This paper presents a mechanical mass sensor with an unprecedented resolution of 1.7 yg (1 yg = 10^-24 g), equivalent to the mass of a proton or hydrogen atom. The sensor is based on a 150 nm long carbon nanotube resonator vibrating at nearly 2 GHz. The device's sensitivity allows for the detection of adsorption events of naphthalene molecules and the measurement of the binding energy of a xenon atom on the nanotube surface (131 meV). The key to achieving this high sensitivity involves using a short nanotube, ultra-high vacuum conditions, and low-noise motion detection. Annealing the nanotube with a large current reduces fluctuations in the resonance frequency, enhancing the mass resolution. The sensor's ability to monitor adsorption events with atomic-level precision opens new possibilities for mass spectrometry, magnetometry, and adsorption experiments.This paper presents a mechanical mass sensor with an unprecedented resolution of 1.7 yg (1 yg = 10^-24 g), equivalent to the mass of a proton or hydrogen atom. The sensor is based on a 150 nm long carbon nanotube resonator vibrating at nearly 2 GHz. The device's sensitivity allows for the detection of adsorption events of naphthalene molecules and the measurement of the binding energy of a xenon atom on the nanotube surface (131 meV). The key to achieving this high sensitivity involves using a short nanotube, ultra-high vacuum conditions, and low-noise motion detection. Annealing the nanotube with a large current reduces fluctuations in the resonance frequency, enhancing the mass resolution. The sensor's ability to monitor adsorption events with atomic-level precision opens new possibilities for mass spectrometry, magnetometry, and adsorption experiments.