ATOM - ELECTRONS ARRANGEMENT

 The arrangement of electrons in an atom is determined by the principles of quantum mechanics. According to the atomic theory, each electron in an atom is described by a set of quantum numbers that determine its energy level, the shape of the region it occupies (its orbital), and its orientation and spin. The Pauli exclusion principle states that no two electrons in an atom can have the same set of quantum numbers, which leads to a specific arrangement of electrons in an atom.

In the case of an oxygen atom, it has 8 electrons. These electrons fill up the energy levels and orbitals in a specific order.

1. The first two electrons fill the 1s orbital,
2. The next two fill the 2s orbital,
3. The remaining four fill two of the three available 2p orbitals. This is often written as 1s² 2s² 2p⁴, following the Aufbau principle, which states that electrons fill lower-energy orbitals before they fill higher-energy ones.

It's true that the electrons form a "cloud" around the nucleus, but this cloud is not uniform. Instead, the cloud is denser in the regions where the electrons are more likely to be found, which are determined by their orbitals. The shape and size of these orbitals, and therefore the shape and size of the electron cloud, are determined by the electrons' quantum numbers.

The number of electrons in an atom (in its neutral state) is equal to the number of protons in the atom's nucleus, which determines the atomic number and identifies the element. So an atom with 8 protons in its nucleus is an oxygen atom, and if it's neutral, it will also have 8 electrons. We can identify the element based on its atomic number (which is the number of protons), and predict its electron configuration based on quantum mechanical principles.

In experimental settings, techniques like spectroscopy can be used to study the electrons in an atom. When an atom is excited (for example, by heating it or exposing it to light), its electrons can absorb energy and move to a higher-energy orbital. When they fall back down to a lower-energy orbital, they emit light of a specific wavelength. By studying the wavelengths of light emitted by an atom, scientists can determine the energy levels of its electrons and thus its electron configuration. Different elements have unique spectral lines which act like their fingerprint.