Chapter 6 Covalent Compounds Section 1 Covalent Bonds

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds Sharing Electrons You learned that electrons are rearranged when an ionic bond forms. When this happens, electrons transfer from one atom to another to form charged ions. Covalent bonds are different because electrons are not transferred from one atom to another, they are shared between two atoms. Summary: Ionic Bonds: a metal will lose an electron and a nonmetal will gain an electron forming ions that are ionicly bonded. Covalent Bonds: atoms will share a pair of electrons between them forming a covalent bond.

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds Atoms Share Electrons Equally or Unequally Your reference table lists the electronegativity values for several elements. In a molecule such as H 2, the values of the two atoms in the bond are equal. Because each one attracts the bonding electrons with the same force, they share the electrons equally. A non polar covalent bond is a covalent bond in which the bonding electrons are shared equally. If the values differ significantly, the two atoms form a different type of covalent bond. Think about a carbon atom bonding with an oxygen atom. The O atom has a higher electronegativity and attracts the bonding electrons more than the C atom does. As a result, the two atoms share the bonding electrons, but unequally. This type of bond is a polar covalent bond. In a polar covalent bond, the shared electrons are more likely to be found nearer to the atom whose electronegativity is higher. If the difference in electronegativity values of the two atoms is great enough, the atom with the higher value may remove an electron from the other atom. An ionic bond will form.

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds Polar Molecules Have Positive and Negative Ends The difference between the electronegativity values of hydrogen and fluorine shows that H and F atoms form a polar covalent bond. The word polar suggests that this bond has ends that are in some way opposite one another, like the two poles of a planet, a magnet, or a battery. In fact, the ends of the HF molecule have opposite partial charges. The electronegativity of fluorine (4. 0) is much higher than that of hydrogen (2. 2). Therefore, the shared electrons are more likely to be found nearer to the fluorine atom. For this reason, the fluorine atom in the HF molecule has a partial negative charge. In contrast, the shared electrons are less likely to be found nearer to the hydrogen atom. As a result, the hydrogen atom in the HF molecule has a partial positive charge. A molecule in which one end has a partial positive charge and the other end has a partial negative charge is called a dipole The HF molecule is a dipole.

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds To emphasize the dipole nature of the HF molecule, the formula can be written as H + F −. The symbol is a lowercase Greek delta, which is used in science and math to mean partial. With polar molecules, such as HF, the symbol + is used to show a partial positive charge on one end of the molecule. Likewise, the symbol − is used to show a partial negative charge on the other end. Although + means a positive charge, and − means a negative charge, these symbols do not mean that the bond between hydrogen and fluorine is ionic. An electron is not transferred completely from hydrogen to fluorine, as in an ionic bond. Instead, the atoms share a pair of electrons, which makes the bond covalent. However, the shared pair of electrons is more likely to be found nearer to the fluorine atom. This unequal distribution of charge makes the bond polar covalent.

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds + H-F -

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds Can Be Classified by Bond Character The diagram below shows the relationship between electronegativity differences and the type of bond that forms between two elements. Notice the general rule that can be used to predict the type of bond that forms. If the difference in electronegativity is between 0 and 0. 5, the bond is probably nonpolar covalent. If the difference in electronegativity is between 0. 5 and 2. 1, the bond is considered polar covalent. If the difference is larger than 2. 1, then the bond is usually ionic. Remember that this method of classifying bonds is just one model. Another general rule states that covalent bonds tend to form between nonmetals, while a nonmetal and a metal will form an ionic bond.

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds Properties of Substances Depend on Bond Type The type of bond that forms determines the physical and chemical properties of the substance. For example, metals, such as potassium, are very good electric conductors in the solid state. This property is the result of metallic bonding. Metallic bonds are the result of the attraction between the electrons in the outermost energy level of each metal atom and all of the other atoms in the solid metal. The metal atoms are held in the solid because all of the valence electrons are attracted to all of the atoms in the solid. These valence electrons can move easily from one atom to another. They are free to roam around in the solid and can conduct an electric current. These mobile electrons are sometime referred to as a “sea of mobile electrons”

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds In ionic substances, the overall attraction between all the cations and anions is very strong. Ionic compounds, such as potassium chloride, KCl, are made up of many K+ and Cl− ions. Each ion is held into place by many oppositely charged neighbors, so the forces—the ionic bonds—that hold them together are very strong and hard to break. In molecular substances, such as Cl 2, the molecules are held together by sharing electrons. The shared electrons are attracted to the two bonding atoms, and they have little attraction for the atoms of other nearby molecules. Therefore, the attractive forces between separate Cl 2 molecules are very small compared to the attractive forces between the ions in KCl.

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds Energy in Bonding Energy is absorbed when a bond is broken. Energy is released when a bond is formed.

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds Section Summary Ionic Bonds form between metals and nonmetals with a transfer of electrons from the metal to the nonmetal forming ions. These ions highly attract ions of opposite charge around them making ionic bond strong. Covalent bonds form between nonmetals with nonmetals when there is not enough electronegativity difference to allow a transfer of electrons. Because electrons can not be transferred, they are shared. If the electrons are shared Equally the bond is nonpolar, if the are shared unequally they are called polar. Metallic bonds form between metals with other metals. The valence electrons in metals are loosely held and tend to be shared with metal atom that are nearby. This sea of mobile electrons is stronger than covalent bonds and is why metals conduct heat and electricity well.

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds Two atoms with an electronegativity difference of 0. 4 form a bond that is 1. ionic, because electrons are shared 2. ionic, because electrons are transferred 3. covalent, because electrons are shared 4. covalent, because electrons are transferred Which factor distinguishes a metallic bond from an ionic bond or a covalent bond? 1. the mobility of electrons 2. the mobility of protons 3. the equal sharing of electrons 4. the unequal sharing of electrons A white crystalline salt conducts electricity when it is melted and when it is dissolved in water. Which type of bond does this salt contain? 1. ionic 2. metallic 3. covalent 4. network

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds In a nonpolar covalent bond, electrons are 1. located in a mobile "sea" shared by many ions 2. transferred from one atom to another 3. shared equally by two atoms 4. shared unequally by two atoms Which diagram best represents a polar molecule?

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds The ability to conduct electricity in the solid state is a characteristic of metallic bonding. This characteristic is best explained by the presence of 1. high ionization energies 2. high electronegativities 3. mobile electrons 4. mobile protons Which structural formula represents a nonpolar molecule?

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds Which formula represents a molecular substance? 1. Ca. O 2. CO 3. Li 2 O 4. Al 2 O 3 Which combination of atoms can form a polar covalent bond? 1. H and H 2. H and Br 3. N and N 4. Na and Br Which of the following elements has the highest electronegativity? 1. H 2. K 3. Al 4. Ca

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds Which bond is most polar? 1. H-F 2. H-Cl 3. H-Br 4. H-I When a chemical bond is broken, energy is 1. absorbed, only 2. released, only 3. both absorbed and released 4. neither absorbed nor released The bonds in the compound Mg. SO 4 can be described as 1. ionic, only 2. covalent, only 3. both ionic and covalent 4. neither ionic nor covalent

Chapter 6 Covalent Compounds Section 1 – Covalent Bonds As two chlorine atoms combine to form a molecule, energy is 1. absorbed 2. released 3. created 4. destroyed Covalent bonds are formed when electrons are 1. transferred from one atom to another 2. captured by the nucleus 3. mobile within a metal 4. shared between two atoms