If I cut a ball of mercury with a knife it forms into other balls why is this
Because mercury is a fluid, and has no rigid shape. Therefore, its form is determined moment-by-moment by the forces acting upon it. These include outside forces like gravity and air pressure, and internal forces such as the cohesion of the mercurial atoms to one another. The original "ball" of mercury is formed by the opposition of forces like gravity gravity pushing down on the substance while the cohesion of the atoms resists being spread out. When the blob of mercury reaches a point where these two forces are equal, it ceases to change shape incomes to rest. Since this point of equilibrium is the same distance from the center of the blob in all directions, the mercury forms into a roughly spherical shape. Separate the original message into two masses, and both are being acted upon by all the same forces as the original mass, and therefore tend to arrive at a very similar final "shape".
I would assume the surface tension in mercury is extremely large (compared to other substances), so that its contact area with the external world becomes as small as possible (i.e., spherical shape for a given volume), more easily than for other substances.
Also, Hg is a metal. That means the interatomic forces (cohesive) are HUGE. Insert this into LURCH's explanation to understand why a drop of Hg always balls up.
PS : Surface tension is a result of cohesive forces - so would also be large. Hence, Hg columns always have a convex meniscus, unlike water columns, which have a concave meniscus.
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