In Newlands’ Octaves, the properties of lithium and ________ were found to be the same.

Correct — A. Newlands arranged the known elements in increasing order of atomic mass and found that every eighth element repeated the properties of the first, as the eighth note repeats the first in a musical octave.

Count from lithium: Li, Be, B, C, N, O, F, Na.
Sodium is the eighth, so it falls directly under lithium in his table.

Both are soft, highly reactive metals of the same family, which is why the pairing the law produces is option (a).

Why the others are wrong

• (b)
  beryllium — Beryllium comes immediately after lithium, so it is second in the count, not eighth. It is an alkaline earth metal and begins a column of its own in the octaves.
• (c)
  aluminium — Aluminium falls under boron in the octaves, not under lithium. Counting eight from lithium reaches sodium, and aluminium lies two places further along.
• (d)
  magnesium — Magnesium is the eighth element counting from beryllium, not from lithium. It repeats beryllium's properties, so this is the right rule applied from the wrong starting point.

John Newlands' Law of Octaves, given in 1866, was the first workable attempt to order elements by property.

He listed the known elements by increasing atomic mass and saw that every eighth element resembled the first, the way the eighth note of a musical scale repeats the tonic.

It held only up to calcium. Past that the pattern broke, and to keep the columns full he pushed unlike elements together, so cobalt and nickel ended up beside the halogens.

Mendeleev's periodic law, which left gaps for undiscovered elements, replaced it.

Key facts

• Newlands stated the Law of Octaves in 1866, ordering elements by increasing atomic mass.
• Every eighth element showed properties like the first, so lithium, sodium and potassium fell in one column.
• The law worked only up to calcium and broke down for heavier elements.
• Newlands placed cobalt and nickel in the same column as the halogens to preserve the pattern.

Common traps

• Counting eight from the wrong starting element, which lands on magnesium
• Answering from the modern periodic table's groups instead of Newlands' own columns

No directly related past PYQ was found.