Reactivity of elements down the group

As you go down group 1 (the alkali metals) in the periodic table, the elements get more reactive.

As you go up group 7 (the halogens), again, the elements become more reactive.

Why do alkali metals get more reactive going down group 1?

Alkali metals from lithium to potassium get more reactive because the force of attraction between the nucleus (core) and the outer electron gets weaker as you go down group 1 elements.

The distance "c" is greater than "a" and the force of attraction between the nucleus and the outer shell (rings) diminishes with distance.

The more electron shells (rings) between the nucleus and outer electron also creates shielding and again this weakens the nuclear attraction.

The outer electron is more easily transferred to say an oxygen atom, which needs electrons to complete its full outer shell.

Why do halogens get more reactive going upwards in group 7?

Halogens from bromide to fluorine get more reactive because the force of attraction between the nucleus (core) and the outer electron get stronger as you go up group 7 elements.

The distance "a" is less than "c" and the force of attraction between the nucleus and the outer shell increases with shorter distances.

The fewer electron shells (rings) between the nucleus and the outer shell (ring) also has less shielding effect and again this increases the electron attraction.

The outer shell will more easily attract another electron, which needs an electron to complete its full outer shell, when there is more attractive force. 

A useful mnemonic picture to help you recall that:

As you go down group 1 (the alkali metals) in the periodic table, the elements get more reactive.

As you go up group 7  (the halogens), again the elements get more reactive.

Is as follows:

To remember how the reactivity of the alkali metals and halogens increases or decreases, put a pin in the middle of the periodic table and spin it anti-clockwise.

Drunks fall down, angels rise.

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.