Oxygen cylinders, when fully charged, are heavier than empty oxygen cylinders because oxygen has mass. Mass is a fundamental property of matter and refers to the amount of material an object contains. Oxygen, like any other substance, has mass because it is made up of atoms and molecules.
In the case of oxygen cylinders, the weight difference between a full cylinder and an empty one can be attributed to the mass of the oxygen gas contained within them. When the cylinder is filled with oxygen, it becomes denser and gains mass. The increase in mass is a result of the oxygen molecules occupying the volume of the cylinder, contributing to its weight.
It's important to distinguish between weight and mass. Mass is an intrinsic property of an object and remains constant regardless of the gravitational field. Weight, on the other hand, refers to the force exerted on an object due to gravity. Weight can vary depending on the strength of the gravitational field. However, when we use the term "weight" colloquially, we often refer to the force of gravity acting on an object, which is proportional to its mass.
So, while oxygen itself does not have weight in the absence of gravity, oxygen molecules do possess mass, and this mass contributes to the weight of an oxygen cylinder when it is filled.