Explanation​:
The electron configuration for S is 1s2 2s2 2p6 3s2 3p4, whereas for $\mathrm{P}$, it is 1s2 2s2 2p6 3s2 3p3.
Due to the increased stability of P’s half-filled p-shell, more energy is required to remove an electron from a $\mathrm{P}$ atom than an $\mathrm{S}$ atom. The electron-electron repulsion in Sulfur makes it easier to remove the first electron.
First ionization energy increases across a period (due to a decrease in atomic radii and an increase in nuclear charge). In period 3, Na is the first element, and Ar is the last to the right.

Explanation:​
The $\mathrm{n}$ atom and Mn^2+ ion have the following orbital diagrams:

The valence electrons and the two lost in the 2+ ion are the $4s$ electrons so the unpaired electrons remain 5.

Explanation:
Isotopes have the same number of protons but different numbers of neutrons.
Species A and B have the same number of protons but also the same number of neutrons so cannot be isotopes.
Species C is the only species in the table with 9 protons so cannot have an isotope with another species in the table.
This leaves species D and E which both have the same number of protons but a different number of neutrons. Therefore, the correct answer is D.

Explanation:

Electrons have negligible mass.

Explanation​:

Isotopes are atoms of the same element with different numbers of neutrons that differ only in terms of their physical properties.

Explanation:
Explanation: Selenium's electron configuration is 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^4. The three $2p$ and three $3p$ orbitals are fully occupied, along with one 4p orbital.

Explanation​:
Electron configuration of the $\mathrm{Cr}$ atom is [Ar] 4s13d^5.
The loss of one s and two d electrons results in [Ar] 3d^3 configuration.

Explanation​:
Choice A is the correct answer because it satisfies Aufbau's principle and the number of electrons in the electron configuration aligns with the same number of electrons in the orbital diagram.

Explanation​:
The student must first understand that each component of the mixture will result in one spot on the paper chromatogram. If the recrystallization was successful, then the purified dye should only have one dot.
Therefore, answer choice B is incorrect since the sample contains more than one component after recrystallization.
Answer choice C indicates that the dye is no longer in the sample, and answer choice D indicates the contaminant was not successfully separated from the dye. So the paper chromatogram is unable to identify any impurities that are still present.
In answer choice A, before recrystallization, shows two components (the dye and contaminant), and after, there is only one component showing successful recrystallization.
This question is categorized as medium since the student must understand the premise and purpose of paper chromatography and apply it to experimental results

The atomic number is the number of protons. The mass number is the sum of the number of protons and neutrons. For a neutral atom, the number of electrons is equal to the number of protons.

Explanation​:
Vanadium has an atomic number of 23 therefore statement I is true. It also has a mass number of 51, therefore statement II is also true as nucleons include both protons and neutrons. Statement III is false as the closest noble gas is Ar, not Ne. The correct condensed electron configuration would be [Ar]4s23d^3. Therefore, the correct answer is A.

Explanation​:
The atomic number of sodium is 11. The atom loses one electron to become a stable ion with a charge of 1+ (10 electrons).
Choice A is incorrect because it shows the electron configuration of a sodium atom, not sodium ion. Choice B is incorrect because it shows an increase instead of a decrease by 1 electron.
Choice D is incorrect because of 2s^1.
Choice C is the correct answer. Ne has an atomic number 10. In a neutral atom, the number of electrons is equal to the number of protons. Therefore, Ne has 10 electrons in its atom. Sodium ion has 10 electrons too.

Explanation:
Liquids have an indefinite shape (take the shape of the container) but have a definite volume. Choice A describes solids. Choices B and D describe the characteristics of gases.

Explanation​:
The number of arrows denotes the number of electrons the atom has. The orbital diagram shown here has 8 arrows which tells you that there are 8 electrons.
Choice A is not correct because C has only 6 electrons.
Choice D is also not correct because F has 9 electrons.
Choice C is incorrect as N has 7 electrons.
Among the choices, oxygen in its neutral form has 8 electrons. Hence, choice B is the correct answer.

Explanation:
For p, $l$ is equal to 1. The number of electrons in a given subshell can be calculated using the formula: $6$ electrons.

The p subshell holds 3 orbitals. Each orbital can fit 2 electrons. Therefore, p subshell holds 6 electrons.

Explanation​:
Choice A is incorrect because the orbital diagram does not satisfy Hund's rule wherein all orbitals must be singly occupied before pairing.
Choice B is incorrect because Pauli's Exclusion principle is not obeyed by having two electrons with the same spin in one orbital.
Choice C is correct because, according to Hund's rule, the orbitals must be half-filled first before pairing, and the paired orbital must have electrons in an opposite spin.
Choice D is incorrect because the 2s orbital must be filled first before filling the 2p orbital, as illustrated by Aufbau's Principle.

Explanation​:
Second ionization energies are higher than the first as the electrons are being removed from a positive ion resulting in a stronger electrostatic attraction.
First ionization energies do increase across a period due to increasing nuclear charge.
First ionization energies also decrease down a group due to increased shielding (number of shells).

Explanation​:
There is an inverse relationship between frequency and wavelength. If the wavelength is doubled, the frequency will decrease by half. Therefore, Choices A, C, and D are all incorrect.

Explanation:
The molar mass of sodium phosphate is 163.94 g mol−1. Divide the mass (20.5 g) of sodium phosphate by the molar mass to calculate the number of moles of the compound. The rest of the choices are incorrect.

Explanation:

Isotopes with fewer neutrons will have a lower density

Explanation:
Seawater is a mixture of water and salts. Sucrose, distilled water, and sodium chloride are all examples of compounds.

Explanation​:
Based on the balanced chemical equation, the number of moles of NO2​ is twice that of N2O4. Avogadro's law states that the volume of a gas is directly proportional to the number of moles of the gas present (25/2= 12.5).

Explanation​:
The lines converge as energy levels increase (increased distance from the nucleus).
Higher energies result in higher frequencies.