What determines the reactivity of the element?

What determines the chemical reactivity of elements?

The chemical reactivity of an element is determined by the number of valence electrons in it and the shell number of the valence shell.

Concept: Modern Periodic Table and Electronic Configuration of Elements

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In chemistry, reactivity is a measure of how readily a substance undergoes a chemical reaction. The reaction can involve the substance on its own or with other atoms or compounds, generally accompanied by a release of energy. The most reactive elements and compounds may ignite spontaneously or explosively. They generally burn in water as well as the oxygen in the air. Reactivity is dependent upon temperature. Increasing temperature increases the energy available for a chemical reaction, usually making it more likely.

Another definition of reactivity is that it is the scientific study of chemical reactions and their kinetics.

The organization of elements on the periodic table allows for predictions concerning reactivity. Both highly electropositive and highly electronegative elements have a strong tendency to react. These elements are located in the upper right and lower left corners of the periodic table and in certain element groups. The halogens, alkali metals, and alkaline earth metals are highly reactive.

• The most reactive element is fluorine, the first element in the halogen group.
• The most reactive metal is francium, the last alkali metal (and most expensive element). However, francium is an unstable radioactive element, only found in trace amounts. The most reactive metal that has a stable isotope is cesium, which is located directly above francium on the periodic table.
• The least reactive elements are the noble gases. Within this group, helium is the least reactive element, forming no stable compounds.
• Metal can have multiple oxidation states and tend to have intermediate reactivity. Metals with low reactivity are called noble metals. The least reactive metal is platinum, followed by gold. Because of their low reactivity, these metals don't readily dissolve in strong acids. Aqua regia, a mixture of nitric acid and hydrochloric acid, is used to dissolve platinum and gold.

A substance reacts when the products formed from a chemical reaction have lower energy (higher stability) than the reactants. The energy difference can be predicted using valence bond theory, atomic orbital theory, and molecular orbital theory. Basically, it boils down to the stability of electrons in their orbitals. Unpaired electrons with no electrons in comparable orbitals are the most likely to interact with orbitals from other atoms, forming chemical bonds. Unpaired electrons with degenerate orbitals that are half-filled are more stable but still reactive. The least reactive atoms are those with a filled set of orbitals (octet).

The stability of the electrons in atoms determines not only the reactivity of an atom but its valence and the type of chemical bonds it can form. For example, carbon usually has a valence of 4 and forms 4 bonds because its ground state valence electron configuration is half-filled at 2s2 2p2. A simple explanation of reactivity is that it increases with the ease of accepting or donating an electron. In the case of carbon, an atom can either accept 4 electrons to fill its orbital or (less often) donate the four outer electrons. While the model is based on atomic behavior, the same principle applies to ions and compounds.

Reactivity is affected by the physical properties of a sample, its chemical purity, and the presence of other substances. In other words, reactivity depends on the context in which a substance is viewed. For example, baking soda and water are not particularly reactive, while baking soda and vinegar readily react to form carbon dioxide gas and sodium acetate.

Particle size affects reactivity. For example, a pile of corn starch is relatively inert. If one applies a direct flame to the starch, it's difficult to initiate a combustion reaction. However, if the corn starch is vaporized to make a cloud of particles, it readily ignites.

Sometimes the term reactivity is also used to describe how quickly a material will react or the rate of the chemical reaction. Under this definition the chance of reacting and the speed of the reaction are related to each other by the rate law:

Where rate is the change in molar concentration per second in the rate-determining step of the reaction, k is the reaction constant (independent of concentration), and [A] is the product of the molar concentration of the reactants raised to the reaction order (which is one, in the basic equation). According to the equation, the higher the reactivity of the compound, the higher its value for k and rate.

Sometimes a species with low reactivity is called "stable", but care should be taken to make the context clear. Stability can also refer to slow radioactive decay or to the transition of electrons from the excited state to less energetic levels (as in luminescence). A nonreactive species may be called "inert". However, most inert species actually do react under the right conditions to form complexes and compounds (e.g., higher atomic number noble gases).

Jul 9, 2022 | Turito Team

The elements of the Periodic Table show various trends. These trends are broadly divided into two categories, i.e., physical properties and chemical properties. There are many observable patterns in elements’ physical and chemical properties as we move across a period or down in a group in the Periodic Table.

Like some elements show more affinity to form bonds with others, while some do not show any affinity. But why does it happen? And what is this affinity? How does reactivity vary in the Periodic Table?

In this section, we shall discuss elements’ reactivity and trends in the Periodic Table.

What is Reactivity?

In general, the reactivity definition is the degree to which a substance shows chemical change when mixed with another substance. It is a measurement of how much a substance reacts with others. The scientific study of chemical changes and their kinetics is another form of reactivity definition.

Reactivity in Chemistry

Reactivity in Chemistry refers to the rate at which a chemical substance undergoes a chemical reaction in time. It is a relative tendency of an element to gain or lose an electron(s) during a chemical reaction.

In pure compounds, reactivity is controlled by the physical properties of the sample. While in a mixture, it depends upon the chemical nature of the atoms present in it.

Reactivity in Chemistry measures how voluntarily a substance experiences a chemical change. This change can occur between the same, whether in the same molecule or within different atoms or molecules. Generally, this change followed the loss of energy during the process.

Highly reactive elements like F, N, O, Na, K, etc., can react vigorously during the chemical process. Sometimes, they might produce explosive reactions and are very reactive in chemistry. Hence, to answer the question of what is reactivity, it can be referred to as how likely or vigorously an atom is to react with other substances.

Factors on Which Reactivity Depends on

Chemical reactivity, or simply reactivity, depends upon many factors. Some of them are discussed below:

The size of an atom is generally considered as the distance between the outer shell electrons and the nucleus, i.e., the atom’s radius.

The larger the atomic radius, the lesser the reactivity of an element. It is because when the electrons are away from the nucleus, there is less electron density on the atom. As a result, it shows less affinity to react with other elements, or you can say that to react with other atoms, you need to provide it with more heat energy.

The energy available for a chemical reaction increases by increasing the temperature, usually making it more likely.

As you increase the temperature of the reaction, the electrons start to vibrate more frequently and collide with other atoms’ electrons more readily. As a result, there is an expansion in the activation energy of the molecules, and hence, they show high chances of forming bonds or reactivity.

The more protons in the nucleus of an atom, the greater the attraction between the nucleus and the outer shell electrons. Hence, greater reactivity decreases.

When the positive charge on the nucleus increases, there will be a stronger nuclear attraction between the nucleus and valence shell electrons. As a result, it pulls the outermost shell electrons towards itself more conveniently. It means the size of the atom decreases, and hence, it will be harder to lose electrons and form bonds. Therefore, reactivity decreases as the positive charge increases on the nucleus. This order gets reversed in case of a negative charge.

Electron guarding (or shielding) blocks the valence shell electrons’ attraction towards the nucleus because of the presence of inner-shell electrons. Reactivity decreases with the increase in shielding electrons in an atom. 

Electrons comprise a negative charge. So, the inner-shell electrons push back outer-shell electrons. This repulsion turns down the attraction among the outer electrons and the nucleus. As a result, reactivity gets reduced because there is a less nuclear attraction to capture an electron from another species.

Trends in Reactivity of Elements

Generally, all chemical and physical properties manifest the electronic configuration of elements. As the electronic configuration differs from element to element, the relation between fundamental properties like ionic and atomic radii, ionisation enthalpy, electron affinity, and electronegativity also differs.

On going along the group, ionic and atomic radii of atoms increase. As a result, reactivity increases. The number of electrons in the outer shell remains the same while moving down the group. But there is a gradual increase in the number of shells of the same group of atoms. Hence, there will be less electron shielding on the valence electron, and it can show high reactivity.

While moving along a period, atomic or ionic radii decrease. The electrons enter the same valence shells, increasing their attraction toward the nucleus. Hence, the outermost shells pull near the nucleus, and there is a decrease in the radii of atoms. There is also an increase in electron density which makes the atom more reactive. Hence, reactivity increases from moving across a period.

Ionisation energy is the amount of energy that is required to pull the electrons from the outermost shell.

When you go through top to bottom, i.e., along with a group, the distance between the nucleus and valence shell increases, and attraction between both decreases. Therefore, ionisation energy decreases because it needs less energy to pull electrons from an atom’s very far valence shell.

When you move along a period, the electron density increases on the atom, and its size decreases. As a result, you need to apply a lot of ionisation energy to remove its electron from the valence shell.

Electron Affinity is the amount of change in energy when an electron in a gaseous state is applied to a neutral atom to form an anion.

On moving down the group, there is a decrease in the electron affinity of elements. It is because, as the size of an atom increases, the valence electrons get further away from its nucleus. Thus, the attraction between them decreases. Hence, pulling an electron from its valence shell becomes easy.

On moving along the period, the amount of electron affinity increases. It is because of the increase in attraction between an atom’s nucleus and valence shell. As a result, removing electrons from a more stable atom is difficult.

The potential of an atom to attract a shared pair of electrons towards itself is called electronegativity.

While moving down the group, the electronegativity of atoms decreases. It is because the number of valence shells is increasing, and the attraction between valence electrons and the nucleus decreases. Therefore, the tendency to pull electrons from other atoms decreases.

On moving across a period, the electronegativity of atoms increases. It is because the attraction between the nucleus and valence shell electrons increases. As a result, the atomic radius decreases. Hence, the tendency to pull electrons from other atoms increases.

Summary

Chemical reactivity is highest at the extremes of the two periods and lowest in the middle of the Periodic Table. The reactivity on the utmost left of a period is because of the simplicity of losing an electron or because of low ionisation enthalpy. Highly reactive elements do not exist in nature in a free state. They usually occur in the combined form.

From the above discussion, you are now well-versed with reactivity in Chemistry. 

Frequently Asked Questions

Q1. What is the factor that determines the chemical reactivity of a molecule?

Answer: Elements react with other substances to achieve stability. And stability can only be gained when they have fulfilled the valence shell. Therefore, how smoothly an element can bring about this calculates its reactivity. This aptness to form stable outer shells depends on various factors.

For example, in the group IA metals (Li, Na, K, and down the column), all elements have a single electron in their ultimate shell, so the easiest way to have their outer shell filled is to give up that one electron.

While moving down the group, the atoms get bigger. And the bigger atoms do not hold on to that one electron as tightly. Therefore, the order of reactivity in this group is Li < Na < K < Rb < Cs.

Q2. How does a molecule’s polarity influence a chemical compound’s reactivity?

Answer: Reactivity and polarity are directly proportional to each other. Higher the polarity, the higher the reactivity. However, in some cases, the compound is very polar but may not react much like in the case of HF. The reaction often depends on reaction conditions. For example,

• Acyl halides react vigorously because of their higher polarity than carboxylic acids.
• Esters have higher reactivity than carboxylic acids.
• Alkyl halides are more reactive than alkanes.

Q3. What are the elements chemically reactive in the atmosphere?

Answer: There are several elements in the atmosphere that are chemically reactive in Chemistry. The most reactive among all is oxygen. It is corrosive, toxic, and can cause blindness in newborns. Chronic exposure to high partial pressures of oxygen above about 5 PSI is toxic to adults.

Some other reactive gases are:

• Nitrogen oxides are assembled by lightning strikes and intense heat combustion, such as in wildfires.
• The metabolisms of many microorganisms produce methane.
• Ground-level ozone is generated by lightning strikes and reacting nitrogen oxides with atmospheric oxygen.
• Methyl bromide is also produced by microbial metabolism.

Q4. How do electrons determine reactivity?

Answer: Chemical reactions involve the transfer or sharing of electrons present in the valence shell. The total number of electrons present in an atom is distributed in shells or orbits. And electrons present in the valence shell determine the nature of the chemical bond made by the atom. Hence, electrons determine reactivity.