What will happen if a small amount of hydrochloric acid is added to a 0.1 m solution of hf?

pH will go down (become more acidic)

Explanation:

This is because HCl is strong acid that means it dissociates completely into H+ and Cl- ions. There's already some H+ ions in solution contributed by HF but addition of HCl supplies more H+ which drives the pH of solution down making the solution more acidic.

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.