Chemistry (Part-II)

Chlorine (Cl) is a nonmetal with a high electronegativity (around 3.16). It tends to gain an electron to achieve a stable electron configuration.

• A positively charged sodium ion ($\text{Na⁺ }$)
• A negatively charged chloride ion ($\text{Cl⁻}$)

These oppositely charged ions are then strongly attracted to each other through electrostatic forces. This type of chemical bond formed by the electrostatic attraction between oppositely charged ions is called an ionic bond.

If the half-life of a radioactive object is six months:

After 6 months (one half-life), half (1/2) of the object will have decayed.

After another 6 months (a total of 12 months, or two half-lives), half of the remaining amount will decay. This means that 1/2 of the remaining 1/2 (which is 1/4 of the original) will decay in the second half-life.

Therefore, after 12 months, a total of 1/2 + 1/4 = 3/4 of the object will have decayed. The correct answer is twelve months.

John Newlands, in 1865, observed that when elements were arranged in order of increasing atomic mass, every eighth element had properties similar to the first. He compared this to the octaves in music.

Therefore, according to the principle of octaves, elements with similar physical and chemical properties occur at the spacing of every seventh of the elements after the first one being considered.

So, the correct answer is  7.

The major limitation of Mendeleev's periodic table was that he could not correctly position hydrogen.

1. Hydrogen resembles alkali metals (Group 1) because it has one valence electron and can lose it to form a positive ion (H+).
2. Hydrogen also resembles halogens (Group 17) because it needs only one more electron to achieve a stable electron configuration and can gain one to form a negative ion (H−). It also exists as a diatomic molecule (H2​) like halogens.

Due to these dual properties, Mendeleev couldn't definitively place hydrogen in either Group 1 or Group 17, and its position remained ambiguous in his table.

Therefore, the answer is Hydrogen.

The scientist who gave the 'Law of Octave' is Newlands. 

The scientist who, in 1817, attempted to arrange elements with similar properties into groups is  Dobereiner.

Johann Wolfgang Döbereiner is known for his Law of Triads. In 1817 and later, he observed that certain groups of three elements (triads) had similar chemical properties, and the atomic mass of the middle  element was approximately the average of the atomic masses of the other two.

The scientist who, in 1817, attempted to arrange elements with similar properties into groups is  Dobereiner.

Johann Wolfgang Döbereiner is known for his Law of Triads. In 1817 and later, he observed that certain groups of three elements (triads) had similar chemical properties, and the atomic mass of the middle  element was approximately the average of the atomic masses of the other two.

Note - Dmitri Mendeleev created the framework that became the modern periodic Table.