1. Energy Levels and Shells:
* Energy Levels: Electrons exist in specific energy levels around the nucleus. These levels are denoted by the principal quantum number (n), which can be any positive integer (1, 2, 3, etc.). Higher numbers indicate higher energy levels.
* Shells: Each energy level corresponds to an electron shell. The first shell (n=1) is closest to the nucleus, and subsequent shells are further away.
2. Subshells and Orbitals:
* Subshells: Each electron shell is divided into subshells, denoted by letters (s, p, d, f).
* s-subshell: Contains only one orbital, which can hold up to 2 electrons.
* p-subshell: Contains three orbitals, each holding up to 2 electrons (total of 6 electrons).
* d-subshell: Contains five orbitals, each holding up to 2 electrons (total of 10 electrons).
* f-subshell: Contains seven orbitals, each holding up to 2 electrons (total of 14 electrons).
* Orbitals: Each subshell is composed of one or more atomic orbitals, which are regions of space where there is a high probability of finding an electron.
3. Filling Order and Hund's Rule:
* Aufbau Principle: Electrons fill orbitals starting from the lowest energy level and moving up.
* Pauli Exclusion Principle: Each orbital can hold a maximum of two electrons, and these electrons must have opposite spins (represented as spin up ↑ and spin down ↓).
* Hund's Rule: When filling orbitals within a subshell, electrons will occupy each orbital individually before pairing up in the same orbital.
4. Electron Configuration:
The electron configuration of an atom describes the distribution of electrons in its orbitals. For example, the electron configuration of oxygen (O) is 1s²2s²2p⁴, meaning:
* 1s²: The first shell (n=1) has 2 electrons in the s-subshell.
* 2s²: The second shell (n=2) has 2 electrons in the s-subshell.
* 2p⁴: The second shell (n=2) has 4 electrons in the p-subshell (two orbitals filled with one electron each, and one orbital filled with two electrons).
Example:
Let's consider the element nitrogen (N), which has 7 electrons.
1. Energy Levels: The first two energy levels (n=1 and n=2) are filled.
2. Subshells: The first shell has only the s-subshell (1s). The second shell has the s and p subshells (2s and 2p).
3. Orbitals: The 1s subshell has one orbital, the 2s subshell has one orbital, and the 2p subshell has three orbitals.
4. Electron Configuration:
* 1s²: The 1s orbital is filled with 2 electrons.
* 2s²: The 2s orbital is filled with 2 electrons.
* 2p³: The 2p orbitals are filled with 3 electrons. Each of the three 2p orbitals receives one electron (Hund's rule).
Therefore, the electron configuration of nitrogen is 1s²2s²2p³.
In summary:
The placement of electrons in orbital shells follows a specific order based on energy levels, subshells, orbitals, and a few key rules: the Aufbau principle, Pauli exclusion principle, and Hund's rule. These rules determine the electron configuration of an atom, which is a fundamental aspect of its chemical behavior.
