Toxic metals are metals that form poisonous soluble compounds and have no biological role, i.e. are not essential minerals, or are in the wrong form. Most often the definition includes at least cadmium, lead, mercury and the radioactive metals. Metals in an oxidation state abnormal to the body may also become toxic: chromium (III) is an essential trace element, but chromium(VI) is a carcinogen. Similarly, cobalt could also be toxic.

Correct Option: B

Toxic metals are metals that form poisonous soluble compounds and have no biological role, i.e. are not essential minerals, or are in the wrong form. Most often the definition includes at least cadmium, lead, mercury and the radioactive metals. Metals in an oxidation state abnormal to the body may also become toxic: chromium (III) is an essential trace element, but chromium(VI) is a carcinogen. Similarly, cobalt could also be toxic.

Iodine is well known to sublime (change from solid to gas without becoming liquid inbetween) when heated. However sodium chloride has a melting point much higher than that of iodine. Therefore, a mixture of iodine and sodium chloride can be separated by using a sublimation apparatus, with solid iodine collected by condensation on a cold surface.

Correct Option: C

Iodine is well known to sublime (change from solid to gas without becoming liquid inbetween) when heated. However sodium chloride has a melting point much higher than that of iodine. Therefore, a mixture of iodine and sodium chloride can be separated by using a sublimation apparatus, with solid iodine collected by condensation on a cold surface.

Mass number = protons or mass of electron (atomic number) + neutrons. So it is mostly greater than the atomic number. At best, it can be equal to the atomic number.

Correct Option: D

Mass number = protons or mass of electron (atomic number) + neutrons. So it is mostly greater than the atomic number. At best, it can be equal to the atomic number.

When an electric field is applied, both the valence electrons and the free electrons move in the same direction. The hole direction is opposite to that of valence electron but the mobility is the same. Even for the same electric field, valence electrons cannot move as freely as the free electrons because its movement is restricted due to collisions. Therefore, the velocity of valence electrons is less compared to free electrons. In other words, the velocity of holes is less compared to free electrons. This means mobility is also less for a hole compared to free electron.

Correct Option: B

When an electric field is applied, both the valence electrons and the free electrons move in the same direction. The hole direction is opposite to that of valence electron but the mobility is the same. Even for the same electric field, valence electrons cannot move as freely as the free electrons because its movement is restricted due to collisions. Therefore, the velocity of valence electrons is less compared to free electrons. In other words, the velocity of holes is less compared to free electrons. This means mobility is also less for a hole compared to free electron.

The total energy of a revolving electron in any orbit is the sum of its kinetic and potential energies. Energy of an electron at infinite distance from the nucleus is zero. As an electron approaches the nucleus, the electron attraction increases and the energy of electron decreases and thus becomes negative. Thus, it can never be positive.

Correct Option: C

The total energy of a revolving electron in any orbit is the sum of its kinetic and potential energies. Energy of an electron at infinite distance from the nucleus is zero. As an electron approaches the nucleus, the electron attraction increases and the energy of electron decreases and thus becomes negative. Thus, it can never be positive.