Periodic Trends in Chemical Properties

Introduction

Moseley gave the Modern Periodic law. It states that the chemical and physical properties of the elements form the periodic functions of their atomic numbers. According to the modern periodic table, elements are arranged based on their atomic numbers, which are directly linked to the physical and chemical properties of the elements. Thus, the elements display periodicity in their properties. For instance, atomic size decreases from left to right in the table; however, certain exceptions that do not follow such periodic table trends are also observed. 

What is a Periodic Trend in Chemical Properties?

Periodic trend is a unique pattern in the periodic table that presents different aspects such as specific element’s size and electrical properties. This periodicity is caused due to recurrence of similar electronic configurations throughout the periodic table. In other words, the elements with similar electronic configurations present similar properties. 

These periodic trends in chemical and physical properties occur due to the periodic table’s arrangement. Periodic trends and chemical reactivity provide chemists with a valuable tool for swiftly predicting an element’s attributes. These trends exist due to the periodic nature of the elements and their identical atomic structure within their various group families. The unknown properties of any element can be deduced in parts due to these periodic trends.

Periodic Trends in Chemical Properties

Major periodic trends are listed as follows:

• Electronegativity
• Melting point
• Metallic character
• Ionisation energy
• Electron affinity
• Atomic radius
• Valency

Electronegativity

Electronegativity is a chemical property that describes the ability of an atom to attract bond pairs of  electrons. A conventional procedure cannot calculate it because it is a qualitative feature. 

Electronegativity measures the tendency of an atom to attract and establish bonds with electrons. The electronic arrangement of atoms gives rise to this feature. The octet rule, stating that the outermost shell has eight electrons, is followed by numerous atoms. 

• Electronegativity increases from left to right and decreases from top to bottom across the periodic table.
• The noble gases, lanthanides, and actinides are notable exceptions to the above two rules.
• A slight variance is observed among the electronegativity values of transition metals.
• Fluorine is the most electronegative element.

Ionisation Energy

Ionisation energy Is needed to remove an electron in its gaseous form from an isolated atom. It is termed as the polar opposite of the electronegativity concept. The lower the ionisation energy, the easier it is for the atom to form a cation.

Thus, higher energy leads to less probability of cation formation. As the valence shells of an atom are nearly complete, elements on the right side of the periodic table have higher ionisation energy. In contrast, elements on the left side of the periodic table have low ionisation energy as they shed electrons and become cations. Thus, ionisation energy increases from left to right in the periodic table.

Electron Affinity

The ability of an atom to take an electron is known as electron affinity. It is a quantitative measurement of the energy shift when an electron is added to a neutral gas atom. The higher an atom’s affinity for electrons, the lower its electron affinity value.

As each atom is larger than the atom above it, the electron affinity decreases below a set of elements. This signifies that an extra electron is far away from the nucleus of the larger atom than it was in the smaller atom. The force of attraction is weaker when the distance between the negatively charged electron and the positively charged nucleus is more significant, causing electron affinity to decrease down an element group.

In contrast, the electron affinity increases across a period because the forces of attraction become stronger from left to right, leading to a decrease in the atom size.

Atomic Radius

The atomic radius is half the distance between the nuclei of two atoms. Although all atoms are distinctly bound together, various elements may form covalent molecules, which are made up of two similar atoms bonded together by a single covalent bond. Atomic radii is another name given to the covalent radii of these molecules. Atomic radius patterns can be found all over the periodic table.

The atomic size of elements decreases from left to right throughout a period because all electrons are added to the same shell. However, protons are simultaneously added to the nucleus, making it more positively charged. As the effect of increasing proton number is larger than that of growing electron number, nuclear attraction is increased, that is, the nucleus attracts more electrons, causing the atom’s shell to come closer and radius to shrink.

Melting Point

The amount of energy necessary to break a bond(s) and turn a substance’s solid phase into a liquid is known as the melting point. A stronger connection between the atoms requires more energy to break. A high bond dissociation energy corresponds to a higher temperature since temperature and energy are directly related. Melting points vary widely and do not follow a consistent pattern across the periodic table. 

• Metals usually have higher melting points as compared to most non-metals.
• Carbon (a non-metal) has the highest melting point in all elements.

Metallic Character

An element’s metallic character can be described as the ease with which an atom can lose an electron. The metallic character and atomic size increase as the attraction between the valence electron and the nucleus weakens, allowing for more superficial electron loss. The outer shells go more apart as the atomic size increases. The average electron density shifts away from the nucleus as the principal quantum number rises. Since the valence shell electrons have less affinity to the nucleus, they can easily lose electrons, resulting in a more metallic character.

Valency

The periodic table’s valency increases and eventually drops over a time period. The heavier elements (those with an atomic number greater than 20), particularly the lanthanide and actinide series, follow this periodic trend less frequently. More core electrons cause better shielding of electrons from the nucleus’s core charge.

Thus, the ionisation energy of elements in a group is lower, and the polarity of species is higher for elements lower in a group. The valency does not alter down a group because the core electrons do not affect the bonding behaviour. In contrast, core electrons affect the non-bonding interactions.

Conclusion

Periodic trends define the element’s chemical and physical properties in various ways and play an essential role in assigning characteristics to the elements. The electronegativity, electron affinity, ionisation energy, and metallic character increase going from left to right across a period, whereas the atomic radius decreases. Melting points do not have a specific trend across the table. Some other periodic trends define the chemical properties of elements, which include valence electrons and periodic table trend reactivity.