Mendeleev examined the properties of elements in minute detail and grouped elements with similar properties together. When Mendeleev published his first periodic table of elements in $1869$ he left gaps in the table for as yet undiscovered elements, and hence elements fell into their correct groups. Mendeelev was therefore able to predict the properties of yet undiscovered elements at the time. The predictive power of Mendeleev's periodic table illustrates the "risk taking" nature of science. What is the distinction between scientific and pseudoscientific claims?