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:
Second ionization energy is the energy required to remove one mole of electrons from one mole of singly positive gaseous ions.

Explanation:
The student must first recall that the component with the greatest affinity for the mobile phase will travel the furthest in paper chromatography. In this case, spot #2 is the furthest from the starting line, so it has the greatest affinity for the mobile phase.
In paper chromatography, the mobile phase is always the solvent (water, in this case), while the stationary phase is the paper.
This question is categorized as easy because the student must only recall basic definitions and concepts for paper chromatography.

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:
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.

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:
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:
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:
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:
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:
An insoluble solid can be separated from a liquid by means of filtration. Distillation is used to separate homogeneous liquid mixtures (solid soluble in a liquid or two miscible liquids) with different boiling points. Sublimation is a technique for separating mixtures with volatile components that can undergo sublimation. Crystallisation is a separation technique that uses solubility properties to separate a solid from a solution.

Explanation:
1 mole is the number of H2 molecules in 2.02 g of H2. Choices A, B, and C all conform to the definition of a mole

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:
To answer this question, the student must understand the properties of the mixture's components.
The wood chips would not dissolve in the water, so they are filtered out first. This will leave the sodium chloride dissolved in the water. The water can be evaporated to remove the water (to isolate the salt).
Paper chromatography would be ineffective in this scenario as it is used for separating substances such as dyes, pigments, and proteins.
This question is categorized as medium complexity because the student must understand the properties of matter and separation techniques.

Explanation:
At high pressures and very low temperatures, the inter-particle forces strengthen, spaces between particles decrease causing the deviation from ideal gas behaviour PV/nRT > 1.

Explanation:
Relative atomic mass of Br=(0.5069)(78.92)+(0.4931)(80.92)=79. The rest of the choices are incorrect relative atomic masses of bromine.

Alternative Method:

79Br atomic mass can be rounded up to 79

81Br atomic mass can be rounded up to 81

Since the relative abundance of both isotopes is about 50% each, the relative atomic mass of Bromine should be close to 80. Choice D corresponds to that estimation.

Explanation:
Electrons fill sublevels according to the Aufbau Principle shown below: 

Following this order, 2p<4s<3d<4p

Explanation:

Isotopes with fewer neutrons will have a lower density

Explanation:
Condensation (gas to liquid) requires making bonds and therefore, releases energy. Sublimation (solid to gas) requires breaking bonds and therefore absorbs energy.

Explanation:
According to Aufbau’s principle, the arrangement of subshell in increasing energy is 1s 2s 2p 3s 3p 4s 3d 4p. Therefore, Choices A and B are both incorrect. Choice C is incorrect because 2d does not exist.

Explanation:
Successive ionization energies are larger, as electrons are removed, there is less electron-electron repulsion, so the remaining electrons are pulled closer to the nucleus, and more energy is required to remove them.

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 s orbital has a spherical shape. In terms of energy, s has the lowest, followed by p and d orbital. Thus, Choices A, B, and D are all incorrect

Explanation:
The shortest wavelength corresponds to a higher frequency and therefore a higher energy transition. The 5p to $1s$ represents the largest energy drop and therefore would produce the shortest wavelength.

Explanation:
Statement I is true because emission spectra are produced when photons are released from excited electrons returning to lower energy levels.
Statement II is true because this is Pauli’s Exclusion principle.
Statement III is also true because it is included in Pauli’s Exclusion principle.