Final answer:
The claim that alloys can only form with metals of similar size is false. Alloys can be substitutional or interstitial and are composed of at least one metal, but do not require similar atomic sizes.
Step-by-step explanation:
The statement that 'Alloys are formed only when the constituting metals are similar in size' is false. An alloy is a mixture composed of two or more elements, at least one of which is a metal. The properties of an alloy, which can include increased strength and corrosion resistance, are often superior to those of its component elements. There are two types of alloys: substitutional alloys, where different atoms replace each other in the crystal structure, and interstitial alloys, where smaller atoms fit in between the larger atoms in the crystal structure. These do not require the atoms to be similar in size. Examples of alloys include steel, made from iron and carbon, brass, from copper and zinc, and bronze, from copper and tin.