Why is the concentration of halides in the ocean more than in lakes?

Answer 1

The hailde component of salts (cl, fl, br, etc) is contributed to seawater mostly from volcanic processes. The continuous activity at the spreading ridges provides an almost constant supply injected directly into the water as gasses. These halides are then available to combine with ions like sodium and potassium (weathered from continental rocks and carried to the ocean by rivers) to form salts (sodium chloride or potassium chloride for example). While lakes receive the sodium and potassium weathering products just as the ocean does, they lack the volcanic processes needed to provide the chloride (and other halide) ions. Here's a good link if you want to know more. Hope this helps a little.
http://www.usask.ca/geology/classes/geol206/geol206rr2.html

Answer 2

In addition to what GatorGal said there is a more fundamental reason. The ocean basins have no outlet so the concentrations of minerals including salts build up over millions of years. Lakes on the other hand have an outlet and are therefore flushed, the mineral content reaches a steady state. Lakes that have no outlet eventually become salty for the same reason as the oceans.