During the 1860s, elements were identified and scientists noticed some elements had similar and properties. these similarities led to organizing the periodic table.

Upto 1860s around 50 elements were identified.

These elements showed some similar chemical and physical properties

Before 1860s there was no need of classification or arrangement of elements as very few elements were discovered, but after some time it was observed that it is becoming difficult to define and remember the exact physical and chemical properties of each individual element.

So scientist thought to arrange them in some order based on similar properties

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer