The first mechanistic step in the direct reaction of an amine with a carboxylic acid to produce an amide is:

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The first step involves protonation of the carbonyl carbon followed by attack of the Nitrogen of amine on it.

Subsequently a water molecule is lost resulting in amide formation.

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.