Chapter 9 Chemical Bonding

Transcription

1 Chapter 9 ALABAMA 8TH GiAD SCINC STANDARDS COVRD IN THIS CHAPTR INCLUD: 5 Differentiate between ionic and covalent bonds Illustrating the transfer or sharing of electrons using electron dot diagrams BONDING OF ATOMS Recall from Chapter 5 that an element is a substance composed of identical atoms When atoms oftwo or more elements combine chemically, they form a compound This new compound has completely different properties than the individual elements from which it is made (Recall the example of hydrogen and oxygen combining to form water) A compound is made up of identical molecules One molecule of water consists of two atoms of hydrogen (H) and one atom of oxygen () The chemical formula that describes each molecule of water is H2 A molecule is the product of two or more r atoms joined by chemical bonds The chemical bond is the force that holds atoms together Recall, atoms of different elements can combine chemically to form molecules by sharing or by transferring valence electrons Valence electrons are either lost, gained or shared when bonds are formed Chemical bonding can occur in two main ways: ionic : bonding and covalent bonding -J atom I I IONIC BONDS An atom with a charge is called an ion, and it is formed by the transfer of electrons When one atom takes electrons from another atom, both are left with a charge The that took electrons has a negative charge (Recall that electrons have a negative charge) The atom that gave electrons has a positive charge The bond formed by this transfer is called an ionic bond Ionic bonds are very strong Ionic compounds have high melting points and high boiling points These compounds tend to have ordered crystal structures and are usually solids at room temperature Ionic compounds will usually dissolve in water, and they have the ability to conduct electricity in a watery solution or a molten state 75

2 Sodium chloride (table salt) is an example of a compound with an ionic bond In salt, one atom positive ions with 1 charge The chlorine atom sodium gives up one electron This forms of sodium reacts with one atom of chlorine The elements share valence electrons They share electrons in such a way that their valence electron orbital is filled The sharing arrangement creates a more stable structure in the bound elements than was present before the bonding occurred In general, there are two rules about elements that form covalent bonds: valent bonds are formed when two or more COVALNT BONDS gains one electron given up by the sodium atom This forms a negative chloride ion with a 1 charge Figure 9 1 illustrates this electron transfer Note that the orbital shape (circular) has been simplified for clarity 1 lements 2 Nonmetals form covalent bonds (CM4) ach of the four hydrogen atoms shares one electron with a single carbon atom Figure 92 Methaie Molecule (CR4) general, they do not conduct electricity well Let s look at an example of a covalent bond bonds The covalent bond between one atom of carbon and four hydrogen atoms is methane with similar electronegativities form covalent bonds valent bonding results in compounds that have low melting points and low boiling points In Carbon and hydrogen have very similar electronegativities and commonly form covalent hydrogen atoms This arrangement gives the carbon atom a full valence shell containing eight hydrogen atom has a full valence shell as well The valence orbitals ofcarbon and hydrogen are Likewise, the carbon atom shares its four valence electrons One is shared with each ofthe four electrons Hydrogen s valence orbital can only contain two electrons This means that each drawn in Figure 92 The overlap of orbitals represents shared electrons Figure 91 Ionic Bonding oftable Salt I; I: _ _ %, % % / 76 ( H H, 1 % Ii C Sodium Chloride =

3 Chapter 9 LCTRON DOT DIAGRAMS An electron dot diagram is the symbol of an element surrounded by dots The dots represent the number of electrons in the elements outer energy level lectron dot diagrams are used to show how atoms bond with each other The dots around the symbol are placed in a particular order: in pairs on all four sides ofthe element symbol Figure 93 shows the electron dot diagram for nitrogen Nitrogen contains five electrons in its outer energy level To draw the dots, place one dot on the top ofthe element symbol Then work your way around clockwise, adding a dot to the right, bottom and left side ofthe symbol Add the fifth dot to the top to make a pair If there were more electrons in the outer shell, you would continue to add dots in this same manner until you complete the level Activity Figure 93 lectron Dot Diagram for Nitrogen Lets practice making some electron dot diagrams ofelements Remember, we are using dots to represent valance electrons Draw dot diagrams around the following elemental symbols: Ne P F In Na Mg S In CO -J a D F H Ar I- z C- a, C- 77

4 valence electrons are shared between the atom of carbon and the four atoms of hydrogen in a molecule of methane Figure 94 below shows how the electrons are shared using an electron Carbon needs eight electrons total to fill its outer shell, so it shares an electron with four dot diagram Carbon has four electrons in its outer shell, so its dot diagram is a C with one dot on each side of its symbol An atom of hydrogen has only one electron in its outer shell Let s return to our example of a covalent bond, methane (CH4) Figure 92 showed us how the NC1: Na:C1: stable electron configuration The sodium atom was left with a positive charge (+f) while the chlorine atom now has a negative charge (l) This electron transfer is illustrated in Figure 95 using an electron dot diagram lectron dot diagrams also illustrate the transfer of electrons during ionic bonding Let s return HCH H H different hydrogen atoms Hydrogen atoms need two electrons to become stable The dot diagram shows that the bonding between carbon and hydrogen atoms creates full valence shells for the carbon atom and each hydrogen atom The white circles in the figure represent the shared electrons In a normal diagram, the dots are all the same color Here, they are shown differently for your learning benefit Figure 94 lectron Dot Diagram for Methane to our earlier example of our ionic bond, sodium chloride Figure 9 1 illustrated the transfer of one electron from a sodium atom The chlorine atom only needs one electron to become stable, so it gained this electron from the transfer This gave both the sodium and the chlorine a more Figure 95 lectron Dot Diagram for Sodium Chionde 78 ) 2 z

5 Chapter 9 Activity For each compound in the table, determine the type ofbond that formed to create the compound Write I for ionic or C for covalent under the second column Draw the appropriate electron dot diagram to represent the bonding of electrons in the space provided (Hint: Refer to the Periodic Table on page 54 to determine the number of electrons in the outer shell of each atom) mpound Type of Bond Dot Structure (IorC) 2 H2 C2H6 c1 C? LU -J I- CaCl2 Cu C) C) ) 79

6 1 Which ofthe following characteristics does a covalent bond have? B atoms share electrons to bond A high melting and high boiling points CHAPTR 9 RvIw -Na Q %A : :Na N\ 5 Two atoms of sodium (Na) and one atom of oxygen undergo ionic bonding One atom B B NaO1a :NaONa: the following electron dot structures shows the correct transfer of electrons? A C ofna has 1 electron in its outer shell, and one atom of has 6 electrons Which of B Li2 C12 C Li2 Cl2 shows the correct charges for the new compound lithium chloride (LiC1)? A Li Cl lithium has one electron in its outer shell and chlorine has 7 Which ofthe following 4 One atom of lithium and one atom of chlorine undergo ionic bonding One atom of B Lf1 C1 A 2 C MgO 3 Which ofthe following molecules is most likely to have a covalent bond? B NaCl D Fe23 B polar molecules C valence electrons B protons A neutrons B all ofthe above C conducts electricity 2 Ionic and covalent bonding take place between 8 Na