The periodic table is a comprehensive tool that organizes elements according to their physical properties, chemical properties, and properties of their compounds, enabling a systematic study of elements.

The modern periodic table organizes elements based on their atomic number, which represents the number of protons in an atom's nucleus. This arrangement reflects the underlying structure and properties of elements.

Valence electrons are the electrons residing in the outermost shell of an atom. These electrons are the most significant in determining the chemical properties of an element, as they are the ones involved in bond formation and chemical reactions due to their higher energy and distance from the nucleus.

Robert Millikan is credited with determining the absolute value of the charge on an electron, a fundamental constant in physics, using his famous oil drop experiment in 1909. This discovery helped establish the electron as a fundamental particle and further confirmed the existence of subatomic particles.

Shielding effect refers to the reduction in effective nuclear charge experienced by valence electrons due to inner electron shells. Across a period, the number of inner electrons remains the same, resulting in a constant shielding effect.

Beta particles are electrons and carry a negative charge. Alpha particles and positrons are positively charged, while gamma rays are neutral photons.

Robert Millikan's oil drop experiment (1909) accurately measured the charge of an electron, a fundamental constant in physics. This groundbreaking work earned him the Nobel Prize in Physics in 1923.

Nonionic surfactants exhibit a wide range of pH and water hardness tolerance, making them suitable for various applications. They do not carry an electric charge, which allows them to interact with both water and oil, resulting in effective cleaning and emulsification.

Nitration of chlorobenzene yields a mixture of o-chloronitrobenzene and p-chloronitrobenzene due to the directing effect of the chlorine substituent, which is an ortho-para director. This makes options A and B the correct products.

Absolute alcohol, also known as anhydrous alcohol or rectified spirit, is a high-purity form of alcohol obtained through a distillation process. To achieve this, rectified spirit is treated with calcium oxide (CaO). Ca(OH)2, CaCO3, and CaCl2 are not suitable for this purpose because they are either calcium hydroxide, calcium carbonate, or calcium chloride, none of which are effective in purifying alcohol through distillation.