Option 4 is correct. Period (I) is the shortest period; it contains only two elements. Period (III) is short; it contains 8 elements. Period (V) is long period; it contains 18 elements. Period (VI) is the longest period; it contains 32 elements.

Magnesium resembles beryllium according to Newlands’ law of octaves.

Metallic character decreases in a period and increases in a group.

The number of shells tells the period and the number of electrons in the last valence shell tells the group.

In 1817, Johann Wolfgang Dobereiner, a German chemist, tried to arrange elements with similar properties into groups called triads having 3 elements each.

Copper is placed along with potassium in group I of Mendeleev’s periodic table.

Mendeleev classified elements on the basis of their atomic mass and chemical properties. 

The properties of an element depend on the valence electrons or the number of electrons in the outermost orbit. In a group of modern periodic table, all the elements have the same number of valence electrons. For example, all alkali elements have one electron in their outermost orbit. Hence, they have similar properties.

Lithium and sodium are placed in the same group of Mendeleev's periodic table because they have the same formulas of hydrides (like LiH and NaH) and oxides (like Li₂O and Na₂O).

The given elements belong to group 17 of the periodic table. As we move from top to bottom in a period in the periodic table, the electronegativity decreases because of increase in atomic size. Hence, from all the given elements, iodine will have the lowest electronegativity. 

The mentioned element should have 3 shells and 6 valence electrons in the outermost shell. It must gain two electrons to attain a stable electronic configuration of 8 electrons in the outermost shell. Such elements which tend to gain one or more electrons are non-metals, so the mentioned element must be a non-metal.

Electronegativity decreases while going down a group.

Newlands, Mendeleev and Dobereiner all tried to correlate the properties of elements with their atomic masses. Newlands, an English scientist, arranged the elements in order of their increasing atomic masses. He started with hydrogen (atomic mass 1 u), the element having the lowest atomic mass and ended with thorium, which was the 56^th element. He found that the properties of the eighth element were similar to the properties of the first element. Dobereiner tried to arrange the elements with similar properties into groups called triads having 3 elements each. According to Dobereiner, when the elements of any particular triad were arranged in order of their increasing atomic masses, the atomic mass of middle element was roughly the mean or average of the atomic masses of the other two elements. Mendeleev, in his periodic table, classified the elements on the basis of their increasing atomic masses and the formula of their oxides and hydrides. Later on, Henry Moseley discovered that atomic number is a more appropriate parameter to determine the properties of an element rather than its atomic mass.

Metals are present in groups 1 and 2 which lie on the extreme left in the periodic table and non-metals are present at the right hand side of the periodic table. Metalloids are present in the middle of the periodic table and noble gases are present in group 18 of the periodic table.

Group 17 elements are known as halogens.