Explain why the relative atomic mass of cobalt is greater than the relative atomic mass of nickel, even though the atomic number of nickel is greater than the atomic number of cobalt. (1) (b) Deduce the numbers of protons and electrons in the ion
. (c) Deduce the electron configuration for the ion
. (1) (1) (Total 3 marks)
The Correct Answer and Explanation is:
(a) Explanation of the Relative Atomic Mass of Cobalt vs Nickel:
The relative atomic mass of an element is determined by the weighted average mass of its naturally occurring isotopes, taking into account both their masses and relative abundances. The atomic number, on the other hand, refers to the number of protons in an element’s nucleus, which defines the element.
Even though nickel (Ni) has a higher atomic number (28) than cobalt (Co) (27), the relative atomic mass of cobalt is greater than that of nickel. This is because the isotopic distribution and the masses of the isotopes of cobalt are such that the weighted average mass is slightly higher than that of nickel. Cobalt has two main isotopes, Co-59 and Co-60, with Co-59 being the most abundant, while nickel has a heavier isotope, Ni-58, and also a significant amount of the isotope Ni-60. The isotopic masses of the elements and the relative abundances of these isotopes influence the final weighted average atomic mass. Therefore, cobalt’s isotopes result in a higher average atomic mass despite its lower atomic number.
(b) Number of Protons and Electrons in the Ion:
In order to deduce the number of protons and electrons in the ion, we need to know the charge on the ion. Let’s assume the ion in question is Co²⁺ (cobalt ion with a +2 charge) and Ni²⁺ (nickel ion with a +2 charge). For both these ions:
- The number of protons in Co²⁺ (cobalt) is the same as the atomic number of cobalt, which is 27.
- Similarly, the number of protons in Ni²⁺ (nickel) is the same as the atomic number of nickel, which is 28.
In the case of a +2 charge, the number of electrons is less than the number of protons by 2. Therefore:
- For Co²⁺, the number of electrons = 27 (protons) – 2 = 25 electrons.
- For Ni²⁺, the number of electrons = 28 (protons) – 2 = 26 electrons.
(c) Electron Configuration for the Ion:
The electron configuration of an atom follows the order of filling of orbitals based on energy levels. For cobalt (Co) and nickel (Ni), the neutral atoms have the following electron configurations:
- Cobalt (Co): [Ar] 3d⁷ 4s²
- Nickel (Ni): [Ar] 3d⁸ 4s²
For the Co²⁺ ion, cobalt has lost 2 electrons. The electrons are removed first from the highest energy orbitals, which are the 4s orbitals. So, the electron configuration of Co²⁺ becomes:
- Co²⁺: [Ar] 3d⁷
For the Ni²⁺ ion, nickel also loses 2 electrons from the 4s orbital. So, the electron configuration of Ni²⁺ becomes:
- Ni²⁺: [Ar] 3d⁸
Thus, the electron configurations for the ions are:
- Co²⁺: [Ar] 3d⁷
- Ni²⁺: [Ar] 3d⁸
