5. (a) Define an atomic orbital - Leaving Cert Chemistry - Question 5 - 2017

The electron configuration of a carbon atom in its ground state is:

1s² 2s² 2p².

This indicates that the carbon atom has two electrons in the first shell (1s), two electrons in the second shell (2s), and two electrons in the 2p subshell.

There are a total of 4 orbitals occupied in a carbon atom:

• 1s has 1 orbital.
• 2s has 1 orbital.
• 2p has 3 orbitals, but only 2 are filled.

Thus, 1 (1s) + 1 (2s) + 2 (in 2p) = 4 occupied orbitals.

It is difficult to specify the absolute boundary of an atom due to Heisenberg's uncertainty principle, which states that it is impossible to accurately measure both the position and momentum of an electron simultaneously. Additionally, the behavior of electrons is described by wave functions that vary in probability rather than define a fixed boundary. Thus, describing an atom's boundary becomes more about understanding regions of probability rather than precise locations.

The covalent radius of an atom is defined as the distance between the nuclei of two atoms of the same element that are bonded together by a single covalent bond, divided by two. This provides an estimate of the size of the atom when it forms a covalent bond.

The covalent radii generally increase down Group 1 of the periodic table. This increase can be accounted for by noting that each successive element adds a new electron shell, which increases the effective shell number. As the atomic number increases, the inner electron shells shield outer electrons, reducing the effective nuclear charge experienced by the outermost electrons. Thus, the atomic size increases, leading to larger covalent radii.

The first ionisation energy is defined as the amount of energy required to remove the most loosely bound electron from a neutral atom in its gaseous state to form a cation.

First ionisation energy values generally increase across the second period of the periodic table due to the increase in effective nuclear charge as protons are added to the nucleus. This results in a stronger attraction between the positively charged nucleus and the negatively charged electrons, making it more difficult to remove an electron and thus requiring more energy.

The table of successive ionisation energies for carbon is as follows:

In a carbon atom, the distribution of electrons among the main energy levels is as follows:

• First energy level (n=1): 2 electrons (1s²)
• Second energy level (n=2): 4 electrons (2s² 2p²)

Thus, there are 2 electrons in the first energy level and 4 in the second energy level.