How and Why Do Atoms Bond?

1. How and Why Do Atoms Bond? Module 12

2. A chemical bond is a mutual electrical attraction between the nuclei and valence electrons of different atoms that binds the atoms together. Chemical Bonds involve valence electrons. Valence electrons are illustrated by electron dot structures or Lewis structures. What are electron dot structures? Another example: The symbol of the element. Cl Na Remember that Na has one valence electron and Cl has seven And a dot for every valence electron

3. What is the electron dot structure for the element Phosphorous? Phosphorous is element number 15. It is in group 15. Therefore a phosphorous atom has 5 valence electrons. Notice that as you place valence electrons around the symbol, you treat the symbol as a rectangle and place one electron on each side until all four sides have an electron. Then you begin to pair the electrons. P

4. Group 13 Group 1 Group 17 Group 15 • • • • • • • • • • • • • • • • • • • • Be • O Na B • C • N F • Ne • • • • • • • • • • • • Group 16 Group 2 Group 14 Group 18

5. Why do atoms bond?? Atoms bond by gaining, losing, or sharing electrons. Why do atoms gain, lose, or share electrons? To attain a noble gas electron electron arrangement.

6. What are the different types of bonds? Ionic Bonding– electrical attraction between large numbers of cations (positive ions) and anions (negative ions). Metals-nonmetals.Metals tend to lose electrons to attain a noble gas electron arrangement (8 valence electrons). They attain a positive charge when they do this. Nonmetals tend to gain electrons to attain a noble gas electron arrangement (8 valence electrons). They attain a negative charge when doing this. Ionic bonds exist between metals which become cations and nonmetals which become anions. How??????

7. Sodium is a metal which has one valence electron. Sodium needs to lose one electron to become like a noble gas. [ ]- + Cl Na Chlorine is a nonmetal which has seven valence electrons. Chlorine needs to gain one more electron to attain a noble gas electron arrangement. When sodium loses its one valence electron, it’s no longer neutral. Now the protons outnumber the electrons by one and sodium has a +1 charge What a great relationship. Sodium needs to get rid of an electron and chlorine needs to attain an electron! When chlorine gains another electron, it’s no longer neutral. The electrons now outnumber the protons by one and chlorine attains a -1 charge Now we all know that opposites attract. Sodium is a positive ion (cation) and chlorine is a negative ion (anion). They are now attracted to each other since they have opposite charges.

9. 1 valence electron 7 valence electrons Sodium Loses its one valence electron Chlorine gains one valence electron

10. Ionic bonds always involves an exchange of electrons. Metals lose electrons and nonmetals gain electrons. What if the exchange does not involve an equal number of electrons??? Calcium has two valence electrons since it is in group 2. +2 [ ]- F F [ ]- Ca Fluorine, like chlorine, has seven valence electrons. Calcium needs to lose two electrons to attain a noble gas electron arrangement, but fluorine only needs to gain 1. What do we do?? Use TWO fluorines, where each fluorine can take one of the two electrons from calcium Now they are attracted to each other and form an ionic bond

11. Potassium and Iodine each become ions when exchanging electrons. Each ion becomes very stable when they attain a noble gas arrangement. Since each ion has an equal but opposite charge, the ions form a bond – an ionic bond Magnesium has two valence electrons and needs to lose them both to become like a noble gas. Chlorine, as we have seen needs only to gain one electron to become stable. Therefore, two chlorines are needed to take the two magnesium electrons. Iodine (I) has seven valence electrons like chlorine. Iodine needs to gain one more electron to attain a noble gas electron arrangement Potassium (K) has one valence electron like sodium. Potassium needs to lose its one valence electron in order to attain a noble gas electron arrangement.

12. It is important to take note of two things: Most neutral atoms are unstable, because they do not have a noble gas electron arrangement (8 valence electrons). Ions (cations or anions) are formed when atoms gain or lose electrons to attain a noble gas electron arrangement. Ions are therefore very stable (like noble gases), whereas neutral atoms are generally unstable.

13. How would aluminum and sulfur form a bond? Aluminum is a metal in group 13. Sulfur is a nonmetal in group 16. Aluminum has 3 valence electrons and needs to lose all 3 to become stable. Sulfur has 6 valence electrons and needs to gain 2 to attain noble gas stability. -2 -2 -2 We can now begin to understand something about chemical formulas. In this compound, two aluminums are needed to transfer six electrons and three sulfurs are needed to receive six electrons. The common multiple between the number of electrons needed to be gained and lost is six (Al loses 3 and S gains 2). The formula for a compound between aluminum and sulfur is S +3 Al Al2S3 S +3 Al You will need 2 aluminums and 3 sulfurs to account for all the electrons transferred. S

14. Compounds with primarily ionic bonds are usually crystalline – made of crystals. Crystals are compounds with very strong bonds at fixed geometric bond angles, so that the atomic structure is reflected in the structure of the compound. www.hull.ac.uk/.../Chap5-2NaCl.htm http://www.yellow-springs.k12.oh.us/ys-mls/microimage_12.htm At the atomic level sodium ions and chlorine ions alternate three dimensionally and form a cubic arrangement. Crystals often form this kind of network of repeating ions. Salt NaCl, for example has a cubic structure. The atomic arrangement of the ions of salt is cubic as well. Nearly all ionic bonds form crystalline arrangements. These bonds are very strong, but brittle. They have very high melting and boiling points.

15. Compounds with ionic bonds usually dissolve in water and strongly conduct electricity when dissolved in water. These are called electrolytes. http://chemed.chem.purdue.edu/genchem/history/gifs/fig2_7.gif A conductivity apparatus like the one below will show whether a solution will conduct electricity or not. Ionic compounds dissolved in water do conduct electricity and the light bulb will glow. If this apparatus were powered and salt was added to the water, the salt water solution would conduct the current and the light bulb would glow.

16. What other types of bonds exist? A second type of bond is called a Covalent Bond. What is a covalent bond? A covalent bond is a bond between two atoms (usually nonmetals) in which electrons are shared to attain a noble gas electron arrangement. How do atoms share electrons?

17. How does a covalent bond form? Hydrogen is a good example:   H• •H Each hydrogen atom has one valence electron. It needs one more electron to attain the noble gas stability of helium. Each hydrogen atom shares its electron with the other so that each atom has “available” 2 electrons, giving it a noble gas electron configuration (like helium) Now both elements are happy 

18. Chlorine can form a bond with another chlorine. Chlorine has seven valence electrons and needs only one more electron to be stable. one more electron needed Cl Cl This is a single covalent bond – a bond formed when a pair of electrons is shared between two atoms. Each chlorine shares its single unpaired electron. Each chlorine now has available its seven valence electrons PLUS an electron shared by the other chlorine.

19. Molecule – a neutral group of atoms held together by covalent bonds Molecular compound - a chemical compound whose simplest units are molecules. Chemical formula– the use of symbols and subscripts to represent the relative number and kinds of atoms making up a compound. Molecular formula – the number and kind of atoms in a molecular compound. Diatomic molecules – a molecule containing two of the same atoms: H2, N2, O2, F2, Cl2, Br2, I2. We have seen the diatomic molecules H2 and Cl2 on the previous slides. Each forms a single covalent bond since a single pair of electrons is shared between each atom.

20. What are double covalent bonds? Bonds formed when two pairs of electrons (4 electrons) are shared between two atoms. The diatomic molecule oxygen, O2, is an example of a molecule with a double covalent bond. Each oxygen has six valence electrons and needs two more electrons to become stable. O O Each oxygen has two unpaired electrons. Each oxygen will share its two unpaired electrons with the other oxygen, so that each oxygen has eight electrons available and becomes stable.

21. What are triple covalent bonds? These bonds form by sharing three pairs of electrons (6 electrons) between two atoms. This is not terribly common. The diatomic molecule, Nitrogen N2, is an example of a molecule with this type of bond. Nitrogen is in group 15 and therefore has 5 valence electrons. Each atom has 3 unpaired electrons and needs 3 more electrons to attain noble gas stability. N N Now each atom has eight valence electrons available and have attained noble gas stability

22. Compounds containing covalent bonds are also called molecular compounds. These compounds are noncrystalline, typically have low boiling and melting points, are nonbrittle, and are nonelectrolytes (do not conduct electricity when dissolved in water). These are opposite of ionic compounds which are typically crystalline, high melting and boiling points, and brittle. Molecular compounds are composed of molecules. Water is made of water molecules. What does water look like at the atomic level? You know the formula H2O! Oxygen is happy like a noble gas!!! When each hydrogen shares it one valence electron with oxygen and oxygen shares its two unpaired electrons, each atom has a noble gas arrangement Oxygen (group 16) has six valence electrons Each hydrogen has one valence electron O H And the hydrogens are happy too!!! H

23. Covalent compounds or molecular compounds do not conduct electricity when dissolved in water. Covalent compounds are composed of only nonmetals. http://chemed.chem.purdue.edu/genchem/history/gifs/fig2_7.gif Table sugar is an example of a covalent compound. Its formula is C12H22O11. Table sugar dissolved in water will not conduct an electric current and is therefore a nonelectrolyte. The lightbulb stays unlit.

24. Covalent bonds are sharing bonds and can share in two ways – equally or unequally. Some atoms have a stronger desire (hunger) for electrons than others. When this difference in electron hunger exists, then these electrons are shared unequally. Polar Covalent Bond – bond in which the electrons are not shared equally. This causes one side of the bond to be slightly negative in charge and the other side to be slightly positive. Water is an example The term “polar” means – containing opposite sides. http://www.brooklyn.cuny.edu/bc/ahp/SDPS/graphics/PolarWater.GIF In the water molecule above, oxygen has a greater attraction for the shared electrons that the hydrogens. Hence, the oxygen side of the bonds are slightly negative and the hydrogen side of the bonds are slightly positive.

25. These are the symbols for slightly positive and slightly negative Here is an example of another polar bond, where chlorine has a stronger attraction for the shared electrons than does hydrogen. So Chlorine is slightly negative and hydrogen is slightly positive.

26. What are nonpolar bonds? Nonpolar bonds are bonds between atoms which share electrons equally. Since the electrons are equally shared, there are no charges and therefore no polarity. All of the diatomic molecules (H2, N2, O2, F2, Cl2, Br2, I2) contain nonpolar bonds since the bond exists between identical atoms which have the same level of attraction for electrons. Molecules are groups of atoms bonded together by covalent bonds. Molecules may also be polar or nonpolar. If one side of the molecule is slightly positive and one side slightly negative, then the molecule is polar. We saw on a previous slide that a water molecule is polar. http://www.brooklyn.cuny.edu/bc/ahp/SDPS/graphics/PolarWater.GIF

27. Ammonia is also a polar molecule.

28. We will learn later that polar substances mix with polar substances. Nonpolar substances mix with nonpolar substances. But polar substances and nonpolar substances don’t mix. Oil is nonpolar and water is polar. They do not mix. This ends Module 12.