classes of organic compounds (carbohydrates, lipids, proteins, and nucleic acids) that are essential to the proper functioning of all living things are known as polymers or macromolecules. All of these compounds are built primarily of carbon, hydrogen, and oxygen but in different ratios.
What are molecules made of? ›
Molecules are made up of one or more atoms. If they contain more than one atom, the atoms can be the same (an oxygen molecule has two oxygen atoms) or different (a water molecule has two hydrogen atoms and one oxygen atom). Biological molecules, such as proteins and DNA, can be made up of many thousands of atoms.