The base unit of standard matter that makes up a chemical element is an atom. Atoms that are neutral or ionised constitute matter whether it is solid, liquid or gaseous. The structure of an atom includes the nucleus, which is made up of neutrons and protons and holds the majority of the atom’s mass. The angstrom (A0), which is specified as 1 x 10-10 metres, is a reliable metric of length for determining atomic sizes and an atom has a diameter of around 2-3 A0.

A complicated configuration of negative electrons in specified shells around a  nucleus containing a positive charge makes up an atom. Every atom is made up of a centre (nucleus) and one or more than one electron connected to it. One or more than one protons and a couple of neutrons make up the nucleus but a type of hydrogen element which is the most prevalent lacks neutrons. 

The nucleus accounts for more than 99% of an atom’s mass. The positively charged protons, the negatively charged electrons and the neutrons (having no charge) make up the structure of an atom.  

The atom is neutral if the number of protons, as well as electrons, is identical. When an atom contains more / fewer electrons than the positively charged protons,  then it has a negative or positive overall charge, and these atoms are known as ions.

Atomic models that describe the structure of an atom

Throughout the development of atomic physics history, there have been a multitude of atomic models. This mostly pertains to the era from the start of the nineteenth century to the first part of the 20th century, when a definitive model of the atom was created. 

The atomic model by John Dalton

John Dalton had been an English physicist who proposed that almost all matter is made up of extremely microscopic particles. This was the first time a full description of all matter was attempted in regards to particles. Dalton used the term “atom” to explain these particles and came up with an atomic hypothesis to describe the structure of an atom

• All matter is composed of atoms, according to this idea, and atoms are indestructible and indivisible.
• Compounds comprise two / or more types of atoms.
• A chemical reaction involves the rearranging of atoms.

Because of the findings of subatomic particles & isotopes, parts of his hypothesis had to be changed. Because atoms are composed of neutrons, electrons, & protons.

JJ Thompson’s plum pudding model

People understood that atoms are composed of even tiny particles with the discovery of the electron in 1897. J. J. Thomson suggested his renowned “plum pudding model” immediately after, in 1904. Atoms were recognized to be composed of negative electrons in this concept, but the atom nucleus was not yet identified. Thomson recognized that an atom’s total charge was neutral. He reasoned that there had to be something to neutralise an electron’s negative charge. He proposed that negative particles float freely inside a puddle of dispersed positive charge because he resembles a famous English dessert, his model is known as the plum pudding model.

The atomic model by Rutherford

The first to argue that Thomson’s plum pudding model is inaccurate was Rutherford. In his novel idea, the nucleus is introduced to the atomic theory. The nucleus has a powerful central charge that is focused on a limited volume. The majority of the atom’s atomic mass is likewise contained in this little volume. Lighter, negatively charged electrons surround the nucleus. His atomic model is also referred to as the planet model. However, there were several significant flaws in this concept. Rutherford, for example, couldn’t describe why atoms exclusively emit light at specific frequencies. Niels Henrik David Bohr, a Danish scientist, later tackled the issue.

The atom model by Niel Bohr

The atom is described by the Bohr model as a core (nucleus that is positively charged) surrounded by electrons. Electrons move in circular paths, with electrostatic forces providing pull. The ground state is the electron’s usually occupied energy level. By absorption of energy, the electron might shift to a less stable state. The excited state refers to this greater energy level. By releasing the energy, the electron may return to its previous state. Overall, when an electron hops between orbits, a certain amount of energy is released/absorbed.

Atomic Quantum Mechanics Model/Electron Cloud Model

The Quantum Mechanics atomic model is now presented as the most realistic atomic model because it describes atomic mechanics as current research believes they operate. It was created by the convergence of several scientific presumptions:

1. All particles have a wavelength and can be seen as matter waves.
2. As a result of the prior assumption, an atomic model was presented that regards electrons as matter waves as well. 
3. We can’t possibly understand both the energy and the location of an electron, according to the concept of uncertainty. As a result, we know less about the electron’s energy as scientists learn more about its location, & vice versa. 
4. In the atom, there are several energy levels for electrons. Electrons are allocated to certain atomic orbitals, which vary in energy from each other.
5. Electrons have an inherent feature called spin, as well as an electron could have either a spin-up/or spin-down value. The spins of any two electrons in the same orbit must be opposing. 

The atomic model in quantum mechanics

According to quantum mechanics, electrons can move all around the nucleus not on particular electron pathways, but in a three-dimensional space called the atomic orbital, in which their incidence has a certain probability. This implies that their position could not be determined with 100% precision.

Conclusion

To conclude, the structure of an atom is made up of a complex arrangement of negatively charged electrons in certain shells surrounding a nucleus with a positive charge. It is hard to predict the behaviour of quantum events using classical physics because of their complexity.

Every atom consists of a core/nucleus and one or more electrons that are attached to it. The atom is composed of one or more protons and a few neutrons.