Explanation:

Isotopes with fewer neutrons will have a lower density

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:
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:
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.

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:

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

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:
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 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:
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:
Electron configuration of Al^+: [Ne] 3s^2
Electron configuration of Zn: [Ar] 4s^2 3d^10
Electron configuration of Ca: [Ar] 4s2

Electron configuration of Sc: [Ar] 4s^2 3d^1.
Sc will lose 2 electrons in the 4s sublevel and 1 from the 3d sublevel, leaving a complete 3rd energy level with 8 valence electrons in Sc^3+ (1s^2 2s^2 2p^6 3s^2 3p^6).

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

Following this order, 2p<4s<3d<4p

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

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

Explanation:
Add 273 to the temperature in Celsius to convert it to Kelvin. −185∘C+273 = 88K. The rest of the choices are incorrect.

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:
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:
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:
There is a large increase between the 3rd and 4th ionization energies which shows that the element is in group 13.
Large increases in ionization energy indicate electrons being removed from a shell closer to the nucleus.

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:
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.