An introduction to the trends of periodic properties in Periodic Table
Moseley gave the Modern Periodic law which states that “Physical and chemical properties of the elements are periodic functions of their atomic numbers”. In the modern periodic table elements have been arranged according to their atomic numbers and as stated above atomic numbers are directly related to their physical and chemical properties. That’s why elements show periodicity in their physical and chemical properties in the periodic table. For example, as we move from left to right in a period, atomic size decreases. The following figure shows the variation of periodic properties of elements.
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Although we find some exceptions which do not follow these periodic table trends. The recurrence of similar electronic configuration in the periodic table is the cause behind periodicity. Thus, we can say that elements having similar electronic configurations have similar properties. Periodic trends provide chemists with a quick and easy tool to quickly predict the properties of elements. In this article, we will discuss periodic properties and their trends in the periodic table in detail.
Periodic Trends of Properties of Elements In Periodic Table
Modern periodic law is the base of periodic trends of properties of elements in the modern periodic table. Following properties of elements show a very clear periodic trend in the periodic table –
Atomic Radius
Ionisation energy
Electron affinity
Electronegativity
Valence electrons
Valency
Metallic character of the elements
Non – metallic character of the elements
Reactivity of elements
Melting and boiling points of elements
Now, understand the trends of properties in detail by knowing the reason for the variation of properties.
Atomic Radius
Atomic radius is the distance between the centre of the nucleus of an atom to its outermost shell.
The periodic trend of atomic radius across a period – As we move from left to right in a period, atomic radius gradually decreases.
Reason – As we move left to right in a period the atomic number of the elements increases so nuclear charge increases while the number of shells in elements remains the same.
Example –
Exceptional Behaviour – Noble gases show exceptional behaviour. The atomic radii of inter gases suddenly increase as compared to its predecessor halogen atom. The reason for this type of exceptional behaviour is that atomic radius refers to van der Waals radius in the case of noble gases while in the case of other elements it refers to the covalent radius.
Across a Group – on moving top to bottom in a group, atomic radii gradually increase as nuclear charge and number of shells also increase.
Ionisation Energy
Ionisation energy is the amount of energy required to remove one electron from an atom. First ionisation energy is the amount of energy required to remove one outermost electron from an atom.
The periodic trend of ionisation energy across a period – As we move from left to right in a period, ionisation energy gradually increases.
Reason – As we move left to right in a period atomic size or atomic radius decreases while nuclear charge increases.
Example -
Exceptional Behaviour – Beryllium possesses more first ionisation energy than Boron. Because beryllium has a half-filled s – orbital and more energy is required to remove an electron from half or completely filled orbitals. That is why noble gases also show exceptionally high ionisation energies.
Across a Group – on moving top to bottom in a group, ionisation energy gradually decreases as atomic radius increases.
Electron Affinity
The amount of energy required to add an electron to an atom is called the electron affinity of that atom. In other words, electron affinity is the change in energy when an electron is added to the atom and a neutral atom changes into a negative ion.
The periodic trend of electron affinity across a period – As we move from left to right in a period, electron affinity gradually increases.
Reason – As we move left to right in a period atomic size or atomic radius decreases while nuclear charge increases.
Exceptional Behaviour – Beryllium does not form a stable anion, so it releases less energy than boron by adding an electron. While nitrogen neither releases nor requires a significant amount of energy on adding an electron so it has electron affinity almost equal to zero.
Across a Group – on moving top to bottom in a group, electron affinity gradually decreases.
Electronegativity
Electronegativity is a measure of the ability of an atom or molecule to attract pairs of electrons in the context of a chemical bond.
Across A Period – As we move left to right across a period, electronegativity increases in the periodic table. Fluorine is the most electronegative element.
Reason – As the nuclear charge increases of an atom, its electron loving character also increases.
Example –
Across A Group – As we move top to bottom in a group, electronegativity decreases.
Valence Electrons
Electrons present in the outermost shell of an atom are called valence electrons of that atom.
Across A Period – As we move left to right across a period in the periodic table, the number of valence electrons increases.
Example –
Across A Group – Across a group, valence electrons remain constant. It means elements present in the same group have the same number of valence electrons. For example, hydrogen, lithium, and sodium elements are present in the 1st group and have the same number of valence electrons which is one.
Valency
Valency is the combining capacity of an atom.
Across a Period – on moving left to right across a period in the periodic table, first valency increases then decrease.
Example –
Across A Group – There is no change in valency across a group. Elements of the same groups show the same valency.
Metallic Character of The Elements
Across a Period – As we move left to right across a period in the periodic table, the metallic character of elements decreases.
Example –
Across a Group – As we move top to bottom in a group of the periodic table, the metallic character of elements increases.
Non-Metallic Character of The Elements
Across a Period – As we move left to right across a period in the periodic table, the non-metallic character of elements increases.
Example –
Across a Group – As we move top to bottom in a group of periodic tables, non-metallic character decreases.
Example -
Reactivity of Elements
The reactivity of metals depends on their electropositive character. So, more is the metallic character, more is the electropositive nature of the element and more is its reactivity. As metallic character decreases across a period left to right, reactivity also decreases. Although reactivity of nonmetals increases on moving left to right across a period. Thus, we can conclude, as we move left to right in a period, the reactivity of elements gradually decreases up to group thirteen and then starts increasing.
Melting And Boiling Points of Elements
Melting and boiling points of metals decrease gradually from top to bottom in a group. While melting and boiling points of nonmetals increase on moving from top to bottom in a group of the periodic table.
Conclusion
The periodic properties in the periodic table develop a base in order to understand the nature of elements in an efficient way. The above article covers all the important trends of properties in the periodic table. These are the basic concepts of chemistry that are helpful in grasping the different concepts of organic chemistry.
FAQs on Trends of Periodic Properties in Periodic Table and Reason of Variation
1. What are the main periodic properties and how do their trends vary across a period?
When moving from left to right across a period in the periodic table, several key properties of elements show consistent trends. This is primarily due to the increasing nuclear charge while electrons are added to the same valence shell. The main trends are:
- Atomic Radius: Decreases due to a stronger pull from the nucleus on the electrons.
- Ionisation Enthalpy: Generally increases because more energy is needed to remove an electron from a smaller atom with a higher nuclear charge.
- Electron Gain Enthalpy: Becomes more negative (more energy is released) as the effective nuclear charge increases, making it easier to add an electron.
- Electronegativity: Increases, as atoms have a greater ability to attract shared electrons in a bond.
- Metallic Character: Decreases, as the tendency to lose electrons reduces.
- Non-metallic Character: Increases, as the tendency to gain electrons grows.
2. How do the properties of elements change on moving down a group?
On moving from top to bottom down a group, the overriding factor is the addition of a new principal energy shell with each successive element. This leads to the following general trends:
- Atomic Radius: Increases because the outermost electrons are in a new shell, farther from the nucleus.
- Ionisation Enthalpy: Decreases because the outermost electron is further away and better shielded from the nucleus, making it easier to remove.
- Electron Gain Enthalpy: Generally becomes less negative as the atom gets larger, reducing the effective nuclear attraction for an incoming electron.
- Electronegativity: Decreases because the larger atomic size and increased shielding reduce the atom's ability to attract bonding electrons.
- Metallic Character: Increases, as the lower ionisation enthalpy makes it easier for atoms to lose electrons.
- Valency: Remains the same for all elements within a group.
3. What is the fundamental reason for the periodicity of properties in the periodic table?
The fundamental reason for periodicity is the recurrence of similar outer electronic configurations at regular intervals. According to the Modern Periodic Law, the properties of elements are a periodic function of their atomic number. As the atomic number increases, electrons fill up orbitals in a specific order. Elements with the same number of valence electrons (electrons in the outermost shell) are placed in the same group, causing them to exhibit similar chemical and physical properties, which then repeat in the next period.
4. Why does atomic radius decrease across a period but increase down a group?
This contrast is due to two different dominating factors. Across a period (left to right), the nuclear charge increases, but electrons are added to the same valence shell. This stronger nuclear pull draws the electron cloud closer, causing the atomic radius to decrease. In contrast, when moving down a group, a new electron shell is added for each element. This new shell is at a greater distance from the nucleus, and the increased shielding effect from inner electrons outweighs the increased nuclear charge, causing the atomic radius to increase significantly.
5. Why is the first ionisation enthalpy of Beryllium greater than that of Boron, which is an exception to the general trend?
This is a classic exception based on electronic configuration. Beryllium (Be, atomic number 4) has a stable, fully-filled 2s orbital (1s²2s²). Removing an electron from this stable configuration requires a significant amount of energy. Boron (B, atomic number 5) has the configuration 1s²2s²2p¹. Its outermost electron is a single electron in the 2p orbital, which is slightly higher in energy and is shielded by the 2s electrons. Therefore, it is easier to remove this 2p electron from Boron than it is to remove a 2s electron from Beryllium, causing Be to have a higher first ionisation enthalpy.
6. How does the valency of elements vary across a period in the periodic table?
Valency, the combining capacity of an element, follows a distinct pattern across a period. On moving from left to right, the valency with respect to the number of valence electrons first increases from 1 to 4 (for Groups 1, 2, 13, and 14). After that, it typically decreases from 3 to 0 for Groups 15 to 18 (calculated as 8 minus the number of valence electrons). For example, in the 3rd period, the valency goes from Na (1), Mg (2), Al (3), Si (4), to P (3), S (2), Cl (1), and Ar (0).
7. What is the key difference between electron gain enthalpy and electronegativity?
The key difference lies in the state of the atom being measured. Electron gain enthalpy is the energy change when an electron is added to an isolated gaseous atom to form a negative ion. It is a measurable property of a single atom. In contrast, electronegativity is a relative measure of the ability of an atom to attract a shared pair of electrons within a chemical bond. It is not a property of an isolated atom but rather a measure of its behaviour when bonded to another atom.
8. How do metallic and non-metallic characters trend in the periodic table?
Metallic and non-metallic characters show opposite trends.
- Metallic character refers to the tendency of an element to lose electrons. It decreases across a period (as ionisation enthalpy increases) and increases down a group (as ionisation enthalpy decreases).
- Non-metallic character refers to the tendency to accept electrons. It increases across a period (as electronegativity increases) and decreases down a group (as electronegativity decreases).
This is why the most metallic elements are found in the bottom-left corner of the periodic table, and the most non-metallic are in the top-right.
9. Why do noble gases have a large positive electron gain enthalpy and an exceptionally high ionisation energy?
Noble gases have these properties due to their exceptionally stable electronic configurations with completely filled valence shells (ns²np⁶).
- High Ionisation Energy: Removing an electron requires breaking this stable arrangement, which demands a very large amount of energy.
- Positive Electron Gain Enthalpy: Adding an electron is also highly unfavourable because the new electron would have to enter the next higher principal energy level, which is very unstable. Instead of releasing energy, the process requires a significant input of energy, resulting in a large positive value for electron gain enthalpy.
