The elements of a delta hf of kj\mole at standard temp and pressure

One mole of a substance forms Instead of standard atmospheric pressure and temperature from its pure elements{ conditions being the same} then this change and enthalpy is known as enthalpy of formation.

Explanation:

The equation of this formation can let us know what actually happens

ΔH = ΔHf(products) - ΔHf (reactants)

Hence we can say this change in enthalpy formation is the difference between the sum of product and sum of reactants.

1.solid

2.carbon dioxide

3.helium

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.