3 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 3
The diagram represents a particulate model of solid magnesium. Which statement best describes the electrons shown in the model?
Answer and reasoning
AThey are negative ions that hold the positive Mg ions in place A student who thinks a metal is held together like an ionic compound picks this. The dots are electrons, not ions; a pure metal contains only one kind of ion, Mg²⁺.
BEach one is held in a fixed bond between one particular pair of ions A student who pictures metallic bonding as shared pairs between particular atoms picks this. The electrons are delocalized among all the ions, not localized in bonds between pairs.
CThey are spread among all the ions and are attached to none of themCorrect The model shows the valence electrons (two from each Mg atom) spread among all the Mg²⁺ ions rather than attached to any one of them: they are delocalized over the whole solid.
DEach one stays with the magnesium atom that it came from A student who pictures a metal as neutral atoms keeping their own electrons picks this. The model shows Mg²⁺ ions with the valence electrons spread among them, not attached to their original atoms.
The table gives data for iron and carbon. Steel is an alloy of iron that contains a small amount of carbon. Based on the data, which statement correctly describes the arrangement of atoms in steel, with a valid reason?
Answer and reasoning
AC atoms fill spaces between Fe atoms, because C atoms weigh much less than Fe atoms A student who thinks atomic mass decides the type of alloy picks this. The arrangement is right, but the reason is not: what matters is that C atoms are much smaller than Fe atoms, as the radii show.
BC atoms fill spaces between Fe atoms, because C atoms are much smaller than Fe atomsCorrect The radius of a C atom (77 pm) is significantly smaller than that of an Fe atom (126 pm), so C atoms fit into the interstices of the iron lattice: steel is an interstitial alloy.
CC atoms take the places of some Fe atoms, as added atoms do in any alloy A student who thinks added atoms always substitute for host atoms picks this. Substitution happens when the atoms have comparable radii; C atoms are much smaller and occupy the spaces between Fe atoms.
DC and Fe atoms form a compound, because C bonds to Fe in a fixed ratio A student who thinks an alloy is a compound picks this. Steel's carbon content can vary; the C atoms are mixed into the iron lattice, not combined with Fe in a fixed ratio.
A student wants to predict whether atoms of an element Q will substitute for copper atoms in an alloy or will occupy the spaces between copper atoms. Which information about Q and Cu does the student need?
Answer and reasoning
AThe density of solid Q and of solid Cu A student who thinks density shows how large atoms are picks this. Density depends on both the mass of the atoms and how they are packed, so it does not give the atomic radius.
BThe mass of a Q atom and of a Cu atom A student who thinks atomic mass decides the type of alloy picks this. Atoms can have very different masses but the same radius (Ag and Au), so mass does not show whether an atom will fit into the spaces.
CThe charge on the ion of Q and on the ion of Cu A student who thinks an alloy is a compound with a fixed formula picks this. An alloy is a mixture of atoms within a metal lattice; no formula needs to balance charges.
DThe atomic radius of a Q atom and of a Cu atomCorrect Whether the Q atoms fill interstices or substitute depends on how their size compares with that of Cu atoms: significantly smaller atoms fill spaces, and atoms of comparable radius substitute.
Working Interstitial alloys form between atoms of significantly different radii; substitutional alloys form between atoms of comparable radius. The quantities needed are the atomic radii of Q and Cu, to compare their sizes.
In preparation: 0 of 3 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
2.4.A.1 Metallic bonding Fix
Metallic bonding
The attraction between an array of positive metal ions and the valence electrons that are delocalized among them. The electrons are not associated with any one ion, and each ion is attracted to the surrounding electrons rather than to particular neighbors.
Delocalized valence electrons ("sea of electrons")
Valence electrons that have left individual metal atoms and are free to move throughout the whole solid. In the model of magnesium each atom contributes two valence electrons, leaving Mg²⁺ ions in the array.
Students often think In a metal, each valence electron is held in a bond between one particular pair of atoms, as in a covalent molecule. In fact No. In a metal the valence electrons are delocalized: they are not associated with any one ion or pair of ions, and they can move throughout the solid.
Students often think A metal is held together like an ionic compound, by attractions between positive ions and negative ions. In fact No. A pure metal contains only one kind of ion, the positive metal ion. It is held together by the attraction between these ions and the delocalized electrons, not by negative ions.
2.4.A.2 Alloy Fix
Alloy
A metallic material made by mixing a metal with one or more other elements. In the alloys modeled in AP Chemistry, the added atoms are mixed among the metal atoms, either in the spaces between them (interstitial) or in place of some of them (substitutional). Its composition can vary, unlike that of a compound.
Interstitial alloy
An alloy formed between atoms of significantly different radii, in which the smaller atoms fill the interstitial spaces between the larger atoms of the metal lattice. Example: steel, in which carbon atoms occupy the interstices in iron.
Interstices (interstitial spaces)
The small spaces between the closely packed atoms of a metal lattice. Atoms much smaller than the metal atoms can occupy them without displacing the metal atoms from their positions.
Students often think An alloy is a compound in which the elements are chemically bonded in a fixed ratio, with its own formula. In fact No. An alloy is a solid mixture: its composition can vary (brass can contain different proportions of zinc, and gold and silver mix in any proportion), and the atoms are mixed within a metal lattice rather than combined in a fixed ratio.
Students often think In any alloy, each added atom takes the place of one atom of the host metal, whatever the sizes of the atoms. In fact No. Atoms of comparable radius substitute for metal atoms, but atoms significantly smaller than the metal atoms occupy the interstitial spaces instead, as carbon does in iron.
2.4.A.3 Substitutional alloy Fix
Substitutional alloy
An alloy formed between atoms of comparable radius, in which atoms of one element take the places of atoms of the other element in the lattice. Example: certain brass alloys, in which zinc atoms substitute for copper atoms.
Atomic radius as the deciding factor
Whether an added element forms an interstitial or a substitutional alloy with a metal depends on how the radii of the two kinds of atoms compare: significantly smaller atoms fill interstices, and atoms of comparable radius substitute for metal atoms.
Students often think In any alloy, the added atoms go into the spaces between the host metal's atoms, whatever their size. In fact No. Only atoms significantly smaller than the metal atoms fill the interstices (as carbon in iron). Atoms of comparable radius take the places of metal atoms in the lattice (as zinc in brass).
Students often think An alloy is a mixture of separate grains of each pure metal, each grain keeping its own arrangement of atoms. In fact Not in the alloys described here. In brass and in gold–silver alloys the atoms of the two elements are mixed atom by atom: Zn or Ag atoms take the places of some atoms of the other metal throughout the lattice, rather than forming separate grains of the pure metals.
4 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 4
An electric current is a flow of charged particles. Solid copper conducts an electric current well. Which feature of the particulate model of solid copper accounts for this property?
Answer and reasoning
AIts valence electrons belong to no single ion and move freely through the metalCorrect In the model, the copper ions sit in an array surrounded by delocalized valence electrons. These charged particles are free to move, so they can flow through the solid when a voltage is applied.
BIts positive ions leave their places and drift through the solid A student who thinks ions carry the current in a metal picks this. In solid copper the ions stay in their places in the array; the moving charges are the delocalized electrons.
CIts electrons are passed along fixed bonds between pairs of neighboring atoms A student who pictures metallic bonding as shared pairs between particular atoms picks this. The valence electrons are not held in fixed bonds; they are delocalized over the whole solid.
DIt takes in electrons from the battery, having no free electrons of its own A student who thinks the battery supplies the moving electrons picks this. Copper already contains delocalized electrons throughout; the battery makes them flow in one direction.
The table gives data for metal M and four other elements. A student plans to add a small amount of one of the other elements to M to make an alloy. For which element should the student predict that the alloy will be an interstitial alloy?
Answer and reasoning
AElement E A student who thinks the mass of the atoms decides the type of alloy picks this, because E has by far the lightest atoms (9 g/mol). E atoms are about the same size as M atoms (136 pm and 140 pm), so they would take the places of M atoms rather than fit into the spaces between them.
BElement G A student who mixes up the two types of alloy, taking an interstitial alloy to be one in which the added atoms take positions in the host lattice, picks this, because G atoms are the closest in size to M atoms. Atoms of comparable radius (141 pm and 140 pm) substitute for one another; that is a substitutional alloy.
CElement J A student who thinks any large difference in radius gives an interstitial alloy picks this, because the radius of J (215 pm) differs the most from that of M (140 pm). J atoms are larger than M atoms, so they cannot fit into the spaces between M atoms; the added atoms must be the significantly smaller ones.
DElement LCorrect Interstitial alloys form between atoms of significantly different radii, with the smaller atoms in the interstitial spaces between the larger atoms. L atoms (75 pm) are the only ones significantly smaller than M atoms (140 pm), so L is the element whose atoms can occupy the spaces between M atoms.
Brass is an alloy of copper and zinc. The atomic radius of Cu is 128 pm and that of Zn is 134 pm. Which diagram best represents one layer of atoms in a sample of brass?
Answer and reasoning
ADiagram 1 A student who thinks added atoms always go into the spaces between the host atoms picks this. Zn atoms are not much smaller than Cu atoms (134 pm compared with 128 pm), so they cannot fit into the interstices; they take the places of Cu atoms.
BDiagram 2 A student who thinks an alloy is a compound picks this. Brass is not made of Cu–Zn molecules or of Cu and Zn in a fixed ratio; the atoms are mixed within one metal lattice.
CDiagram 3Correct Zn and Cu atoms have comparable radii, so Zn atoms substitute for some Cu atoms in the lattice: every position is occupied by an atom of about the same size, either Cu or Zn. This is a substitutional alloy.
DDiagram 4 A student who pictures an alloy as separate grains of each metal picks this. In brass the Zn atoms are mixed among the Cu atoms in a single lattice, not gathered in a separate region.
Gold and silver can be mixed in any proportion to form a solid alloy. The atomic radii of Au and Ag are both about 144 pm. Which claim about the structure of the alloy is supported by this evidence?
Answer and reasoning
AAg atoms take the places of Au atoms in the lattice, as the two atoms are equal in sizeCorrect Atoms of comparable radius form a substitutional alloy: Ag atoms (about 144 pm) take the places of Au atoms (about 144 pm) in the lattice. Because any position can hold either atom, the two metals can mix in any proportion.
BAg atoms fit into the spaces between Au atoms, as Ag atoms are much lighter than Au atoms A student who thinks atomic mass decides the type of alloy picks this. An Ag atom is lighter than an Au atom, but it is the same size, so it cannot fit into the spaces between Au atoms.
CAg atoms fit into the spaces between Au atoms, as added atoms go into the gaps of any metal A student who thinks added atoms always fill the spaces between host atoms picks this. Only atoms significantly smaller than the host atoms fit there; Ag atoms are the same size as Au atoms.
DAu and Ag atoms stay in separate grains, as each metal keeps its own arrangement A student who pictures an alloy as separate grains of each metal picks this. In the alloy the Au and Ag atoms are mixed atom by atom within one lattice, which the comparable radii make possible.
Compiled from the AP Chemistry Course and Exam Description (effective Fall 2024) and our question bank · Specialist review in progress. How these pages are made · Free, no account
