Mass number = protons + neutrons
mass number = 5 + 7
mass number = 12
Answer:
1. hydrogen - H
2. helium - He
3. sodium - Na
4. magnesium - Mg
5. potassium - K
Explanation:
Hydrogen is the element of group 1 and first period. The atomic number of hydrogen is 1 and the symbol of the element is H.
The electronic configuration of the element hydrogen is:-

Helium is the element of group 18 and first period. The atomic number of helium is 2 and the symbol of the element is He.
The electronic configuration of the element helium is:-

Sodium is the element of group 1 and third period. The atomic number of sodium is 11 and the symbol of the element is Na.
The electronic configuration of the element sodium is:-

Magnesium is the element of group 2 and third period. The atomic number of magnesium is 12 and the symbol of the element is Mg.
The electronic configuration of the element magnesium is:-

Potassium is the element of group 1 and forth period. The atomic number of potassium is 19 and the symbol of the element is K.
The electronic configuration of the element potassium is:-

Answer: a. The concentrations of the reactants and products have reached constant values
Explanation:
The reactions which do not go on completion and in which the reactant forms product and the products goes back to the reactants simultaneously are known as equilibrium reactions. For a chemical equilibrium reaction, equilibrium state is achieved when the rate of forward reaction becomes equal to rate of the backward reaction.
Equilibrium state is the state when reactants and products are present but the concentrations does not change with time and are constant.
Equilibrium constant is defined as the ratio of concentration of products to the concentration of reactants each raised to the power their stoichiometric ratios. It is expressed as 
K is the constant of a certain reaction when it is in equilibrium, while Q is the quotient of activities of products and reactants at any stage other than equilibrium of a reaction.
For a equilibrium reaction,

![K_{eq}=\frac{[B]}{[A]}](https://tex.z-dn.net/?f=K_%7Beq%7D%3D%5Cfrac%7B%5BB%5D%7D%7B%5BA%5D%7D)
Thus the correct answer is the concentrations of the reactants and products have reached constant values.
The artificial fixation of nitrogen (N2) has enormous energy, environmental, and societal impact, the most important of which is the synthesis of ammonia (NH3) for fertilizers that help support nearly half of the world's population.
<h3>Artificial fixation of nitrogen</h3>
a) The equilibrium constant expression is Kp=PCH4 PH2 OP CO×PH 23.
(b) (i) As the pressure increases, the equilibrium will shift to the left so that less number of moles are produced.
(ii) For an exothermic reaction, with the increase in temperature, the equilibrium will shift in the backward direction.
(iii) When a catalyst is used, the equilibrium is not disturbed. The equilibrium is quickly attained
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