Let's work out the force exerted per square metre of 0.76m of mercury. The density of mercury at room temperature is 13591 kg/m$^3$, so the mass of a block of mercury 1 square metre by 0.76 metres high is 10329 kg, and multiplying by the acceleration due to gravity 9.81 m/s$^2$ gives the weight as 101325N. The pressure is therefore 101325Pa, which is one atmosphere.

So one atmosphere is exactly the pressure exerted by 0.76m of mercury.

Let's work out the force exerted per square metre of $0.76m$ of mercury. The density of mercury at room temperature is 13591 kg/m$^3$, so the mass of a block of mercury 1 square metre by $0.76$ metres high is $10329 kg$, and multiplying by the acceleration due to gravity $9.81 m/s^2$ gives the weight as $101325N$. The pressure is therefore $101325Pa$, which is one atmosphere.

So one atmosphere is exactly the pressure exerted by $0.76m$ of mercury.