First, isotopes <u>are the atoms of a single element whose nuclei have a different number of neutrons</u>, and therefore, differ in mass numbers. You should know that atoms are formed by a nucleus that has a small size and is made up of protons and neutrons. The nucleus is surrounded by a cloud of electrons, which are found in a region of the atom called the cortex.
The mass number, represented as A, <u>is the sum of the number of protons and neutrons in the nucleus</u>. On the other hand, the atomic number (Z) is <u>the number of protons that exist in the nucleus.
</u>
The isotopes of an element X are represented as follows,
<em>(see first attached picture)</em>
It should be noted that the number of neutrons of a chemical element can be calculated as the difference A-Z.
<u>The atomic and mass numbers of bismuth with 125 neutrons are</u>:
Z = 83
A = 83 + 125 = 208
Thus, the atomic symbol of the bismuth isotope with 125 neutrons is:
<em>(see second attached picture)</em>
Answer:
They test it using the scientific method.
Explanation:
Here I send you all the 3 elements that you are asking for.
Notice that first on the left you will find lewis structure, then condensed and finally chemical formula of each of the compound you enlisted.
Answer : The correct option is, (D) 
Explanation :
(A) 
This reaction is a double displacement reaction in which the cation and anion of two reactants are exchange their places to give two different products.
(B) 
This reaction is a decomposition reaction in which the larger molecule decomposes to give two or more products.
(C) 
This reaction is a neutralization reaction in which an acid and a base react to give a salt and water as a product.
(D)
This reaction is a redox reaction in which the oxidation and reduction reaction occur simultaneously.
Oxidation reaction is the reaction in which a substance looses its electrons. In this oxidation state increases.
Reduction reaction is the reaction in which a substance gains electrons. In this oxidation state decreases.
In this reaction, magnesium shows oxidation due to change in oxidation number from (0) to (+2) and hydrogen shows reduction due to change in oxidation number from (-1) to (0).
Hence, the correct option is, (D)
