<u>Answer: </u>The correct statement is X is the effective nuclear charge, and it increases across a period.
<u>Explanation:</u>
We are given that:
X = number of protons − number of core electrons
Effective nuclear charge is defined as the actual nuclear charge (Z = number of protons) minus the screening effect caused by the electrons present between nucleus and valence electrons. These electrons are the core electrons.
The formula used for the calculation of effective nuclear charge given by Slater is:

where,
= effective nuclear charge
Z = atomic number or actual nuclear charge or number of protons
= Screening constant
The effective nuclear charge increases as we go from left to right in a period because nuclear charge increases with no effective increase in screening constant.
Hence, the correct answer is X is the effective nuclear charge, and it increases across a period.
To convert a number without the use of the expression 10^-4 you would have to treat it like an integer line for instance on an integer line the negative is to the left and the positive number is to the right so 3.91*10^-4 means that the decimal would have to move 4 places to the left so the answer would be .000391 instead of 3.91*10^-4.
I hope this helps you :)
atomic number is equal to proton number
so the proton number will be 87
Answer:
a) H2S:SO2 = 2:2 and O2:H2O = 3:2
Explanation:
⇒ H2S:SO2 = 2 mol H2S / 2 mol SO2 ≡ 2 : 2
⇒ O2:H2O = 3 mol O2 / 2 mol H2O ≡ 3 : 2