Which two elements were put in the same slot by Newland?

Correct option is D

​The correct answer is (D) Co and Ni.

Newland's Law of Octaves:

•  In 1865, John Newlands, an English chemist, arranged the elements in order of increasing atomic mass and noticed that every eighth element had similar properties.
•   He put Cobalt (Co) and Nickel (Ni) in the same slot, even though they were different elements, as they exhibited similar chemical properties.
• Newlands' approach was one of the early steps toward the development of the periodic table, though it was later revised by Dmitri Mendeleev.

Information about other options:

• Na and Al: Sodium (Na) and Aluminum (Al) were not placed in the same slot by Newlands. They are in different groups with distinct properties.
• Ca and Cu: Calcium (Ca) and Copper (Cu) were not placed in the same slot by Newlands. They belong to different groups and have different chemical behaviors.
• As and Se: Arsenic (As) and Selenium (Se) have similarities, but they were not placed in the same slot in Newlands' arrangement.

Information Booster:

• This was an early attempt at classifying elements and was later improved upon by Mendeleev's Periodic Table.
• Newlands' observation that properties repeat every eighth element was pivotal in the development of the modern periodic table.