Question

In the 19th century, Dmitri Mendeleev predicted the existent of a then unknown element, X, with a mass of 68. He also predicted that an oxide of X would have the formula X2O3, on the modern period table. What is the group number and period of the element?

Answer 1

1.) One valence electron
2.) 3.
3.) Group 13, period 4
4.) 1s2, 2s2, 2p6, 3s2, 3p1

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Answer 2

Answer: Group number is 13 and the period number is 4.

Explanation: Given formula of the compound is $X_2O_3$ . From this, we can tell the charge of X. The charge of X is +3 and the charge of O is -2.

It's oxidation state (charge) is calculated as:

$2x+3(-2)=0$

$2x-6=0$

$2x=6$

$x=\frac{6}{2}$

$x=3$

Charge of X is +3 means it has 3 valence electrons and we know that elements having 3 valence electrons are present in group 13.

It's atomic number would be 31 and the electron configuration will be:

$1s^22s^22p^63s^23p^64s^23d^1^04p^1$

Electronic configuration also shows that it has 3 valence electrons(two in 4s and one in 4p).

We can find out its period number with the help of its mass number. It's mass number is 68 which is closer to the atomic mass of Ga, an element of 4th period.

So, the group number for X is 13 and the period number is 4.