Do you think scientists have "discovered" all the elements on Earth or do you think there are other elements to be found or created?
An atom is the smallest component of an element having the chemical properties of the element.
The nice thing about the periodic table is that there are no gaps present. We start with the smallest element in nature with only one proton (hydrogen) and work our way up to elements with over 100 protons without any gaps or missing elements if you will. Since protons are found only in discrete numbers (1,2,3,4,etc.) we know that there is nothing in the middle that we have missed. For example, sulfur has 16 protons and chlorine has 17 protons. There will never be found an element between the two because it would require a fraction of a proton which is not found.
The heaviest stable atom found in nature is Bismuth (Bi: 83 protons). All elements on the periodic table with higher atomic numbers are ultimately unstable and decay to smaller particles. The heaviest atom found in nature is Uranium (U: 92 protons) but we all know that element is unstable. So all of the naturally occurring elements on Earth have already been discovered.
However, scientists have been making heavier and heavier elements in a laboratory setting for years now. Particle colliders are used to smash atoms together to creater new, heavier elements but they only exist for a small amount of time and only under these highly restrictive conditions. The heaviest element on record has 118 protons and is called Ununoctium (Uuo) but it can only exist for a fraction of a second after being created. Is there an upper limit to how large an atom can be made in a lab? We don't know but scientists will certainly continue to try and create new, even heavier elements. So new elements will probably be created in the future although they will only exist for a fraction of a second.
No, I don't think scientist have discovered all elements on earth. When the periodic table was first made there were only a few elements, but now we are already in the hundreds. I believe there are more elements, they just haven't been discovered yet. It is my opinion that as time goes on and technology keeps improving there are chances they will discover more elements.