SNC2D CHEMISTRY 2/18/2013. CHEMICAL REACTIONS L Ionic & Molecular Compounds (P & ) Compounds. Compounds

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1 SNC2D CHEMISTRY CHEMICAL REACTIONS L Ionic & (P & ) Compounds A compound is a pure substance made up of two or more elements, in which the elements are chemically combined. The connection between the elementsis known as a bond. February 18, DCHEM - Ionic & 1 Compounds A small change in the way the atoms combine can make a big difference in the chemical and physical properties of compounds. February 18, DCHEM - Ionic & 2 1

2 Compounds 1. What is the main difference between an element and a compound? an element is a pure substance made up of all the same atoms while a compound is a pure substance made up of two or more elements February 18, DCHEM - Ionic & 3 Although millions of compounds have been discovered, almost all of them can be classified as one of two type: ionic or molecular. February 18, DCHEM - Ionic & 4 The key to understanding the formation of compounds is understanding the arrangement of the electrons about the nucleus. The farther away an electron is from the nucleus, the greater is its energy and the more likely it is to be involved in a chemical change. Thus, the electrons in the outer orbit (i.e. the valence electrons) are the electrons involved in bonding. February 18, DCHEM - Ionic & 5 2

3 ELECTRONS ARE THE KEY!! Î Electrons can move from one orbit to another. Ï The farther an electron is from the nucleus, the greater is its energy and the more likely it is to be involved in a chemical change. Ð The electrons involved in bonding are located in the outer shell (i.e. the valence electrons). Ñ The number of electrons in the outer shell are determined by their location in the periodic table. Ò When elements form compounds they either lose/gain/share electrons so that they have the same electron arrangement of the closest noble gas. Ó Noble gases do not easily form compounds because their arrangements of electrons are particularly stable (i.e. their outer shell is full). February 18, DCHEM - Ionic & 6 ATOMS moststable when valence shell is full (2 or 8 e s) L He, Ne, get a stable valence shell by: Î losing e s (becomes +ve ion L cation) ionic bond Ï gaining e s (becomes ve ion L anion) Ð sharing e s covalent bond In either case the ionic/molecular compound formed is electrically neutral. February 18, DCHEM - Ionic & 7 In ionic compounds, electrons are transferred from one atom to another (or to several others) so that the atoms can have the stable electron arrangements of the closest noble gases. Salt (NaCl) is an example of an ionic compound made up of ions of opposite charge that attract each other. The attraction between the ions is called an ionic bond. February 18, DCHEM - Ionic & 8 3

4 2. What do you notice about the arrangements of ions? ionic compounds form crystals that have an alternating arrangement of positively charged ions and negatively charged ions February 18, DCHEM - Ionic & 9 IONIC COMPOUNDS occur when there is a transfer of e s from a metal to a non-metal forms an ionic bond Lewis diagrams can be used to show the transfer of electrons. NaCl X Na + Cl CaCl 2 X Cl + XCa + Cl February 18, DCHEM - Ionic & 10 Ionic compounds have a number of properties in common including hardness, melting points, solubility, and conductivity. PROPERTIES OF IONIC COMPOUNDS are solids at room temperature have high melting points (i.e. strong force of attraction between charged ions) are electrolytes (i.e. form solutions that conduct electricity) February 18, DCHEM - Ionic & 11 4

5 3. Use Lewis diagrams to show how ionic bonding occurs in each of the following pairs of atoms: (a) K + F (KF) (d) Na + N (Na 3 N) (b) Na + O (Na 2 O) (e) Be + S (BeS) (c) Mg + I (MgI 2 ) (f) Li + O (Li 2 O) February 18, DCHEM - Ionic & 12 On the other hand, the atoms in molecular compounds share electrons to form the stable electron arrangements of the closest noble gas. As a result, the molecules they form are neutral. Ice (H 2 O) is an example of a molecular compound made up of neutral molecules. The bonds between shared pairs of electrons is called a covalent bond. February 18, DCHEM - Ionic & There is a less structured arrangement of the molecules within a molecular compound. Why? the force of attraction between the atoms within the molecule is strong but the molecules themselves are neutral so the force of attraction between the molecules is weak February 18, DCHEM - Ionic & 14 5

6 MOLECULAR COMPOUNDS occur when two or more non-metals(h included) share e s forms a covalent bond Lewis diagrams can also be used to show the sharing of electrons. HCl X H + Cl February 18, DCHEM - Ionic & 15 Molecular compounds also have a number of properties in common. PROPERTIES OF MOLECULAR COMPOUNDS can be solids, liquids, or gases at room temperature low melting points (i.e. weak force of attraction between neutral molecules) are nonelectrolytes (i.e. form solutions that do not conduct electricity) While the force of attraction between the molecules is weak (since the molecules are neutral), it is important to remember that the bonds between the atoms in the molecule itself are strong. February 18, DCHEM - Ionic & Use Lewis diagrams to show how covalent bonding occurs in each of the following pairs of atoms: (a) H + H (H 2 ) (c) C + O (CO 2 ) (b) N + N (N 2 ) (d) H + O (H 2 O) February 18, DCHEM - Ionic & 17 6

7 H 2, N 2, and F 2 are examples of diatomic molecules a molecule that is made from two atoms of the same element. Since they form the shape of a 7 in the periodic table they are sometimes referred to as the magnificent seven. February 18, DCHEM - Ionic & 18 DIATOMIC MOLECULE molecular compound made from two atoms of the same element the magnificent seven February 18, DCHEM - Ionic & 19 7