That which causes an object to have inertia

Inertia. The first law of motion is often called the law of inertia. ... Inertia causes a moving object to stay in motion at the same velocity (speed and direction) unless a force acts on it to change its speed or direction. It also causes an object at rest to stay at rest.

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.