The positions of five elements, A, B, C, D and E, are shown in the periodic table - Edexcel - GCSE Chemistry - Question 5 - 2012 - Paper 1

Elements A and B have similar reactions because they are in the same group of the periodic table. Elements within the same group share similar chemical properties due to having the same number of electrons in their outer shell.

Mendeleev predicted the properties of element D based on the trends observed in the periodic table. He noted that elements in the same group exhibited similar properties. By examining the properties of surrounding elements and their atomic masses, he was able to infer the characteristics of element D, such as its reactivity and valence.

Since element C has 29 protons, the periodic nature of the table suggests that element D, being next in the sequence, would likely have more protons. Assuming that the elements are arranged in order of increasing atomic number, we can estimate that element D has 31 protons.

An atom with an atomic number of 9 has 9 protons located in the nucleus. The number of neutrons is found by subtracting the atomic number from the mass number: 19 (mass number) - 9 (atomic number) = 10 neutrons. This means there are 10 neutrons also in the nucleus. The electrons, which are equal to the number of protons, are arranged in energy levels around the nucleus. For an atomic number of 9, the electron configuration would be 2 electrons in the first energy level and 7 electrons in the second energy level.