An atom is most stable when it has eight electrons in its outermost energy level.
Answer:
Scientific models are used to explain phenomena that can not be experience directly and it is also used for prediction.
Explanation:
An example of idea model is the Eistein equation. An idea model shows how things interact together to produce a particular result.
An example of physical model is the solar system.
Computer model is used to predict long term events and an example of this is computer simulations.
It is called a polyatomic ion.
Hope this helps!!!
As long as the equation in question can be expressed as the sum of the three equations with known enthalpy change, its 
can be determined with the Hess's Law. The key is to find the appropriate coefficient for each of the given equations.
Let the three equations with given be denoted as (1), (2), (3), and the last equation (4). Let 
, 
, and 
be letters such that 
. This relationship shall hold for all chemicals involved.
There are three unknowns; it would thus take at least three equations to find their values. Species present on both sides of the equation would cancel out. Thus, let coefficients on the reactant side be positive and those on the product side be negative, such that duplicates would cancel out arithmetically. For instance, 
shall resemble the number of 
left on the product side when the second equation is directly added to the third. Similarly
Thus
and
Verify this conclusion against a fourth species involved- 
for instance. Nitrogen isn't present in the net equation. The sum of its coefficient shall, therefore, be zero.
Apply the Hess's Law based on the coefficients to find the enthalpy change of the last equation.
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