3 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 3
The table gives data for three group 1 metals and what is observed when a small piece of each metal is added to water. In each reaction the metal atoms lose their valence electrons. Which statement best explains the trend in reactivity shown?
Answer and reasoning
AFrom Li to K each atom has more electrons in total and so has more electrons that it is able to lose. A student who links reactivity to the total number of electrons picks this. Each atom has one valence electron (2s¹, 3s¹, 4s¹) and loses only that one; what changes is how strongly it is held.
BFrom Li to K the attraction of the nucleus is shared among more electrons, each getting a smaller part. A student who thinks a nucleus has a fixed attraction that is shared out picks this. The force on an electron is not divided among the electrons; the valence electron is held less strongly because it is farther from the nucleus and more shielded.
CFrom Li to K the atoms are larger, and so a greater area of each atom is in contact with the water. A student who transfers the idea of surface area from lumps of solid to single atoms picks this. The size of the atom matters because the valence electron is farther from the nucleus and less strongly attracted, not because of contact area.
DFrom Li to K the valence electron is farther from the nucleus and less strongly attracted to it.Correct The valence electron is in 2s, 3s and 4s, each farther from the nucleus and shielded by more inner electrons. The falling ionization energy (520, 496, 419 kJ/mol) shows that it is held less strongly, so the atoms lose it more readily, in line with the more vigorous reaction.
The table gives the formulas of the chlorides and oxides of two metals. Gallium is in group 13 and period 4, and sulfur is in group 16. Based on periodicity, what is the ratio of gallium to sulfur in the compound that they form?
Answer and reasoning
AGa : S = 2 : 3Correct Gallium is in the same column as aluminum, and sulfur is in the same column as oxygen. The compound is analogous to Al₂O₃, so its formula is Ga₂S₃.
BGa : S = 1 : 1 A student who looks for an analogy along the period picks this, modeling the compound on CaO because calcium is also in period 4. Analogous compounds are formed by elements in the same column: gallium matches aluminum.
CGa : S = 1 : 3 A student who matches gallium with aluminum but does not check the group of the nonmetal picks this, copying AlCl₃. Sulfur is in the group of oxygen, not of chlorine, so the model is Al₂O₃.
DGa : S = 3 : 2 A student who writes each ion's own charge as its subscript picks this (Ga³⁺ and S²⁻ giving Ga₃S₂). By analogy with Al₂O₃, and so that the charges balance, the ratio is 2 : 3.
Working Ga is in the same column as Al, and S is in the same column as O, so the compound is analogous to Al₂O₃: Ga₂S₃, a ratio Ga : S = 2 : 3.
An element has the ground-state electron configuration 1s²2s²2p⁶3s²3p⁴. What is the typical charge of the ions of this element in its ionic compounds?
Answer and reasoning
A−6 A student who attaches the negative charge of the electron to the number of valence electrons picks this. The atom has six valence electrons but gains only two more, so the charge is −2.
B−2Correct The atom has six valence electrons, 3s²3p⁴. It gains two electrons, giving a valence shell of eight and two more electrons than protons: a charge of −2, as in sulfide.
C+2 A student who thinks gaining electrons gives a positive charge picks this. Two electrons are gained, and electrons are negative, so the charge is −2.
D+6 A student who takes the charge to equal the number of valence electrons, as for a group 1 or group 2 metal, picks this. A nonmetal with six valence electrons gains two electrons instead of losing six.
Working The atom has 6 valence electrons (3s²3p⁴). It gains 8 − 6 = 2 electrons to give a valence shell of eight, so the ion has two more electrons than protons: a charge of −2.
In preparation: 0 of 3 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
1.8.A.1 Valence electrons and bonding Fix
Valence electrons and bonding
The valence electrons are the outer electrons of an atom. Whether two elements are likely to form a chemical bond is determined by the interactions between the valence electrons and the nuclei of their atoms.
Reactivity
How readily an element takes part in chemical reactions. For the metals of groups 1 and 2 it generally increases down the group, as the valence electrons are held less strongly by the nucleus; for the halogens it generally decreases down the group, as an added electron is farther from the nucleus.
Periodicity
The recurrence of similar properties at regular intervals when the elements are arranged in order of atomic number. It arises because similar arrangements of valence electrons recur.
Students often think An atom with more electrons in total has more electrons available to lose, so it reacts more readily. In fact No. Each atom loses only its one valence electron. Potassium is more reactive because that electron is farther from the nucleus and less strongly attracted, not because the atom has more electrons.
Students often think A nucleus has a fixed amount of attraction that is shared among all of its electrons, so each electron of a larger atom receives a smaller share. In fact No. The force on an electron is given by Coulomb's law and is not divided among the electrons. A valence electron is held less strongly when it is farther from the nucleus and shielded by more inner electrons.
1.8.A.2 Group (column) Fix
Group (column)
A vertical column of the periodic table. The atoms of the main-group elements in one group have the same number of valence electrons (helium, with two, is the exception in group 18).
Analogous compounds
Compounds with formulas of the same type formed by elements of the same group with a given other element, such as NaCl and KCl, or MgO and CaO. Elements in the same column tend to form them because their atoms have the same number of valence electrons.
Students often think Elements in the same row of the periodic table form compounds with formulas of the same type, so a formula can be predicted from a neighbor in the same period. In fact No. Elements in the same column tend to form analogous compounds, because their atoms have the same number of valence electrons. Across a period the number of valence electrons changes, and so do the formulas (CaO but Ga₂O₃).
Students often think Once a metal from the same group has been found, any of its compounds gives the pattern for the formula, whichever group the other element belongs to. In fact Not directly. An analogy needs both elements to be replaced by elements from their own groups. Sulfur is in the same group as oxygen, so a sulfide is analogous to an oxide, not to a chloride.
1.8.A.3 Ionic compound Fix
Ionic compound
A compound made of cations and anions. The ions are present in the ratio that makes the total charge zero, and the formula gives that ratio.
Typical ionic charge
The charge an element usually has in its ionic compounds, predicted from its position in the periodic table: 1+ for group 1, 2+ for group 2 and 3+ for aluminum (the valence electrons are lost); 3− for the nonmetals of group 15, 2− for those of group 16 and 1− for group 17 (electrons are gained until the valence shell holds eight).
Charge balance in a formula
The rule that the positive and negative charges in the formula of an ionic compound add to zero. For Al³⁺ and O²⁻, two Al³⁺ (6+) balance three O²⁻ (6−), giving Al₂O₃.
Students often think Only the electrons in the highest-energy subshell are valence electrons, so the charge of a metal ion equals the number of electrons in its last-filled subshell. In fact No. The valence electrons of aluminum are the three electrons in its outer shell, 3s²3p¹, and aluminum forms Al³⁺ in its ionic compounds.
Students often think Electrons are negative, so an atom with n valence electrons forms an ion with a charge of n−. In fact No. The charge of an ion is the number of protons minus the number of electrons. An aluminum atom that loses its 3 valence electrons has a charge of 3+; a sulfur atom, with 6 valence electrons, gains 2 electrons and has a charge of 2−.
5 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 5
The complete photoelectron spectrum of an element is shown. What is the typical charge of the ions of this element in its ionic compounds?
Answer and reasoning
A−5 A student who thinks every atom completes its octet by gaining electrons picks this: 3 valence electrons, so 5 are gained. A metal atom with three valence electrons loses them instead, forming a +3 ion.
B−3 A student who attaches the negative charge of the electron to the number of valence electrons picks this. The three valence electrons are lost, leaving three more protons than electrons: +3.
C+1 A student who counts only the electron in the highest-energy subshell (the smallest peak, 3p¹) as a valence electron picks this. The two 3s electrons are also valence electrons, so three electrons are lost.
D+3Correct The five peaks, with relative heights 2, 2, 6, 2 and 1, correspond to 1s²2s²2p⁶3s²3p¹. The atom has three valence electrons in its third shell and loses them to form a +3 ion, as aluminum does.
Working The peaks correspond to 1s² 2s² 2p⁶ 3s² 3p¹ (relative heights 2, 2, 6, 2, 1), so the atom has 13 electrons and 3 valence electrons (3s²3p¹). Losing the 3 valence electrons gives a charge of +3.
A metal M in group 2 forms an ionic chloride that is 63.9% chlorine by mass. Based on the typical charges of the ions, what is the molar mass of M?
Answer and reasoning
A10.0 g/mol A student who writes each ion's own charge as its subscript picks this, using M₂Cl: 35.45 × (36.1/63.9) = 20.0 g for two moles of M. One M²⁺ needs two Cl⁻, so the formula is MCl₂.
B20.0 g/mol A student who thinks ions always combine one to one picks this, using MCl: 35.45 × (36.1/63.9). An M²⁺ ion needs two Cl⁻ ions to balance its charge, so the formula is MCl₂ and the molar mass is 40.1 g/mol.
C40.1 g/molCorrect A group 2 metal forms M²⁺, so the chloride is MCl₂. The 70.90 g of Cl in one mole is 63.9% of the mass, so M is 70.90 × (36.1/63.9) = 40.1 g/mol.
D80.1 g/mol A student who keeps chlorine as Cl₂ units in the formula picks this, using M(Cl₂)₂ with four Cl per M: 141.8 × (36.1/63.9). The chloride contains two separate Cl⁻ ions per M²⁺.
Working Group 2 metals form M²⁺ and chlorine forms Cl⁻, so the formula is MCl₂. In 1 mol of MCl₂ the mass of Cl is 2 × 35.45 g = 70.90 g, which is 63.9% of the total. Mass of M = 70.90 g × (36.1/63.9) = 40.1 g, so the molar mass of M is 40.1 g/mol.
Sodium reacts with chlorine and with iodine. In each reaction, a halogen atom gains an electron from a sodium atom. Which halogen atom has the stronger attraction for the electron it gains, and why?
Answer and reasoning
AChlorine, because the electron gained enters a shell closer to the nucleus and is attracted more strongly.Correct The electron gained by a chlorine atom enters the third shell; in iodine it enters the fifth shell, farther from the nucleus and shielded by many more inner electrons. The charge experienced is similar, so by Coulomb's law the smaller distance gives chlorine the stronger attraction.
BIodine, because its nucleus contains many more protons and so attracts an additional electron more strongly. A student who applies only the charge term of Coulomb's law picks this. Iodine's additional protons are shielded by its additional inner electrons, and the added electron is much farther from the nucleus, so it is attracted less strongly.
CIodine, because its larger atom has more room in the outer shell to take in an additional electron. A student who pictures an atom as a container picks this. Both atoms have room for one more electron in the valence shell; in the larger atom that electron is farther from the nucleus and attracted less strongly.
DChlorine, because it is a gas, and the atoms of a gas gain electrons more easily than those of a solid. A student who explains the behavior of atoms with a bulk property picks this. The state of the element does not decide the attraction between a nucleus and an added electron; the distance of the valence shell from the nucleus does.
The particulate diagram represents the relative numbers of the ions in a sample of an ionic compound of aluminum and a nonmetal, Z. How many valence electrons does an atom of Z have?
Answer and reasoning
A2 A student who takes the size of an ion's charge to be the atom's number of valence electrons picks this, after finding that Z is 2−. A nonmetal that forms a 2− ion gains two electrons, so its atom has 8 − 2 = 6 valence electrons.
B5 A student who reads the subscript of Z as its own charge picks this: three Z for every two Al taken as Z³⁻, an atom with five valence electrons. Two Al³⁺ (6+) are balanced by three Z²⁻, so Z gains two electrons.
C6Correct There are two Al ions and three Z ions, a ratio of 2 : 3. Two Al³⁺ ions (6+) are balanced by three Z²⁻ ions. An atom that gains two electrons to complete its valence shell has six valence electrons.
D8 A student who gives the atom the valence shell of its ion picks this. The ion Z²⁻ has eight valence electrons; the neutral atom has two fewer.
Working The diagram shows 2 Al ions and 3 Z ions, a ratio of 2 : 3. Aluminum forms Al³⁺, so 2(+3) + 3(charge of Z) = 0 and the charge of Z is −2. An atom that gains 2 electrons to reach eight has 8 − 2 = 6 valence electrons.
A student wants to test the claim that elements in the same column of the periodic table tend to form analogous compounds. Which procedure is best aligned with testing this claim?
Answer and reasoning
ADetermine the empirical formulas of the chlorides formed by sodium, magnesium and aluminum A student who confuses a column with a row picks this. Sodium, magnesium and aluminum are in the same period, not the same column, and their chlorides (NaCl, MgCl₂, AlCl₃) are not analogous.
BDetermine the empirical formulas of the chlorides formed by lithium, potassium and rubidiumCorrect Lithium, potassium and rubidium are in the same column. If their chlorides have formulas of the same type (LiCl, KCl, RbCl), the compounds are analogous, which is what the claim predicts.
CCompare the melting points and the boiling points of fluorine, chlorine and bromine A student who reads 'analogous' as 'having similar physical properties' picks this. The claim is about the formulas of the compounds the elements form, and these measurements involve no compounds.
DCompare how vigorously lithium, sodium and potassium react when each is added to water A student who expects the elements of a group to behave alike in every respect picks this. How vigorously the metals react does not show the formulas of the compounds formed, which is what the claim concerns.
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
