Question

From each of the following triads, name the element with characteristics specified below:

Triad	Greatest atomic size	Chemically most reactive
F, Cl, Br	…	…

Answer 1

Step-1: Introduction

The elements are arranged in horizontal rows and vertical columns. This arrangement is called periodic table.

The scientists namely, Dobereiner, Newlands and Mendeleev worked on the periodic table arrangement. They gave three types of law.

1. The law of triads
2. The law of octaves
3. The periodic law

Step-2: The law of triads

Groups of three elements are triads. The middle element atomic mass was equal to the average of the first and third elements atomic mass.

Apply the law of triads to the given elements Fluorine ($\mathrm{F}$), Chlorine ($\mathrm{Cl}$) and Bromine ($\mathrm{Br}$),

$\begin{array}{rcl}\mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{Chlorine}& =& \frac{\mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{Fluorine}+\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{Bromine}}{2}\\ & =& \frac{19+80}{2}\\ & =& 49.5\end{array}$

But the actual mass of Chlorine is 35.5

So, the law of triads does not obey for these elements.

Step-3:

Atomic radii: Atomic radii is the distance of the electrons in the outermost shell from the atomic nucleus. It increases from top to bottom in a group and decreases from left to right in a period but is highest for a noble gas.

So, Bromine will have largest atomic size.

Chemical reactivity: The chemical reactivity rises from top to bottom. But in case of halogens, Fluorine is more reactive while Iodine is least reactive.

Final answer:

Therefore, the most reactive and largest atomic size element is Fluorine and Bromine respectively.

Triad	Greatest atomic size	Chemically most reactive
F, Cl, Br	Br	F