BONDING AND CHEMICAL REACTIONS GAVS SHARED RESOURCES

CHEMICAL BONDS BONDING & CHEMICAL REACTIONS MODULE GAVS SHARED RESOURCES

CHEMICAL BONDS What do this lump of coal, diamond, and what we call pencil lead, which is not really lead at all, have in common? All three substances are forms of carbon. Carbon atoms chemically bond together in different ways to form these three substances. atoms seldom exist on their own A chemical bond is formed when atoms are held together by attractive forces. This attraction occurs when electrons are shared between atoms, or when electrons are exchanged between the atoms that are involved in the bond. The sharing or exchange of electrons takes place so that the outer energy levels of the atoms involved are filled, making the atoms are more stable. If an electron is shared, it means that it will spend its time moving in the electron orbitals around both atoms. If an electron is exchanged it means that it is transferred from one atom to another. In other words one atom gains an electron while the other loses an electron.

ELECTRON DOT DIAGRAMS What is this and what does it tell us?

ELECTRON DOT DIAGRAMS Recall that the valence electrons of an atom are the electrons that reside in the highest occupied energy level. Valence electrons are primarily responsible for the chemical properties of various elements. The number of valence electrons can be easily determined from the position of the element on the periodic table. Remember from the previous module that within each column, or group, of the table, all the elements have the same number of valence electrons. For elements in groups 1-2 and 13-18, the number of valence electrons is easy to tell directly from the periodic table. This is illustrated in the simplified periodic table shown below.

ELECTRON DOT DIAGRAMS Electron dot diagrams (sometimes referred to as Lewis Dot Diagrams) are diagrams in which the valence electrons of an atom are shown as dots distributed around the element's symbol. Since electrons repel each other, the dots for a given atom are distributed evenly around the symbol before they are paired. The table below shows the electron dot diagrams for the entire second period of elements on the periodic table. Electron dot diagrams would be the same for each element in the representative element groups.

THE OCTET RULE The octet rule states that elements tend to form compounds in ways that give each atom eight valence electrons. There are two ways in which atoms can satisfy the octet rule. One way is by sharing their valence electrons with other atoms forming Covalent Bonds. The second way is by transferring valence electrons from one atom to another forming Ionic Bonds. Atoms of metallic elements tend to lose all of their valence electrons, which leaves them with an octet from the next lowest principal energy level. Atoms of nonmetallic elements tend to gain electrons in order to fill their outermost principal energy level with an octet.

IONIC BONDS BONDING & CHEMICAL REACTIONS GAVS SHARED RESOURCES

IONIC BONDS Ionic compounds are formed when atoms transfer electrons. The electrons actually move from one atom to another. When atoms transfer electrons in this way, they become charged particles called ions. The ions are held together by ionic bonds. An ionic bond is the force of attraction that holds together positive and negative ions. It forms when atoms of a metallic element give up electrons to atoms of a nonmetallic element.

IONIC BONDS Instead of drawing the entire atom to show the transfer of electrons in an ionic bond, you can use the Electron Dot diagrams. Using the same example of Sodium Chloride (Table Salt), the ionic bond is the attraction between the Na + ion and the Cl - ion. A single electron is transferred from the sodium atom to the chlorine atom, as shown below. It is conventional to show the cation without any dots around the symbol, since the energy level that originally contained the valence electron(s) is now empty. The anion is now shown with a complete octet of electrons.

WHY DO IONIC BONDS FORM? Ionic bonds form only between metals and nonmetals. Metals "want" to give up electrons, and nonmetals "want" to gain electrons. As in our example before, Sodium (Na) is an alkali metal in group 1. Like other group 1 elements, it has just one valence electron. If sodium loses that one electron, it will have a full outer energy level. Chlorine is a halogen in group 17. It has seven valence electrons. If chlorine gains one electron, it will have a full outer energy level. After sodium gives up its valence electron to chlorine, both atoms have a more stable arrangement of electrons.

PROPERTIES OF IONIC COMPOUNDS Ionic compounds have a number of properties: 1.Ions are arranged in a crystalline lattice structure. 2.Ionic solids are crystalline at room temperature. 3.The ionic bond is a strong electrostatic attraction. This means that ionic compounds are often hard and have high melting and boiling points because it takes a lot of energy to break apart. 4.Ionic compounds are brittle and bonds are broken along planes when the compound is put under pressure (stressed). So even though they are very hard they are brittle because once struck by a large force it causes the ions of the same charge next to each other. The repulsive forces between ions of the same charge causes the crystal to shatter. When an ionic crystal breaks, it tends to do so along smooth planes because of the regular arrangement of the ions. 5.Solid crystals do not conduct electricity, but ionic solutions do. This is because the strong bonds between ions lock them into place in the crystal. However, in the liquid state, ionic compounds are good conductors of electricity. 6.Most ionic compounds dissolve easily in water. When they dissolve, they separate into individual ions. The ions can move freely, so they are good conductors of electricity. Dissolved ionic compounds are called electrolytes.

USES OF IONIC COMPOUNDS Many ionic compounds are used in industry. The human body also needs several ions for good health. Having low levels of the ions can endanger important functions such as heartbeat. Solutions of ionic compounds can be used to restore the ions.

COVALENT BONDING BONDING & CHEMICAL REACTIONS GAVS SHARED RESOURCES

COVALENT BONDS A covalent bond is the force of attraction that holds together two atoms that share a pair of electrons. In covalent bonds, the outermost orbitals of the atoms overlap so that unpaired electrons in each of the bonding atoms can be shared. By overlapping orbitals, the outer energy shells of all the bonding atoms are filled. The shared electrons move in the orbitals around both atoms. As they move, there is an attraction between these negatively charged electrons and the positively charged nuclei.

COVALENT BONDS Covalent bonds form only between atoms of nonmetals. The sharing occurs because both atoms have a strong electronegativity and do not want to lose any electrons. Because neither want to lose any electrons, they share instead. This is why metals do not form covalent bonds as they easily will lose their electrons. The two atoms may be the same or different elements. A molecule is the smallest particle of a covalent compound that still has the properties of the compound.

COVALENT BONDS In covalent bonding, the atoms acquire a stable octet of electrons by sharing electrons. When only one pair of electrons are shared between two bonded atoms a single bond is formed. However, it is possible for atoms to share more than one pair of electrons. When two or three pairs of electrons are shared by two bonded atoms, they are referred to as double bonds or triple bonds, respectively. In an Electron Dot structure, each valence electron is represented by a dot, and bonds are shown by placing electrons in between the symbols for the two bonded atoms and circling the shared pair. Often, a pair of bonding electrons is further abbreviated by a dash. For example, we can represent the covalent bond in the F 2 molecule by either of the Electron Dot structures shown below.

PROPERTIES OF COVALENT COMPOUNDS 1.The melting and boiling points of covalent compounds are generally lower than those of ionic compounds. This is because it takes less energy for individual molecules than ions in a crystal to pull apart. 2.Covalent compounds are generally more flexible than ionic compounds. The molecules in covalent compounds are able to move around to some extent and can sometimes slide over each other. This results in many covalent compounds being gases or liquids at room temperature. In ionic compounds, all the ions are tightly held in place and most are crystalline solids. 3.Covalent compounds generally are not very soluble in water, for example plastics are covalent compounds and many plastics are water resistant. 4.Covalent compounds generally do not conduct electricity when dissolved in water, for example iodine dissolved in pure water does not conduct electricity. This is because covalent compounds have shared electrons. Since these are not free to move like the transferred electrons of ionic compounds, they are poor conductors.

POLAR & NONPOLAR COVALENT BONDS In some covalent bonds, electrons are not shared equally between the two atoms. These are called polar bonds. The picture below shows this for water. The oxygen atom attracts the shared electrons more strongly because its nucleus has more positively charged protons. As a result, the oxygen atom becomes slightly negative in charge. The hydrogen atoms attract the electrons less strongly. They become slightly positive in charge. As you can see this creates two poles on the molecule, a positive pole (side) and a negative pole (side). In other covalent bonds, electrons are shared equally. These bonds are called nonpolar bonds. Neither atom attracts the shared electrons more strongly. As a result, the atoms remain neutral and there are no poles (+ or - sides).

IONIC VS. COVALENT BONDS REVIEW GAVS MODULE PG. 6

BONDS REVIEW Go to pg. 6 of the module and practice reviewing ionic and covalent bonds.

NAMING CHEMICAL COMPOUNDS & WRITING CHEMICAL FORMULAS GAVS MODULE PG. 7

NAMING COMPOUNDS Water is a chemical compound, and you probably know that its chemical formula is H 2 O. That formula tells you that water is composed of the elements hydrogen and oxygen. However, the physical and chemical properties of liquid water are nothing like the properties of hydrogen and oxygen, which are both gases. Water is an example of a common name that is given to a compound because it is something that everybody is accustomed to seeing and using every day.

NAMING COMPOUNDS The name of a compound is a combination of the names of the elements that make it up with some slight changes. There are specific rules for naming and writing formulas for both ionic and covalent compounds.

NAMING COVALENT COMPOUNDS