Forming Compounds

1. Forming Compounds • When two atoms collide, the valence electrons of each atom interact. • Elements try to get to the electron configuration of the closest noble gas - full valence shell.

2. Valence Shell • Three ways for an atom to acquire a full valence shell. • An atom may give up electrons • An atom may gain electrons • An atom may share electrons • When atoms give up and gain electrons in a reaction the resulting compound is known as an ionic compound with an ionic bond. • The third way to acquire a full valence shell will be talked about later in the course.

3. Ionic Compounds • Ionic compounds involve bonds between a metal cation and a non-metal anion. • If just two different elements are involved, than you have a binary compound. • Binary compounds require that the total charge (sum of the element’s charges) of the compound is equal to zero. • We represent the compound by writing down the element symbol for cation first and then the anion • Subscripts after each symbol identify how many ions are required for a total charge of zero. • The representation of the compound is known as the chemical formula

4. Lewis Structures • Sodium (Na+) and Chlorine (Cl-) • Now Chlorine has a full valence shell and the ionic compound NaCl is formed. Na Cl [ ]+ [ ]- Na Cl

5. Example • NaCl (Table Salt) • How do we name this compound? • Sodium Chloride • The suffix “ide” is put at the end of the name for the element that is the electron acceptor (anion) Na+ + Cl- NaCl • The sodium has a +1 charge and the chlorine has a -1 charge therefore +1 + -1 = 0.

6. Another Example • What would happen if we combined Magnesium and Chlorine? • Charges do not add up to zero. • Therefore we need more of one of the elements, but which one. • Magnesium has a 2+ charge and Chlorine has a 1- charge so we need two Chlorine. • MgCl2 (Magnesium Chloride) • Can also be done by drawing out the required number of atoms to get a total charge of zero.

7. Polyatomic Ions • Polyatomic Ions consist of two or more non-metal atoms grouped together. • There is only one common polyatomic cation • Ammonium NH4+ • There are several common polyatomic anions • Hydroxide OH- • Carbonate CO32- • Nitrate NO3- • Sulfate SO42- • Chlorate ClO3- • Phosphate PO43-

8. Polyatomic Ions • Compounds are named the same way • Writing the chemical formula is a little different - If more than one polyatomic ion is needed, than brackets must be put around the ion • Example: The chemical formula for ammonium oxide is (NH4)2O not NH42O • Do not forget the brackets!!!!

9. Polyatomic Example • Calcium Nitrate • Calcium (Ca2+) and Nitrate (NO3-) • Need two Nitrate ions to balance charges. • Ca(NO3)2

10. Transition Metals • Transition metals can form more than one ion - except for silver(+1), zinc (+2) and aluminum (+3). • For example Sodium can only produce the Na+ ion. Iron on the other hand can produce two ions. Fe  Fe2+ or Fe3+ • A roman numeral is placed after the atom in brackets to identify the charge • Iron that produces the +2 ion is iron(II) • Iron that produces the +3 ion is iron(III)

11. Examples • 1. Iron(III) Oxide (Rust) Fe3+ O2- Fe3+ O2- O2- • Charge of +6 from the iron and -6 from the oxygen. Chemical Formula - Fe2O3 • 2. CuCl2 Cu Cl- Cl- • Cu must have a +2 charge to balance the -2 from the 2 Cl. Copper(II) Chloride

12. Covalent Bonds • Two or more non-metallic elements. • Electrons must be shared since both atoms are looking to gain electrons. • When atoms share electrons they are joined by a covalent bond. • A neutral particle that is composed of atoms joined together by covalent bonds is called a molecule. • Substances that are composed of molecules are called molecular compounds.

13. Molecular Compounds • Water (H2O) • Two H+ atoms and a O2- atom. O H H O H H

14. Naming Molecular • H2O • Start with the element that is farther left on the periodic table (Hydrogen). • The rules for the second element still apply, suffix of “ide”. • Different is that the elements require prefixes depending on how many are in the compound. • So water’s chemical name is dihydrogen monoxide.

15. Prefixes • Prefix mono is only used for the second element. • “a” or “o” is left off of the prefix when used with an element starting with a vowel

16. Diatomic Molecules • Atoms can share electrons with the same atom. • These molecules have two of the same atoms joined by a covalent bond. • Since there are two of the same atoms the word diatomic is used. (“di” meaning two) • Seven elements exist as diatomics: • Hydrogen • Oxygen • Nitrogen • Fluorine • Chlorine • Bromine • Iodine

17. Ionic Compounds • Ionic compounds form large structures called lattices • Attraction between oppositely charged ions is strong.

18. Ionic Properties • Characteristics of an ionic compound: • Tend to have relatively high melting and boiling points because of the large amount of energy is needed to break the strong force of attraction in an ionic bond. • Conduct electricity when they are liquid or when they are dissolved in water. Melting or dissolving allow ions to move freely. In a solid state the ions are not able to move and therefore cannot conduct electricity.

19. Molecular Compounds • Bonds within the molecule are strong but forces of attraction between the molecules is weak.

20. Molecular Properties • Characteristics of a molecular compound: • Have relatively low melting points because little energy is needed to break the forces of attraction between molecules. • Relatively soft • Tend not to conduct electricity when they are in solid or liquid state. Do not conduct when dissolved in water because ions are not formed.

21. Electrolytes • An electrolyte is a substance the dissolves in water to produce a solution that conducts electricity. • Ionic substances tend to be electrolytes. • Molecular substances tend to be non-electrolytes.