የኬሚስትሪ መግቢያ ፈተና፣ 2005 ዓ.ም. (2013)

The seven SI base units include the candela for luminous intensity, the mole for amount of substance and the second for time. The base unit of mass is the kilogram, so the gram is a fraction of a base unit and not a base unit itself. The mole tempts a lot of students because it feels like a counting label rather than a measurement, but amount of substance is one of the seven base quantities.

Wavelength and frequency are tied together by \(c = \nu\lambda\), and the speed of light \(c\) is fixed. Rearranged, \(\nu = c/\lambda\), so when the wavelength gets shorter the frequency must get higher. Option B tempts because energy and wavelength sound like they should rise together, but \(E = hc/\lambda\) shows the energy goes up when the wavelength goes down.

Magnesium is in group IIA and loses two electrons to give \(\mathrm{Mg^{2+}}\). Nitrogen gains three electrons to give \(\mathrm{N^{3-}}\). The compound must be electrically neutral, so three \(\mathrm{Mg^{2+}}\) ions carrying \(+6\) pair with two \(\mathrm{N^{3-}}\) ions carrying \(-6\), which gives \(\mathrm{Mg_3N_2}\). Option A tempts if you copy the 2 from the \(\mathrm{N_2}\) gas, but the subscripts in an ionic formula come from the charges.

A bond is non-polar when both atoms pull on the shared pair with the same strength. Two chlorine atoms have identical electronegativity, so the difference is zero and the electron pair sits in the middle. C-Cl is the tempting choice because carbon and chlorine are both non-metals, but their electronegativities differ by about 0.5, so that bond is polar covalent.

Paramagnetic means the atom holds at least one unpaired electron. Group IVA atoms end in \(ns^2np^2\), and by Hund's rule those two p electrons go into separate orbitals with parallel spins, so two electrons stay unpaired. Group zero, IIA and IIB all finish with completely filled subshells, so every electron is paired and they are diamagnetic. Group IIB tempts because of its d electrons, but that \(d^{10}\) subshell is full.

A cubic lattice can arrange its points in three ways: at the eight corners only (simple cubic), corners plus one point at the centre of the cube (body centred), or corners plus one point at the centre of each face (face centred). No other arrangement keeps full cubic symmetry. Four is a common guess from students who add an edge centred cell, but that pattern is the same as one of the three already counted.