Ions Prentice-Hall Chapter 7.1 Dr. Yager

Objectives  Determine the number of valence electrons in an atom of a representative element  Explain how the octet rule applies to atoms of metallic and nonmetallic elements  Describe how cations form  Explain how anions form

Overview  We have learned about the electron structure of atoms (Chapter 5) and organization of the periodic chart (Chapter 6).  Now we will use this information to understand bonding between atoms, as well as properties of many compounds.

 Valence electrons are the electrons in the highest occupied energy level of an element’s atoms.  The number of valence electrons largely determines the chemical properties of an element.  To find the number of valence electrons in an atom of a representative element, simply look at its group number.

Valence Electrons Applications of Group 4A Elements CarbonSiliconGermanium

Electron dot structures are diagrams that show valence electrons as dots.

 Noble gases, such as neon and argon, are chemically unreactive. In 1916, chemist Gilbert Lewis used this fact to explain why atoms form certain kinds of ions and molecules.  He called his explanation the octet rule: In forming compounds, atoms tend to achieve the electron configuration of a noble gas. The Octet Rule

 Each noble gas (except He) has 8 valence electrons: ns 2 np 6, n = principal energy level ns 2 np 6, n = principal energy level  Atoms of metals obey this rule by losing electrons.  Atoms of some nonmetals obey the rule by gaining electrons or sharing electrons with another nonmetal. The Octet Rule

 An atom’s loss of valence electrons produces a cation, or a positively charged ion.  The most common cations are those produced by the loss of valence electrons from metal atoms.  You can represent the electron loss, or ionization, of the sodium atom by drawing the electron configuration of the atom and the ion formed. Formation of Cations

The electron configuration of the sodium ion is the same as that of a neon atom.

Using electron dot structures, you can show the ionization more simply.

The sodium atoms in a sodium-vapor lamp ionize to form sodium cations.

A magnesium atom attains the electron configuration of neon by losing both valence electrons. The loss of valence electrons produces a magnesium cation with a charge of 2+.

Walnuts are a good dietary source of magnesium. Magnesium ions (Mg 2+ ) aid in digestive processes.

Cations of Group 1A elements always have a charge of 1+. Cations of group 2A elements always have a charge of 2+.

A copper atom can ionize to form a 1+ cation (Cu + ). By losing its lone 4s electron, copper attains a pseudo noble gas electron configuration.

 The gain of negatively charged electrons by a neutral atom produces an anion.  An anion is an atom or a group of atoms with a negative charge.  The name of an anion typically ends in -ide. Formation of Anions

These are symbols of anions formed by some elements in Groups 5A, 6A and 7A.

A gain of one electron gives chlorine an octet and converts a chlorine atom into a chloride ion. It has the same electron configuration as the noble gas argon.

Both a chloride ion and an argon atom have an octet of electrons in their highest occupied energy levels.

In this equation, each dot in the electron dot structure represents an electron in the valence shell in the electron configuration diagram.

1. How many valence electrons are there in an atom of oxygen? a)2 b)4 c)6 d)8

1. How many valence electrons are there in an atom of oxygen? a)2 b)4 c)6 d)8

2. Atoms that tend to gain a noble gas configuration by losing valence electrons are a)metals. b)nonmetals. c)noble gases. d)representative elements.

2. Atoms that tend to gain a noble gas configuration by losing valence electrons are a)metals. b)nonmetals. c)noble gases. d)representative elements.

3. When a magnesium atom forms a cation, it does so by a)losing one electron. b)gaining one electron. c)losing two electrons. d)gaining two electrons.

3. When a magnesium atom forms a cation, it does so by a)losing one electron. b)gaining one electron. c)losing two electrons. d)gaining two electrons.

4. When a bromine atom forms an anion, it does so by a)losing two electrons. b)gaining two electrons. c)losing one electron. d)gaining one electron.

4. When a bromine atom forms an anion, it does so by a)losing two electrons. b)gaining two electrons. c)losing one electron. d)gaining one electron.