How might atoms of the same element vary?

Atoms of the same element can vary based on their atomic masses, basically the atomic mass units or amu. If there are isotopes of a given element present, then the atoms of all the elements posses the same atomic number and thus the same element, but can vary based on their atomic mass, due to the different number of neutrons that corresponding atom has. Isotopes are atoms of the same element, but with different number of neutrons and ultimately different atomic mass.

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.

As newton's first law states a object in motion stays in motion