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Chemistry 7.1

© Copyright Pearson Prentice Hall

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Ions

Pyrite (FeS2), a common mineral that emits sparks when struck against steel, is often mistaken for gold—hence its nickname, “fool’s gold.” Pyrite is an example of a crystalline solid. In this chapter, you will learn about crystalline solids composed of ions that are bonded together. But first you need to understand how ions form from neutral atoms.

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Valence Electrons

Valence Electrons

How do you find the number of valence electrons in an atom of a representative element?

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Ions > Valence Electrons

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.

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To find the number of valence electrons in an atom of a representative element, simply look at its group number.

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Applications of Group 4A Elements

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Carbon Silicon Germanium

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

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The Octet Rule

The Octet Rule

Atoms of which elements tend to gain electrons? Atoms of which elements tend to lose electrons?

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Ions > The Octet Rule

Noble gases, such as neon and argon, are unreactive in chemical reactions. 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.

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Ions > The Octet Rule

Atoms of metals tend to lose their valence electrons, leaving a complete octet in the next-lowest energy level. Atoms of some non-metals tend to gain electrons or to share electrons with another nonmetal to achieve a complete octet.

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Formation of Cations

Formation of Cations

How are cations formed?

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Ions > Formation of Cations

An atom’s loss of valence electrons produces a cation, or a positively charged ion.

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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 complete electron configuration of the atom and of the ion formed.

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

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

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

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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+.

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Walnuts are a good dietary source of magnesium. Magnesium ions (Mg2+) aid in digestive processes.

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

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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.

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Formation of Anions

Formation of Anions

How are anions formed?

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Ions > Formation of Anions

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.

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The figure shows the symbols of anions formed by some elements in Groups 5A, 6A, and 7A.

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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.

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

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

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The negatively charged ions in seawater—the anions—are mostly chloride ions.

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The ions that are produced when atoms of chlorine and other halogens gain electrons are called halide ions.

• All halogen atoms have seven valence electrons.

• All halogen atoms need to gain only one electron to achieve the electron configuration of a noble gas.

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Oxygen is in Group 6A.

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Ions > Formation of Anions 7.1

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Conceptual Problem 7.1

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