Name: Date: Topic: Bonding Classwork. Listening to Some Metal

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1 Name: Date: Topic: Bonding Classwork Learning Objective: Listening to Some Metal 2.20 The student is able to explain how a bonding model involving delocalized electrons is consistent with macroscopic properties of metals (e.g., conductivity, malleability, ductility, and low volatility) and the shell model of the atom. Science Standards: Essential knowledge 2.C.3: Metallic bonding describes an array of positively charged metal cores surrounded by a sea of mobile valence electrons. a. The valence electrons from the metal atoms are considered to be delocalized and not associated with any individual atom. b. Metallic bonding can be represented as an array of positive metal ions with valence electrons drawn among them, as if the electrons were moving (i.e., a sea of electrons). c. The electron sea model can be used to explain several properties of metals, including electrical conductivity, malleability, ductility, and low volatility. d. The number of valence electrons involved in metallic bonding, via the shell model, can be used to understand patterns in these properties, and can be related to the shell model to reinforce the connections between metallic bonding and other forms of bonding. Essential knowledge 2.D.2: Metallic solids are good conductors of heat and electricity, have a wide range of melting points, and are shiny, malleable, ductile, and readily alloyed. Metallic solids are often pure substances, but may also be mixtures called alloys. 1. Some properties of alloys can be understood in terms of the size of the component atoms: Interstitial alloys form between atoms of different radius, where the smaller atoms fill the interstitial spaces between the larger atoms. (Steel is an example in which carbon occupies the interstices in iron.) The interstitial atoms make the lattice more rigid, decreasing malleability and ductility. Substitutional alloys form between atoms of comparable radius, where one atom substitutes for the other in the lattice. (Brass is an example in which some copper atoms are substituted with a different element, usually zinc.) The density typically lies between those of the component metals, and the alloy remains malleable and ductile. Common Core Standards: RST2. Determine the central ideas or conclusions of a text; trace the text s explanation or depiction of a complex process, phenomenon, or concept; provide an accurate summary of the text. WHST1: Write arguments focused on discipline-specific content. c. Use words, phrases, and clauses to link the major sections of the text, create cohesion, and clarify the relationships between claim(s) and reasons, between reasons and evidence, and between claim(s) and counterclaims. Background: Metallic bonding in sodium Metals tend to have high melting points and boiling points suggesting strong bonds between the atoms. Even a metal like sodium (melting point 97.8 C and boiling point 883 C) melts and boils at a considerably higher temperature than the element (neon) which precedes it in the Periodic Table. Sodium has the electronic structure 1s 2 2s 2 2p 6 3s 1. When sodium atoms come together, the electron in the 3s atomic orbital of one sodium atom shares space with the corresponding electron on a neighboring atom to form a molecular orbital - in much the same sort of way that a covalent bond is formed. The difference, however, is that each sodium atom is being touched by eight other sodium atoms - and the sharing occurs between the central atom and the 3s orbitals on all of the eight other atoms. And

2 each of these eight is in turn being touched by eight sodium atoms, which in turn are touched by eight atoms - and so on and so on, until you have taken in all the atoms in that lump of sodium. All of the 3s orbitals on all of the atoms overlap to give a vast number of molecular orbitals which extend over the whole piece of metal. There have to be huge numbers of molecular orbitals, of course, because any orbital can only hold two electrons. 1) Based on your knowledge of chemistry discuss the term molecular orbital. 2) Even though there is orbital overlap, discuss how the orbital overlap in a metallic bonding different than that of a covalent bond. The electrons can move freely within these molecular orbitals, and so each electron becomes detached from its parent atom. The electrons are said to be delocalized. The metal is held together by the strong forces of attraction between the positive nuclei and the delocalized electrons. 3) Define delocalized electrons. Discuss how this delocalization is related to a metal s ability to conduct electricity. 4) In the box to the right draw a possible model of 8 sodium atoms engaged in metallic bonding. This is sometimes described as "an array of positive ions in a sea of electrons". If you are going to use this view, beware! Is a metal made up of atoms or ions? It is made of atoms. Each positive center in the diagram represents all the rest of the atom apart from the outer electron, but that electron hasn't been lost - it may no longer have an attachment to a particular atom, but it's still there in the structure. Sodium metal is therefore written as Na - not Na +. 5) Discuss in terms of protons and electrons why metallic sodium is considered an atom. Metallic bonding in magnesium If you work through the same argument with magnesium, you end up with stronger bonds and so a higher melting point.

3 6) Predict why magnesium would have stronger bonds then sodium. 7) Describe how magnesium s melting and boiling point will compare to sodium s melting and boiling point. Justify. Magnesium has the outer electronic structure 3s 2. Both of these electrons become delocalized, so the "sea" has twice the electron density as it does in sodium. The remaining "ions" also have twice the charge (if you are going to use this particular view of the metal bond) and so there will be more attraction between "ions" and "sea". More realistically, each magnesium atom has 12 protons in the nucleus compared with sodium's 11. In both cases, the nucleus is screened from the delocalized electrons by the same number of inner electrons - the 10 electrons in the 1s 2 2s 2 2p 6 orbitals. That means that there will be a net pull from the magnesium nucleus of 2+, but only 1+ from the sodium nucleus. So not only will there be a greater number of delocalized electrons in magnesium, but there will also be a greater attraction for them from the magnesium nuclei. Magnesium atoms also have a slightly smaller radius than sodium atoms, and so the delocalized electrons are closer to the nuclei. Each magnesium atom also has twelve near neighbors rather than sodium's eight. Both of these factors increase the strength of the bond still further. 8) Based on the reading discuss the reason that the metallic bonding in magnesium is stronger than in sodium. Give multiple bulleted reasons. Metallic bonding in transition elements 9) Based on your knowledge of chemistry the strength of the metallic bond in the transition metals. Justify. 10) Describe how a typical transition metal s melting and boiling point will compare to an alkali or alkaline earth metal s melting and boiling point. Justify.

4 Going Further: Based on research discuss why mercury is a liquid at room temperature. Metals are described as malleable and ductile. This is because of the ability of the atoms to roll over each other into new positions without breaking the metallic bond. If a small stress is put onto the metal, the layers of atoms will start to roll over each other. If the stress is released again, they will fall back to their original positions. Under these circumstances, the metal is said to be elastic. 11) Discuss how the delocalization of a metal s electrons gives rise to the substances malleability and ductility. Alloys are metallic materials prepared by mixing two or more molten metals. They are used for many purposes, such as construction, and are central to the transportation and electrics industries. Some common alloys are present in the following table. Composition of Typical Alloys The structures of alloys are more complicated than those of pure metals because they are built from atoms of two or more elements with different atomic radii. Because the metallic radii of the d-block elements are all similar, they can form an extensive range of alloys with one another with little distortion of the original crystal structure. An example is the copper-zinc alloy used for some copper coins. Because zinc atoms are nearly the same size as copper atoms and have similar electronic properties (they belong to neighboring groups), they can take the place of some of the copper atoms in the crystal. An alloy in which atoms of one metal are substituted for atoms of another metal is called substitutional alloys.

5 Elements that can form substitutional alloys have atoms with atomic radii that differ by no more than about 15%. Because there are slight differences in size and electronic structure, the less abundant atoms in substitutional alloy distort the shape of the lattice of the more abundant atoms of the host metal and hinder the flow of electrons. Because the lattice is distorted, it is harder for one plane of atoms to slip past another. Therefore, although a substitutional alloy has lower electrical and thermal conductivity than pure element, it is harder and stronger. 12) Define the term alloy. Discuss why we are investigating alloys. 13) Discuss the term substitutional alloy. Include a reason why metals are alloyed in substitutional alloy. Steel is an alloy of about 2% or less carbon in iron. Carbon atoms are much smaller than iron atoms, and so they cannot substitute for iron in the crystal lattice. Indeed, they are so small that they can fit into the interstices (the holes) in the iron lattice. The resulting material is called an interstitial alloy. For two elements to form an interstitial alloy, the atomic radius of the solute element must be less than about 60% of the atomic radius of the host metal. The interstitial atoms interfere with the electrical conductivity and with the movement of the atoms forming the lattice. This restricted motion makes the alloy harder and stronger than the pure host metal would be. 14) Define the term interstitial alloy. Include a description of the properties of an interstitial alloy. 15) Using two different colors create a model of a substitutional alloy of copper and zinc and an interstitial alloy of iron and carbon. The atoms of each substance can be represented by different color circles. Substitutional Alloy Interstitial Alloy

6 Passivation is the process of making a material passive, usually by the deposition of a layer of oxide on its surface. In air, passivation affects the properties of almost all metals notable examples being aluminum, zinc, titanium, and silicon. In the context of corrosion, passivation is the spontaneous formation of a hard non-reactive surface film that inhibits further corrosion. This layer is usually an oxide that is a few nanometers thick. Pure aluminum naturally forms a tough resistant oxide, almost immediately that protects it from further oxidation in most environments. 16) Describe how passivation makes normally reactive aluminum non-reactive. Going Further: Discuss how passivation is used to make stainless steel. Include any other ways stain steel can be protected against corrosion. Reflection: A) Describe the delocalization process found in metals. Discuss how this delocalization gives rise to the properties of a metal. B) Discuss how the number of delocalized electrons affects a metal s boiling point. C) Describe the two types of alloys. Discuss how these alloys alter the properties of the base metal in the alloy. D) This activity was designed to engage the student in the following science practices: 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. For each of these practices, discuss how this activity hit upon each of these practices.