Concept of Atom, Element, Molecule and Compound

 

The study of matter and its interactions with one another is the focus of the scientific field of chemistry. Anything with mass and space is considered to matter. It is composed of atoms, which are minuscule building blocks that come together to create molecules and compounds. In order to plan experiments, anticipate chemical reactions, and comprehend the characteristics of matter, it is essential to understand the fundamental notions of atom, element, molecule, and compound.

Matter:

Anything that has mass and takes up space is considered to be matter. It is the material that makes up the cosmos and it manifests in many shapes with distinct properties. The types of matter include:

1. Solid: A material that is solid has a specific form and volume. Solids are made up of densely packed particles that vibrate about their fixed locations.

2. Liquid: A condition of a substance known as liquid has a specified volume but no distinct form. Liquids' constituent particles can circulate around one another because they are less tightly packed than those of solids.

3. Gas: A condition of a substance known as gas lacks both a definite form and a specified volume. Gases are made up of free-moving particles that take up the full capacity of their container.

Matter

Substance:

A substance is a category of matter that possesses unique physical and chemical qualities as well as a consistent makeup. Pure form of matter is known as substance. This indicates that a material has a unique set of properties that identify it and set it apart from other substances, even if it is composed of the same fundamental particles whose ratio of composition does not vary. Although substances can exist in several states of matter, including solid, liquid, and gas, and may change state and experience chemical reactions, they always retain their identity. Water, sodium chloride, copper, oxygen, and carbon dioxide are a few examples of substances.

Atom:

The fundamental building block of matter, an atom nonetheless has the chemical characteristics of an element. It is the tiniest elemental particle capable of taking part in a chemical process. A nucleus, protons, neutrons, and electrons make up an atom. The protons and neutrons that make up the atom's nucleus are located in its center, while the electrons that surround it are negatively charged particles. The mass number of an element is determined by the total number of protons and neutrons, whereas the atomic number of an element is determined by the number of protons in the nucleus.

Element 

Element:

A pure material with only one kind of atom makes up an element. Because of the uniqueness of its atomic number, its atom's nucleus always has the same number of protons. The distinctive chemical symbol for each element, such as H for hydrogen, O for oxygen, or Na for sodium, is used to identify it. There are 118 known elements, 92 of which are found naturally while the rest are created artificially.

The fundamental components that make up all matter in the universe are known as elements. By using standard chemical processes, they cannot be broken down into simpler chemicals. The elements are arranged according to their atomic structure in a chart called the periodic table. Each element has a distinct symbol made up of one or two letters, and the atomic number of an element determines where it belongs in the periodic table. The 118 elements in the periodic table are categorized into groups and periods based on their related characteristics and electron configurations. Based on their chemical and physical characteristics, the elements in the periodic table are separated into three main groups: metals, nonmetals, and metalloids (or semimetals).

Molecule:

Two or more atoms linked together by chemical bonds make up a molecule. These chemical connections might be ionic, polar, or covalent. Ionic bonds are created when electrons are transferred from one atom to another, whereas covalent connections are created when electrons are shared across atoms. When atoms share their electrons inequitably, polar covalent connections are created. Both homonuclear and heteronuclear molecules are possible. A heteronuclear molecule combines atoms of several elements, whereas a homo-nuclear molecule has only atoms of the same element.

Molecule 

Compound:

A compound is a pure material that is composed of two or more components that are chemically joined in a certain order. A compound's chemical formula identifies its constituent parts. For instance, water (H2O) is a substance made up of two hydrogens and one oxygen atom. While some compounds contain more than two components, others contain only two elements. Chemical processes that include the forming and breaking of chemical bonds are what create compounds.

Compound 

In conclusion, the fundamental components of matter are the atom, element, molecule, and compound. The smallest component of an element that takes part in a chemical reaction is called its atoms. Pure compounds made up of only one kind of atom are called elements. When two or more atoms are bound together by chemical bonds, molecules are created. Pure substances known as compounds are created when two or more elements are chemically bonded together. Understanding the characteristics of these fundamental units enables us to plan experiments, anticipate chemical reactions, and comprehend and predict the characteristics of matter.

Shifts in UV-Visible Spectroscopy (Absorption and Intensity)

 

Intensity and Absorption Shifts in UV-Visible Spectroscopy 

There are different terms related to UV-visible spectroscopy that describe the types of spectral shifts observed in the sample, including hyperchromic, bathochromic, hypsochromic, and hypochromic shifts. These shifts are defined below:

Hyperchromic Shifts

A hyperchromic shift is an increase in the absorbance intensity or the magnitude of the electronic transition after changes in the environment, concentration, or other factors. A hyperchromic effect occurs when the absorbance intensity of the electronic transition is increased due to a change in the environment.

One example of a hyperchromic shift is the denaturation of proteins, such as albumin, caused by increase temperature or other factors. It has been found that the absorbance of albumin increases with temperature, leading to a hyperchromic shift. Hyperchromic shifts have also been observed in particular organic dyes that exhibit a positive solvatochromic effect.

Hypochromic Shifts

A hypochromic shift is a decrease in the absorbance intensity or magnitude of the electronic transition for a given concentration of an analyte. A hypochromic effect occurs when light absorption intensity decreases due to the interactions between the molecules in the solution. 

The hypochromic shift, also known as a blue shift, refers to a decrease in the absorption intensity or molar absorptivity (ε) of a molecule. This shift is observed as a reduction in the peak height or the absorbance value in the UV-Vis spectrum. The hypochromic effect can be caused by various factors, including changes in the molecular environment, intermolecular interactions, or alterations in the electronic structure of the molecule.

An example of a hypochromic shift is the non-additive behavior of spectral shifts observed in binary mixtures of dyes. The interaction between the two dyes reduces the absorbance intensity due to the lowering of the dye’s transition dipole moment.

Bathochromic Shifts

A bathochromic shift is a spectral shift to lower energy or longer wavelength. A bathochromic shift takes place when a conjugated system inspires the energy levels to lower energy than those of an isolated molecule.

The term "bathochromic" is derived from the Greek words "bathos" meaning depth and "chroma" meaning color. A bathochromic shift, also known as a red shift, refers to a shift in the absorption wavelength of a molecule to longer wavelengths, resulting in a change towards the red end of the visible spectrum. In UV-Vis spectroscopy, the absorption of light by a molecule causes the promotion of electrons from lower energy levels (ground state) to higher energy levels (excited states). The energy difference between these levels corresponds to a specific wavelength of light.

A classic example of a bathochromic effect is the shifting of an azo dye's maximum absorbance towards a longer wavelength upon the addition of an electron-donating substituent to the molecule. This long-wavelength shift is due to the donation of electrons extending the π-electron system, which lowers the energy levels of the molecular orbitals involved in the electronic transition.

Hypsochromic Shifts

A hypsochromic shift is a spectral shift towards higher energy or shorter wavelength. Hypsochromic shifts happen whenever a transition is motivated into higher energy levels of an organic chromophore.

An example of a hypsochromic effect is the shift of the maximum absorbance of a -CN group in phenyl-CN towards shorter wavelengths compared to phenyl-H.

Shifts in UV-Visible Spectroscopy 

In conclusion, the shifts observed in UV-visible spectroscopy are due to changes in the molecular electronic structure, concentration, environment, and intermolecular interactions in the sample. Spectral shifts, such as hyperchromic, bathochromic, hypsochromic, and hypochromic, provide valuable information on the molecular characteristics of the sample, making UV-visible spectroscopy a powerful analytical technique.

Why Absorption and Intensity Shifts Occur in UV-Visible Spectroscopy

 

Introduction

The number of double bond and aromatic conjugations inside a molecule is determined using ultraviolet and visible spectroscopy, sometimes referred to as electronic spectroscopy.

 In this process, electrons are moved from the HOMO to the LUMO orbital (HOMO stands for Highest Occupied Molecular Orbital and LUMO for Lowest Unoccupied Molecular Orbital).  As conjugation increases, the HOMO-LUMO gap narrows.

A popular analytical method known as UV visible spectroscopy uses the absorption of light in the ultraviolet to visible region to provide details about the electrical structure of molecules. Inorganic chemistry, biology, and analytical chemistry all often employ UV-visible spectroscopy.

A sample is exposed to a variety of light wavelengths in the ultraviolet-visible range during UV-visible spectroscopy. This may affect how some sample components absorb or do not absorb photons of particular energies. The molecular structure and chemical make-up of the sample can be ascertained by the absorption or non-absorption of specific light wavelengths.

Absorption and intensity shifts caused by changes in the electronic structure of a sample or changes in the experimental circumstances are important issues to address in UV visible spectroscopy. This article will go into depth about these two events, including their causes and how they impact the spectrum analysis process.

Absorption Shifts

Absorption shifts are variations in the wavelength of a species' maximum absorbance or the placement of absorption peaks within the UV-visible spectrum induced by changes in the sample's electronic structure. Changes in the electrical structure of the sample might cause absorption shifts. 

Absorption shifts refer to changes in the wavelength of the maximum absorbance of a species or the location of absorption peaks within the UV-visible spectrum, which are caused by changes in the electronic structure of the sample. Absorption shifts can occur due to changes in the electronic structure of the sample. A shift to a longer wavelength indicates an increase in energy required for excitation, while a shift to a shorter wavelength indicates a decrease in energy required. The causes of these shifts are varied and include changes in the oxidation state of a species, the presence of conjugated systems within the sample, and changes in coordination number.

A) Oxidation State

One cause of absorption shifts in UV-visible spectroscopy is the change in oxidation state of the sample. Oxidation is a process that involves the addition of electrons to a molecule by others, leading to the conversion of this molecule or an atom to its higher oxidation state. The reverse reaction is called reduction, which involves the loss of electrons from a molecule.

In cases where a molecule undergoes oxidation, the transition energy of the molecule may change, causing the absorption maxima to shift to a longer wavelength. An example of this is the shifting of the Fe^2+ ion from 507 nm to 535 nm upon oxidation to Fe^3+. The electronic structure of Fe^3+ involves a single unpaired electron based on the complex ion's d orbitals.

The shifting of the absorption maxima of Fe^2+ to Fe^3+ is due to the formation of a new molecular orbital (MO) when electrons are added to the complex ion. In Fe^2+ ions, the energy of the MO is lower, corresponding to a maximum absorbance at 507 nm. Upon oxidation, the energy of the MO increases, which shifts the maximum absorbance to a longer wavelength (535 nm).

b) Conjugation

We know that, E = hc As the conjugation increase transition energy (E) between the orbitals is decrease and therefore wavelength (max) increase. If double bonds (chromophore) present in the molecule are in conjugation, then absorption shift towards longer wavelength. In compound 'A', double bonds are in conjugation therefore 'A' possessing higher wavelength (max) as compared to compound 'B' (non-conjugated derivative).

Effect of Conjugation

The set of atoms in a molecular structure that alternately possess single and double bonds is referred to as a conjugated system. Conjugation alters the molecule's electrical structure significantly, changing the absorption of UV and visible light.

There is considerable -conjugation in inorganic dyes such food colouring, azo dyes, and acid-base indicators. As a result, the system interacts with light, producing the distinctive colours that can be seen. As an example, the presence of a conjugated double bond in the system causes azo dyes to absorb at longer wavelengths than their comparable aniline dyes.

c) Coordination Number

A third cause of absorption shifts in UV-visible spectroscopy is a change in coordination number. Coordination compounds such as metal complexes can undergo changes in their coordination number that lead to changes in their UV-visible absorbance.

For example, the coordination number of Co in a complex containing a diamine ligand is four. Its electron configuration is d6, and this requires ligands with low-energy orbitals, such as halides. However, when the number of ligands increases, the energy required to excite an electron from the t2g set to the eg set decreases. The energy required for the d-d transition is lowered upon the formation of the hexacoordinate complex, leading to a maximum absorbance at a longer wavelength (from 515 to 550 nm).

 

 

Intensity Shifts

Intensity shifts refer to changes in the intensity of the absorbance peaks. Intensity shifts are possible and can indicate changes in the sample or experimental conditions. An increase in intensity may be due to changes in the concentration of the analyte or the introduction of a co-solvent. In contrast, a decrease in intensity may be indicative of sample degradation, decreased analyte concentration, or the presence of a scavenging agent.

a) Concentration

A change in concentration can lead to an intensity shift in the UV-visible spectrum. When a sample's concentration is too low, the signals may be weak or barely detectable. On the other hand, high concentrations of a sample can lead to overloading of the detector or produce signal saturation. Both effects reduce the sensitivity of the analysis and can lead to erroneous results.

The sensitivity of the analysis can be controlled by changing the sample concentration, but it is challenging to find the optimum concentration. The optimum concentration is usually dependent on the instrument's sensitivity, the wavelength range, and the sample's identity.

b) Environment

The sample environment can also play a role in intensity shifts in UV-visible spectroscopy. The environment may include the type of solvent used, temperature, or pressure. These factors can lead to changes in the sample's structure or chemical configuration, leading to a change in the sample's spectral behavior.

For example, an increase in temperature may cause a shift in the maximum absorbance wavelength due to a change in the electronic structure of the sample. The pH of the solvent used can also affect the maximum absorbance, depending on the pKa of the sample's constituents.

c) Aggregation

Aggregation can also cause intensity shifts in UV-visible spectroscopy. Aggregation occurs when molecules in solution come together to form larger particles. The formation of aggregates can cause a shift in the absorbance spectrum since the interaction between the particles can affect the electronic structure of the sample.

For example, aggregation of proteins occurs when proteins come together to form multimers. This can lead to changes in the maximum absorbance of the protein due to the changes in the electronic structure of the protein as the proteins aggregate.

Conclusion

In conclusion, UV-visible spectroscopy is a powerful analytical technique used in the identification and quantification of analytes in a sample. Spectral changes in absorption and intensity of UV-visible light can be used to identify and quantify the analytes concentration present in the sample. Shifts in the absorbance maxima and intensity of UV-visible spectra can be caused by changes in the electronic structure of the sample, solution environment, and sample concentration. Analysis of these shifts can be crucial in providing insights into the molecular structure and composition of the sample under investigation.