9. Magnesium 9 (Mg), Calcium (Ca) and Barium (Ba) are all in the same group on the periodic
table. What do these elements have in common?
A. They all have the same atomic number
B. They all have the same number of electrons
C. They all react with substances in similar ways
D. They were all discovered by Dmitri Mendeleev

C. They all react with substances in similar ways

Explanation:

First of all, let's define a few quantities:

- Atomic number: it is the number of protons in the nucleus of an atom. For neutral atoms, the atomic number is also equal to the number of electrons in the atom (since neutral atoms have same number of protons and electrons)

- Valence electrons: these are the electrons in the most external shell of the atom. When the outermost shell is full, the element does not react, since it does not easily gain/give away electrons. When the outermost shell is not full, the atom tends to gain or give away electrons in order to complete this shell, so the atom is more reactive.

Now we can analyze the different statements:

A. They all have the same atomic number  --> false: magnesium has atomic number 12, calcium has atomic number 20, barium has atomic number 56

B. They all have the same number of electrons  --> false: since the atomic number of the 3 elements is different, they have different number of electrons

C. They all react with substances in similar ways  --> true: these 3 elements are all in the same group of the periodic table, this means they have the same number of valence electrons, so they all react in the same way

D. They were all discovered by Dmitri Mendeleev --> false: these elements were already known before Mendeleev, he just re-arranged them into the periodic table