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Occurs when two or more substances have the same molecular formula, but different structures and properties.
The term Isomery was created by Berzelius in 1830. He considered the Greek words (iso = same(s); meres = parts) to give the idea of "same composition".
(https://www.manualdaquimica.com/quimica-organica/o-que-isomeria.htm, access on: 09/10/21)

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Lewis structures, devised in 1916 by the physicist-chemist Gilbert Newton Lewis, explore the nature of covalent bonds and the structure of matter.
These are formulas that represent the bonds and all other electrons in the valence shell in each atom by dashes and dots, respectively.
(SOLOMONS, 2018)

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Resonance structures are Lewis structures that consists on the fact that there are many ways to represent one single molecule, due to the movement of pairs of free electrons within the molecule.

In order to identify and represent the resonance structures, there are five patterns that need to be recognized: (1) an isolated allylic pair; (2) an allylic positive charge; (3) an isolated pair adjacent to a positive charge; (4) a π bond between two atoms with different electronegativities; (5) π bonds conjugated in a ring.
(KLEIN, 2017)

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imagem 1 exemplo de estrutura de ressonância

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A formal charge is associated with any atom that does not exhibit the appropriate number of valence electrons. When such atom is present in a Lewis structure, the formal charge must be drawn. To identify a formal charge it is required two discrete tasks:
(1) Determine the appropriate number of valences electrons for an atom.
(2) Determine whether the atom exhibits the appropriate number of electrons.
Different forms of resonance of a chemical species can have atoms with different formal charges.

(KLEIN, 2013 and https://regradetres.wordpress.com/2013/05/12/calculo-da-carga-formal/ , access on: 09/10/2021)

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Chirality is an important geometric property related to the symmetry of the molecule. A chiral molecule does not overlap its mirror image (they are like our hands). (SOLOMONS, 2018)

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The chemists Justus von Liebig and Friedrich Wöhler were responsible for discovering the first isomers: silver fulmanate and silver cyanate. They found out that the formulas of the two molecules were identical (AgCNO), but the position of the ligands were different.
(mundoeducacao.uol.com.br , access on: 09/10/2021)

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Example of how curved arrows are used in resonance.

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Curved arrows are used to simulate a hypothetical electron movement, as if they were leaving one atom and going to another one.
In the representation, the tail of the arrow is at the origin position of the electrons, and the tip is indicating where they will be in the next structure.
(SOLOMONS, 2018)

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In large molecules, instead of using Lewis structures, bond-line structures are more commonly used for simplifying the drawing process and being easier to read.

Bond-line structures are drawn in a zigzag format, where each corner or endpoint represents a carbon atom. Simple bonds are shown with one line, double bonds are shown  with two lines and triple bonds are shown with three lines. Hydrogen atoms bonded to carbon are not shown in this type of structure.

(KLEIN, 2013)

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foto 5 exemplo de estrutura ligação-linha

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Resonance hybrids, that happen between pairs of free electrons and bonds, are the combination of all the main resonance structures of a given molecule.
Their representation are usually made by partial charges and partial bonds. For this, we use dashed lines and the lowercase delta with the reference charge to represent the partiality of bonds and charges, respectively.
(SOLOMONS, 2018)

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Assessment of the importance and contributions of resonance structures

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Assessment of the importance and contributions of resonance structures

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There are many resonance structures for just one molecule. So an order of evaluation was established so that we know the most important ones. The assessment (described in descending order) are:
1 - those with the highest number of chemical bonds
2 - those that have all atoms with the complete valence layer
3 - those that don't have load separation
4 - and those with negative charge on the most electronegative elements
(SOLOMONS, 2018)

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In drugs, one stereoisomer may be the main responsible for the desired therapeutic action and the other for the side effects. These isomers are marketed in their enantiomerically pure forms. For example: In the case of L-Dopa, used as an antiparksonian, most side effects, such as granulocytopenia, are due to the D isomer.
(LIMA, Vera Lucia Eifler. Os fármacos e a quiralidade: uma breve abordagem, 2008)

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What are localized and delocalized electrons?

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What are localized and delocalized electrons?

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The localized free electron pairs are those that do not allow us a new resonance structure when changing their position. On the other hand, as we can imagine, delocalized free electron pairs are those who make new structures possible by changing their position.
(SOLOMONS, 2018)

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Cyclohexane has a dynamic structure. A synchronized rotation around the carbon-carbon bonds changes a chair conformation to another chair conformation. This interconversion involves an energy barrier of 10.8 kcal mol⁻¹.
(SOLOMONS, 2018)

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Cis-trans isomerism occurs especially in open-chain compounds that have a pi bond between two carbon atoms. When there are identical ligands on the same side, it's a cis isomer. When ligands on the same side are different, it's a trans isomer. (SOLOMONS, 2018)

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Chiral molecules can show their laterality in many ways, including how they affect humans. One of the enantiomeric forms of a compound called limonene is primarily responsible for the odor of oranges, and another enantiomer is responsible for the odor of lemons.
(SOLOMONS, 2018)

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When the resonance occurs, the atom who loses the pair of free electrons takes on a positive charge while the atom who gains the pair of  free electrons takes on a negative charge. So, with that there is the spreading of loads, wich is called deslocalization. The deslocalization of a positive or negative charge stabilizes a molecule, and that stabilization is also called resonance stabilization.
(KLEIN, 2017)