4.1 IONIC BONDING AND STRUCTURE

ESSENTIAL IDEA Ionic compounds consist of ions held together in lattice structures by ionic bonds. NATURE OF SCIENCE (2.2) Use theories to explain natural phenomena – molten ionic compounds conduct electricity but solid ionic compounds do not. The solubility and melting points of ionic compounds can be used to explain observations.

THEORY OF KNOWLEDGE General rules in chemistry (like the octet rule) often have exceptions. How many exceptions have to exist for a rule to cease to be useful? What evidence do you have for the existence of ions? What is the difference between direct and indirect evidence?

UNDERSTANDING/KEY IDEA 4.1.A Positive ions (cations) form by metals losing valence electrons.

UNDERSTANDING/KEY IDEA 4.1.B Negative ions (anions) form by non- metals gaining electrons.

An ion is a charged particle. Ions form when atoms gain or lose electrons. A cation is a positive ion. ( Think of Pulsy CAT) Formed when an atom loses one or more electrons. An anion is a negative ion. Formed when an atom gains one or more electrons. The charge on an ion is equal to the number of electrons gained or lost.

UNDERSTANDING/KEY IDEA 4.1.C The number of electrons lost or gained is determined by the electron configuration of the atom.

CATIONS FROM GROUPS 1, 2 AND 3 Group 1 elements have one valence electron so they tend to lose one electron and form 1+ ions. Group 2 elements have two valence electrons so they tend to lose two electrons and form 2+ ions. Group 3 elements have three valence electrons so they tend to lose three electrons and form 3+ ions. NOTICE THAT FOR IONS, THE CHARGE IS LISTED AFTER THE NUMBER (2 +, 3 - ). OXIDATION STATES ARE LISTED WITH THE CHARGE BEFORE THE NUMBER ( + 2, - 3). Group 4 elements do not tend to lose or gain electrons due to the amount of energy involved so they do not normally form ionic bonds.

ANIONS FROM GROUPS 5, 6 AND 7 Group 15 elements have five valence electrons so they tend to gain 3 electrons and form 3- ions. Group 16 elements have six valence electrons so they tend to gain two electrons and form 2- ions. Group 17 elements have seven valence electrons so they tend gain one electron to form 1- ions. ELECTRONS ARE GAINED OR LOST TO HAVE 8 ELECTRONS IN THEIR OUTER SHELL.

TRANSITION ELEMENTS Transition elements have electron configurations that allow them to lose different amounts of electrons from their d-sublevels. They can form more than one stable ion. The ions can be distinguished by color. Here are some common examples: Lead forms 4+ and 2+ Copper forms 2+ and 1+ Tin forms 4+ and 2+ Hydrogen can form the hydride H - as well as H +.

UNDERSTANDING/KEY IDEA 4.1.D The ionic bond is due to the electrostatic attraction between oppositely charged ions.

IONIC CHARACTER You can predict ionic character by two ways: Position on the Periodic Table Electronegativity differences Position on the Periodic Table Metals on the left side of the zigzag line tend to lose electrons. Non-metals on the right side of the zigzag line tend to gain electrons. The highest tendency to form ionic compounds will be between elements on the bottom left with elements on the top right. Electronegativity Electronegativity values are given in Table 8 of the IB Data Booklet. Differences greater than 1.8 are considered to be ionic.

UNDERSTANDING/KEY IDEA 4.1.E Under normal conditions, ionic compounds are usually solids with lattice structures.

LATTICE STRUCTURE The ions in ionic compounds surround themselves with ions of the opposite charge. Because of this, ionic compounds have three-dimensional crystalline structures known as ionic lattices. The strength of force between the ions is known as lattice enthalpy and depends upon the size of the ion and the charge on the ion. The smaller the ion and the greater the charge, the higher the lattice enthalpy. The coordination number expresses the number of ions that surround a given ion in the lattice. The formula unit is the smallest particle of an ionic cmpd.

APPLICATION/SKILLS Be able to explain the physical properties of ionic compounds (volatility, electrical conductivity and solubility) in terms of their structure.

PHYSICAL PROPERTIES Remember physical properties can be observed without chemically altering the substance. MELTING AND BOILING POINTS Ionic compounds have high melting and boiling points because the electrostatic attraction between the ions is very strong and requires large amounts of energy to break the bond. The higher the charge and smaller the ion, the greater the melting point.

VOLATILITY Volatility is the tendency of a substance to vaporize. Ionic compounds have very low volatility.

SOLUBILITY Solubility is the ease with which the solid dissolves in a liquid to become a solution. Solubility trends are based on the similarity of the chemical nature of the solute and the solvent. Polar compounds dissolve in polar solvents. Nonpolar compounds dissolve in nonpolar solvents “Like dissolves like”

ELECTRICAL CONDUCTIVITY Electrical conductivity is the ability to conduct electricity. Freely moving ions must be present to conduct electricity. Molten or dissolved ionic compounds conduct electricity. Solid ionic compounds do not because the ions are locked into place and are not free to move about.

BRITTLENESS Brittleness means the crystal will shatter when force is applied. Ionic compounds tend to be brittle because ions of like charge can be next to each other in the lattice structure and the repulsive charges cause the structure to split easily.

APPLICATION/SKILLS Be able to deduce the formula and name of an ionic compound from its component ions, including polyatomic ions.

GUIDANCE Know these polyatomic ions: NH 4 +, OH -, NO 3 -, HCO 3 -, CO 3 2-, SO 4 2-, PO 4 3-

Citations International Baccalaureate Organization. Chemistry Guide, First assessment Updated Brown, Catrin, and Mike Ford. Higher Level Chemistry. 2nd ed. N.p.: Pearson Baccalaureate, Print. Most of the information found in this power point comes directly from this textbook. The power point has been made to directly complement the Higher Level Chemistry textbook by Catrin and Brown and is used for direct instructional purposes only.