Nuclear Chemistry and Electrons - MCAT Physical

The mass number is the total number of nucleons (protons and neutrons) in an atom's nucleus. An atom that undergoes alpha decay will lose a helium nucleus. This decreases its mass number by four. All other forms of radioactive decay listed alter the atomic number of the atom, but not the mass number.

Beta emission involves the conversion of a neutron to a proton and an electron, but only expels the electron.

Electron capture involves the conversion of a proton and an electron into a neutron.

Positron emission involves the conversion of a proton to a neutron and a "positron," or positively charged electron.

In fission a large atom, with low binding energy per nucleon, splits into two approximately equal smaller atoms, which are more stable due to greater binding energy per nucleon. All the other properties mentioned are accurate.

The subatomic particles in an atom's nucleus, protons and neutrons, are held together by their own force called the strong nuclear force. Strong nuclear forces act in a somewhat counterintuitive way, and only over very short distances. The strong force is what allows protons to overcome their electrical repulsion to one another and exist together in the nucleus.

This is a tricky question, because it requires two steps. The first step is to recognize that the question specifies that U-235 absorbs a neutron via fission, so it becomes U-236. U-236 then breaks down into three neutrons, Ba-141, and Kr-92, because the total mass must not change.

Uranium has 92 protons, meaning that U-236 has 144 neutrons (236 – 92 = 144). Barium has 56 protons, meaning that Ba-141 must have 85 neutrons.

The identity of the unknown compound is given by the total protons (92) minus the protons in barium (56).

92 – 56 = 36

The unknown compound has 36 protons, meaning it must be Kr. The remaining neutrons must make up the remaining mass of the krypton isotope. There are 144 neutrons total, 85 given to barium, and 3 lost during reaction.

144 – (85 + 3) = 56

Krypton has 56 neutrons and 36 protons, giving it a total mass of 92.

Both reactions have the potential to produce energy, and both reactions have unstable intermediates. Fission reactions have unstable intermediates in the form of species like Xe-135, which the passage describes as being reactive with neutrons to become a different chemical species. Similarly, fusion reactions fuse smaller nuclei into unstable larger nuclei that subsequently give off energy upon their decay.

The concept of mass deficit dictates that nuclear subatomic particles typically have lower mass in their bound state than they do individually. If you took the total mass of each unbound constituent, it would be higher than the mass of the bound nucleus. This is the case regardless of whether Xe-135 interacts with another chemical species.

The first two statements are true. Statement III is false becuse radioactive decays tend to produce nuclei with higher binding energy per nucleon than the parent nucleus, since nuclei with higher binding energy are held together more tightly and thus are more stable.

The binding energy of atoms is due in large part to the nuclear force, or strong force, which is the most powerful of the four universal forces.

Because the magnitude of the strong force is so large, it is able to overcome the electromagnetic force that is generated by the repulsion of like charges. Atoms exist in other parts of the universe beyond Earth, so the gravitational force is clearly not responsible. Neutrons have "neutral" charge, but that does not mean they are able to "neutralize" positive or negative charges.

When protons and neutrons combine to form the nucleus of an atom, they generally have less total energy than the energies of the individual nucleons combined. This difference is known as the mass defect. In this case, the nucleus has 24 protons, each with a mass of , and 27 neutrons, each with a mass of .

The mass defect will be the difference between this predicted mass and the actual measured mass.

Fission of heavy elements is exothermic; it releases energy because the products are closer to the peak of the binding energy curve than the original nucleus was. In other words, the products are more tightly bound and have more negative binding energies.

In contrast, fission is endothermic for lighter elements because their fission products are farther from the peak of the binding energy curve (the products are less tightly bound). So, fission of lighter elements would require an energy input in order to occur, making light-element fission endothermic.

All other listed answers are true statements.

The mass of a proton is and the mass of a neutron is . The mass of the given nucleus would be:

Because of mass deficit, it is not possible to get a mass greater than this value. The mass of a nucleus is always less than the sum of the masses of the parts.

Nuclear attraction is a force that holds together the molecules in the nucleus of an atom. Remember that there are a total of three subatomic particles in an atom: protons, neutrons, and electrons. Protons are positively charged, neutrons are neutral, and electrons are negatively charged. Of the three subatomic particles, only protons and neutrons are found inside the nucleus. Nuclear attraction must occur between a neutron and a proton. This force counteracts repulsion between protons to hold the nucleus together. Electrons are found in electron shells outside the nucleus and do not participate in nuclear attraction.

Nuclear forces occur between protons and neutrons in the nucleus of an atom. Recall that quarks are elementary particles that make up composite particles called hadrons. Protons and neutrons are types of hadrons; therefore, nuclear forces occur between two hadrons.

There are four major types of fundamental forces: gravitational force, electromagnetic force, weak force, and strong force. Gravitational force is the attractive force between two bodies, electromagnetic force is a combination of the force between charges and magnetic force, weak force is the force between a W boson and a Z boson, and strong force is the force that occurs between two quarks. Nuclear forces between protons and neutrons are an effect created by the strong force between two quarks; however, nuclear forces are not classified as a type of strong force.

Since the half-life of the sample is 3 days long, throughout the course of 9 days, the sample will undergo a half-life cycle 3 times. This means we have to half the mass of the original sample 3 times.

We originally start with

After 3 days:

After 6 days:

After 9 days:

Another way to solve these types of problems is using this formula: where is the number of half lives.

thus or of the original sample remains.

Therefore after 9 days, there will be of the original sample remaining.

Potassium (K) is orignially in the electron configuration of \[Ar\]4s1. To obtain a +1 charge it loses an electron, resulting in a configuration of \[Ar\].

When an element loses an electron it is generally taken away from the highest electron shell. The electron configuration of iron (Fe) is . The 4s orbital is farther away from the nucleus than the 3d orbital, therefore the electron configuration of Fe+ will be .

This particular configuration denotes a particle with ten total electrons. The sodium atom, with eleven electrons, is the only one listed that could not have this configuration. Ionized sodium, however, symbolized as Na+, does apply. (Be careful to distinguish neutral atoms and ions).

When determining electronic configuration, the answer is made much easier by starting with the next smallest noble gas in brackets. As a result, \[Ar\] is an appropriate way to incorporate every previous electron before argon.

After argon, vanadium has five other electrons to distribute, and because vanadium is a transitional element, it will fill its 3d subshells before filling the 4p subshells. The 4s subshell is filled first, and the last three electrons are placed into the 3d subshells.

\[Ar\]3d34s2

Were this question asking for the electronic structure of magnesium (Mg) in its ground state, would be the correct answer; however, the charge on means that the molecule shed two valence electrons to achieve a more stable orbital. Those electrons will be shed from the outermost valence shell, which in this case is the shell; therefore, is correct.

Note that the ground state of magnesium will have twelve electrons (the same as its atomic number), while the ion will have ten.

Nuclear orbitals will always fill from the innermost to the outermost subshells. Using the rule, we can approximate the order in which these orbitals will fill. Because electrons fill starting with the centermost orbitals, the electron that is closest to the nucleus will belong to the orbital that fills first.

corresponds to the principle quantum number, the first number in the given orbital location. is the azimuthal quantum number, and dictates the shape of the orbital.

5s and 3d produce the same number from the equation, but in the event of a tie we always pick the orbital with the lowest letter (s < p < d < f). The innermost orbital will be 5s.