Mina Lebitz - 500 AP Chemistry Questions to Know by Test Day

. (B) The mass number of cadmium is 112, not the atomic mass (the weighted average of the naturally occurring isotopes). The mass number will always be a whole number because it is the sum of the number of protons and neutrons (collectively called the nucleons, referring to their location in the nucleus) in an atom. The number of electrons and protons will always be the same in a neutral atom because they are the only negatively and positively charged (respectively) particles in the atom. The atomic number is the number of protons. It determines the identity of the atom, so finding cadmium on the periodic table will tell us its atomic number. If we subtract the atomic number from the mass number, we get the number of neutrons in that particular isotope.

. (E) Location E on the periodic table is in the vicinity of francium. Francium has a half-life of just 22 minutes and is the second rarest naturally occurring element (astatine is the rarest). It is doubtful anyone has actually reacted francium with water, but if it behaves as expected, it would certainly be a spectacle. In general, but moreso for the alkali metals at the bottom of the group (with the largest atomic radii and the lowest first ionization energies), the reaction is highly exothermic, partly because the reaction produces a strong base whose total dissociation in water is highly exothermic. The reaction also produces energy in the form of light and H2(g). When added to water, the interior of a piece of sodium metal, for example, will melt before it is consumed due to the high temperature produced by the reaction (the melting point of Na is ~800C). In addition, the high heat ignites the flammable H2(gg) that is produced.

to compare the first ionization energies of elements 120.)

. (B) The location of B on the table is in the vicinity of the halogens, specifically fluorine. Fluorine is the element with the highest electronegativity. Electronegativity is a measure of an atoms ability to attract electrons to itself while in a bond (within a molecule). Because the measurement of electronegativity of an atom relies on the atom being in a bond, the noble gases He, Ne, and Ar do not have measured values for electronegativity. The ionization energies of the valence electrons in Kr, Xe, and Rn are sufficiently low, allowing these noble gases to form covalent bonds with other atoms (mostly those with a high electronegativity, like F, whose electron-attracting abilities are strong enough to force the Kr, Xe, or Rn atoms to share their electrons) and therefore have their electronegativities assessed.

) and chlorine, not fluorine, has the highest electron affinity. Electron affinity doesnt change significantly within a group, and it can be thought of as the reverse ionization energy of an atoms 1 anion. For example, the energy change to remove an electron from Cl (to produce Cl) is 349 kJ mol1 (endothermic), and the energy change to add an electron to Cl to produce Cl is 349 kJ mol1 (exothermic).

for an explanation of the periodic trend regarding atomic radius.)

. (E) Metallic character is not something that is specifically measured. It is a set of properties given to metals, but the properties are due to one of the most basic properties of metalstheir readiness to lose electrons. This is due to metallic bonding, which can be thought of as the most sharing form of bonding. Metals are lattices of positively charged ions that are bathing in a sea of electrons. These electrons are highly mobile and account for nearly all the properties of metals, especially their electrical conductivity. The weak pull on the valence electrons by the nucleus allows them to be pulled off easily, resulting in the low ionization energies and the relatively strong tendency of metals to lose electrons and take on (almost) exclusively positive oxidation states. Metals with the highest metallic character can be considered as those having the lowest ionization energies, though this is a bit of a simplification (but it will work for the AP Chemistry exam).

is similar.)

is similar.)

for an explanation of how atomic radius is measured.)

for a description of the reaction of alkali metals with water.)

A general strategy for except questions: The except questions are tricky, even if what they are asking is not. Our brain doesnt think in the negative, so a good habit to get into is to circle the word except in the question to remind us that we are looking for a false statement, then treat each answer choice as either true or false, marking each choice as we go. At the end of choice (E), we choose the false one as our answer.

. (D) This question is asking if we know that ground state elements in the same group have similar properties. Phosphorus and astatine are both in group 15 (5A) so their valence shell electron configurations are both s2p3, conferring on them similar chemical reactivities. Sulfur, selenium, and oxygen are in group 16 (s2p4), while silicon is a group 14 semimetal (metalloid) with valence shell electron configuration of s2p2.

. (B) This has a simple mathematical solutiontake the atomic number (which will tell us the number of electrons in a neutral atom) of the element and add the absolute value of the negative oxidation states (more electrons) and subtract the absolute value of the positive oxidation states. F (9+1) and Na+ (111) both have 10 electrons and are therefore isoelectronic.

We can also arrive at this answer by finding one of the elements in each pair on the periodic table and moving one element to the right for each negative charge and one to the left for each positive charge. If the two elements we are comparing lead us to the same element once weve accounted for their oxidation state, then theyve got the same number of electrons. For Na+ and F, this element would be neon. The same number and configuration of electrons does not correlate with similar chemical reactivity in ions. F and Na+ are like neon in that they have a full valence shell and both are more stable and less reactive than in their ground state, but they are charged and therefore behave like ions. Their ionic radius also differs due to their different nuclear charge (see second paragraph of ).

, keeping in mind that the iodide ion has 54 electrons.

). In an ion, the number of electrons does not equal the number of protons. Atoms become ions because they gain or lose electrons (not protons), so ions that are positively charged will be smaller than what they are when in their ground state (same effective nuclear charge pulling on fewer electrons), whereas negatively charged ions will be larger when they are in their ground state (same effective nuclear charge pulling on more electrons).

For isoelectronic ions (ions with the same number of electrons), the ionic radius decreases with increasing nuclear charge. For example, O2> F > Na+ > Mg2+ > Al3+. This is because the same number of electrons are being pulled by an increasing number of protons. (See for a description of the factors that affect lattice energy).

for an except question strategy.)

. (B) The operative word in this question is diatomic. The noble gases are monatomic and so any answer choice that contains a group 18 gas is incorrect. With the exception of astatine, which is more metallic than the rest of the elements in the group, all of the halogens (group 17) are diatomic in their standard states, but only F2 and Cl2 are gases. Br2 is a liquid and I2 is a solid.