Chemistry 2202 Unit 2Section 2Homework Portfolio (2006-2007)page 1
CHEMISTRY 2202
Unit 2Section 2Homework Portfolio -- Solution Key
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(2)1.Why do atoms of low electronegativities normally form cations?
Atoms with low electronegativities do not attract their valence electrons strongly. Therefore an atom with a high electronegativity has a greater attraction for these valence electrons and a transfer of electrons occurs. The atom with low electronegativity loses its valence electrons, and becomes a positively charged ion, a cation, because it now has less electrons than protons.
(6)2.Classify each species as stable or unstable and neutral or charged and explain each of your choices.
(a) potassium atom: unstable and neutral. The potassium atom is unstable because it has only one valence electron, but is neutral because it has an equal number of electrons and protons.
(b) oxygen atom: unstable and neutral. The oxygen atom is unstable because it has six valence electrons, but is neutral because it has an equal number of electrons and protons.
(c) neonatom: stable and neutral. The neon atom is stable because it has eight valence electrons and is neutral because it has an equal number of electrons and protons.
(d) magnesium ion: stable and charged. The magnesium ion is stable because its valence orbitals are filled. It has a 2+ charge because it lost 2 electrons.
(4)3.(a)Write a Lewis diagram equation for the reaction between each pair of elements:
(i) sodium and oxygen
(ii) magnesium and nitrogen
(2)(b)Write the empirical formula (formula unit) for each ionic compound formed in 3a.
(i) Na2O (ii) Mg3N2
(2)4.(a)What is an ionic bond?
An ionic bond is the electrostatic attraction of positively charged ions and negatively charged ions.
(2)(b)When describing potassium chloride, KCl, why is the term “formula unit”used instead of “molecule”?
An ionic compound such as KCl consists of many positively charged and negatively charged ions arranged in a three dimensional structure. The term “formula unit” describes the lowest ratio (or ‘empirical formula’ ) of these ions. The term molecule is used to describe compounds that bond by sharing valence electrons to form a stable group of atoms.
(2)(c)Why doionic compounds often fracture to form pieces with flat sides?
Consider the diagrams below. Ions are held rigidly in place in the crystal. When a force causes ions within the crystal structure to move, it often results in ions of similar charge repelling each other along the plane of movement and thus creates a plane along which the crystal fractures.
(3)5.What are the properties expected of a compound that is held together by network covalent bonding?
A network of covalent bonding creates structures in which the atoms are rigidly held in place by numerous covalent bonds. These atoms require a tremendous amount of force or heat to break these bonds and dislodge them from their positions. Therefore network solids will be very hard, have very high melting points and be poor conductors of heat. Because the valence electrons are shared between two atoms and are not mobile it is also expected that network solids will be poor conductors of electricity
8.Four solids are tested in order to determine the type of bonding in each sample. These observations were made during each test.
Sample / Electrical Conductivity / Hit with Hammer / Melting Point / AppearanceA / did not conduct under any conditions / shattered into many pieces / 75°C / white, opaque, soft texture
B / did not conduct under any conditions / did not break / 3550°C / colorless, shiny, opaque
C / conducted in the liquid state only / shattered into many sharp pieces / 801°C / white, shiny, transparent
D / conducted in solid and liquid states / dented, changed shape / 232°C / grey, shiny, opaque
(4)Identify the bond type within each sample. Justify each choice.
Sample A: Covalent Bonding: It’s low melting point and soft texture are the best evidence to support that the substance consists of molecules.
Sample B: Network Covalent Bonding: Its very high melting point and its hardness suggest that the substance is a network solid.
Sample C: Ionic Bonding: The high melting point, the sharpness of the pieces and the fact that it conducted electricity only after being melted strongly suggest that this compound is an ionic compound.
Sample D: Metallic Bonding: The fact that it conducted electricity as a solid was a strong clue as well as its tendency to dent suggest this substance is a metal.