Answer:
3. Equal numbers of  protons and neutrons
Explanation:
The nucleus becomes unstable if the ratio of protons to neutrons is less than 1:1 or more than 1:1.5.
The most stable nucleus has a neutron proton ratio of 1:1 which means that they can not release a neutron or a proton to decay. 
Nucleus 3 is therefore the most stable.
 
        
             
        
        
        
Answer:
Their positive charge is located in the small nucleus
Explanation:
Ernest Rutherford performed the gold foil experiment in 1911 where he used alpha particles generated from a radioactive source to bombard a thin gold foil. 
In his experiment, he observed that the bulk of the alpha particles passed through the gold foil, just a tiny fraction was deflected back. To explain his findings, Rutherford proposed that an atom is made of positively charged centre where nearly all the mass is concentrated called nucleus. Surrounding the nucleus is a large space containing electrons. 
 
        
                    
             
        
        
        
Answer:
its atomic number
Explanation:
the mass number/atomic mass is how many protons and neutrons are combined in an element. But, the atomic number is just protons.
 
        
             
        
        
        
Answer:
The unknown element is Carbon.
(a) X is Carbon, its oxide is of acidic nature
(b) C+ O2 → CO2
Explanation:
We are given an unknown element that reacts with oxygen.
 X + O2 → XO2
It is given that the oxide XO2 turns blue litmus red which is shown by acids, so XO2 is an acidic oxide. Also X will be a Non metal because Non metallic oxide is Acidic in nature.
 The Oxide XO2 is dissolving is water. The oxide upon reacting with water will give an Acid.
XO2 type of oxide is found in the 14th group of the periodic table generally 
So we have options like C, Si and Ge as these only have the Acidic oxide.
But when it comes to reactivity with water Only Carbon satisfies the need so the unknown element X is C.
CO2 + H2O → H2CO3 which is an acid.
(a) Element X is Carbon (C), The nature of the compound formed is Acidic.
(b) The reaction given will be as follows
      C+ O2 → CO2
Therefore the unknown element is Carbon.