Answer the following question in one or two sentences:
(i) What is the percentage of gold present in ‘Hallmark’ gold?
(ii) What is the meaning of Chalcogens?
(iii) What are the metals used in the manufacture of science equipment?
(iv) Name the metal present in chlorophyll which is used in photosynthesis?
(v) When iron is exposed to moist air, a reddish brown substance is deposited on it. What is it? What is its composition?

Hint: The term Hallmark gold is used to certify the quality of gold. The word "chalcogen" originates from the combination of the Greek word “khalkόs” which means copper and the Latinised Greek word “genes” which means born.

Complete step by step solution:
(i) The term Hallmark gold is used to certify the quality of gold. The Bureau of Indian standards give hallmarks on gold jewellery. The number ‘916’ is used to denote how pure the gold is. It means that there is 91.6 gram of pure gold in 100 gram of alloy. It is also 22 carat gold.
(ii) The chemical elements in group 16 of the periodic table are called chalcogens. This group is also commonly called the oxygen family and consists of the elements oxygen (O), sulphur (S), selenium (Se), tellurium (Te), and the radioactive element polonium (Po). All the chalcogens are found in copper ores.
(iii) The following elements are used for the manufacture of science equipment:
Aluminium, brass and bronze, carbon steel, copper, lead, nickel.
(iv) For oxygen photosynthesis, chlorophyll a is used which is a specific form of chlorophyll. The molecular structure of chlorophyll a consists of the following components:
 A chlorin ring, a magnesium ion and a hydrocarbon tail. The chlorin ring is a heterocyclic compound derived from pyrrole wherein the four nitrogen atoms of the chlorin bind the magnesium ion. There are also side chains attached to the chlorin ring of the various chlorophyll molecules. Different chlorophyll molecules have different side chains which alters the absorption spectrum of light.
(v) When iron is exposed to moist air, a reddish brown substance is deposited on it which is called rust. Its chemical composition is: $ { Fe }_{ 2 }{ O }_{ 3 }.x{ H }_{ 2 }O $

Note: Often, oxygen is considered separately from the other chalcogens; it is even excluded from the scope of the term chalcogen sometimes because of its very different chemical behaviour from other chalcogens such as sulphur, selenium, tellurium, and polonium.