For statements I and II, state for each if they are true or false. I) Atoms with an electron configuration of [Ne] 3s² have larger radii than chlorine atoms in their ground state. II) Chlorine atoms have a higher nuclear charge in their ground state than atoms with an electron configuration of [Ne] 3s². If both are true, state if II is a correct explanation for I.
Let’s first have a look at which element atom would have an electron configuration of [Ne] 3s². When a noble gas symbol is written in square brackets, this refers to the core shells of the element. This element has 10 electrons in its core shells. The configuration written outside the square brackets refers to the valence shells. This element has two electrons in its valence shell. Adding the 10 and two electrons, we get a total of 12 electrons, which corresponds to the element magnesium because magnesium has an atomic number of 12. Atomic number is the number of protons in the nucleus of an atom or ion. For magnesium, there are 12 protons.
In statement a), they refer to atoms, which are electrically neutral. And in atoms, the number of protons must equal the number of electrons. So, the number of electrons in a magnesium atom is 12. And this is how we identify the element as being magnesium, from the 12 electrons in the electron configuration. We now know that the atoms referred to in statement I are magnesium atoms. Let’s now compare magnesium and chlorine in terms of their atomic radii. Magnesium has 12 protons in its nucleus. And chlorine with atomic number 17 has 17 protons in its nucleus. Magnesium’s 12 electrons and chlorine’s 17 electrons fill in into shells in the following manner. Two electrons into the first shell, eight electrons into the second shell, two electrons into the third shell for magnesium, and seven into the third shell for chlorine.
The positively charged protons attract the electrons in the shells towards the nucleus. Because there is a large number of protons in the nucleus of chlorine relative to that of magnesium. The electrons in the outer shell of chlorine are attracted much more strongly towards the nucleus of chlorine than the electrons in magnesium’s outer shell are attracted to magnesium’s nucleus. This is as a result of a large effective nuclear charge in chlorine. Effective nuclear charge or 𝑧 subscript eff is the net positive charge experienced by valence electrons. The effect of nuclear charge of chlorine is much higher than that of magnesium.
The results of this high effective nuclear charge in chlorine and the strong attraction for outer electrons in chlorine is a pulling effect on those electrons towards the nucleus of chlorine. Thus, a chlorine atom is in reality much smaller than a magnesium atom. So, for statement I, it is true to say that atoms with an electronic configuration of [Ne] 3s² or magnesium have a larger radii than chlorine atoms in their ground state. Statement II is also true. Chlorine atoms do indeed have a higher nuclear charge in their ground state than atoms with an electronic configuration of [Ne] 3s². So, statement II is a correct explanation for statement I. And our last statement is thus also true.