Chemistry: Chemistry Bonding & Chemical compound formula

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# h3chefor1 → Molecule Formulae
Element	reacts with	, and will get the compound molecule.
What is the mass of this molecular compound?

1. 168		2. 248
3. 256		4. 328

(Problem credited by -- Thailand Entrance High School)

Strategy

the rule "Must have" of Mr.Zhang ®
Find the molecular formula ,
and later calculate the mass.

Solution

Element A; configuration atomic no=38
Energy level → 2, 8, 18, 8, 2
A is an ion,lose 2 electrons,is in group 2, 5 shells
outermost shell of A,there is 2 electrons
1s² 2s²2p⁶ 3s²3p⁶3d¹⁰ 4s²4p⁶ 5s²
Energy level → 2, 8, 18, 8, 2
5th period,group 2

Element B; configuration atomic no=35
atomic no=35
1s² 2s²2p⁶ 3s²3p⁶3d¹⁰ 4s²4p⁵
Energy level → 2, 8, 18, 7
4th period,group 7

Empirical formula of the molecular compound, A has a +2 charge
and B has a -1 charge , therefore 1A²+ ion is required to
balance 2 B^-

Now how to balance take the two ions... in this case A and B. That makes group IIA
and group VIIA. Therefore A is +2 and B is -1. Now reverse the numbers of the charges...
A now has 1 and B is now 2.

Therefore...two atoms of B and one atom of A has the chemical equation;