How many elements were uncovered during the discovery of Mendeleev’s periodic table?

Which of the following element was discovered after the discovery of Mendeleev’s periodic table?

Describe how Mendeleev convinced the scientific community that his way of organising the elements into a periodic table was correct.

What breakthrough did Mendeleev make in devising a periodic table that was accepted by the scientific community?He arranged the elements in order of atomic weightHe left gaps in the table for unknown elementsHe devised the ‘law of octaves’He published the first full list of all the elements2Which of the following statements about the modern periodic table is not true?The number of shells decreases down a groupElements in the same group have the same number of electrons in the outer shellElements are arranged in atomic number orderElements in the same period have the same number of shells3Where are the non-metal elements found in the periodic table?On the rightAt the topAt the bottomOn the left4Which of the following elements, shown by its electronic structure, is found in group 2 of the periodic table?2, 8, 22, 72, 8, 12, 8, 45Why do fluorine, chlorine and iodine all have similar chemical properties?They are all metalsThey all have 7 electrons in their outer shellThey are all in the same periodThey all have odd numbers of electrons

Mendeleev left gaps in his periodic table for elements that hadn't been discovered yet.Identify which features of these elements he made predictions about.Their atomic weights onlyTheir atomic weights and propertiesTheir properties only

Why did Mendeleev leave gaps in his version of the periodic table?