Trends in physical and chemical properties

What are metals?

Approximately three-quarters of all known elements on Earth, metals are substances or minerals usually having a lustre and high electrical conductivity, readily losing electrons to form positive ions(cations). Aluminium is an example of metal.

General physical and chemical properties of metals:

• Lustrous
• High thermal and electrical conductivity
• High malleability (can be beaten into thin sheets)
• High ductility (can be drawn into thin wires)
• High melting and boiling points
• High density

What are nonmetals?

The simplest way to define nonmetals would be to say that those elements/minerals which are not metals are called nonmetals.

These elements are structurally brittle and do not conduct electricity, except graphite. Most of the nonmetals, except noble gases, readily gain electrons to fill their outer shells and form negative ions (anions). Oxygen is an example of nonmetal.

General physical and chemical properties of nonmetals:

• Non-Lustrous 
• Poor thermal and electrical conductivity
• Low density
• Brittle

Periodic Trends

The arrangement of the elements is based on the periodic law, which Dmitri Mendeleev came up with. This law says that when the elements are organised according to their atomic numbers, the atomic structure and most of their properties display a periodic variation, i.e., they recur periodically. The periodic table tells us loads about the physical and chemical properties of metals and nonmetals along the periods and the groups.

Some of those crucial trends in physical and chemical properties and physical and chemical properties of metals and nonmetals are:

• Atomic Radius Trends

The atomic radius is roughly half the distance between two atoms’ nuclei (just like a radius is half the diameter of a circle). Some atoms are held together in metallic crystals by covalent connections, while others are attracted to each other in ionic crystals. Most elements can form covalent molecules, which are made up of two similar atoms bonded together by a single covalent bond. Covalent radii are generally used to describe the atomic radii of these compounds. Atomic radius patterns can be found all across the periodic table.

• In a period, the atomic radius decreases from left to right. The rise in the number of protons and electrons over time causes this. Electrons are drawn closer to the nucleus because one proton has a more significant effect than one electron; hence, the radius decreases.

• Within a group, the atomic radius increases from the top to the bottom. The reason is the shielding of electrons (the valence shell electrons are weakly attracted to the nucleus; thus, they stray further away, increasing their distance). This happens because of the attraction of inner-shell electrons, which block the attraction of valence shell electrons.

• Metallic Character Trends

Metals are known to lose electrons and form a cation quickly. Therefore, the metallic character of an element is defined as how readily it can lose an electron. 

• Metallic properties decrease from left to right over a period. This is triggered by the decrease in the atom’s radius (induced by Zeff), which prevents the outer electrons from ionising as rapidly as they would otherwise.

• Metallic features become more dominant as you move down the group. Because electron shielding causes the atomic radius to grow, electrons in the outermost shell ionise more quickly due to lesser attraction.

• Electronegativity Trends

It is reasonable to think of electronegativity as a chemical attribute that describes an atom’s ability to attract and bind with electrons. There is no conventional procedure for determining electronegativity in a given situation. However, the Pauling scale, named after the chemist Linus Pauling, is the most commonly used scale for assessing the electronegativity of an atom. The electronegativity of elements increases as we move from left to right across a period. The loss of an electron is less energy-consuming than the gain of an electron when the atom’s valence shell is less than half-full. Conversely, it is easier to pull an electron into the valence shell than donate one when the atom’s valence shell is more than half full.

• The electronegativity in a group reduces as we descend from the top to the bottom. This is because the atomic number increases as one moves down the group, resulting in a wider distance between the valence electrons and the nucleus. So, it becomes more convenient to lose those electrons than to gain new ones.

• Noble gases, lanthanides, and actinides are examples of important deviations to the rules mentioned above of electronegativity. The noble gases possess a full valence shell and, as a result, do not usually attract electrons to themselves. The lanthanides and actinides have more convoluted chemistry, and their behaviour does not generally follow any trends in terms of periodicity. As a result, the electronegativity values of noble gases, lanthanides, and actinides are unavailable.

• In the case of the transition metals, despite having electronegativity values, there is a slight variation between them across the period and down a group. This is mainly because their metallic qualities affect their capacity to attract electrons as readily as other elements.

• Electron Affinity Trends

The ability of an atom to gain an electron is referred to as electron affinity. On the contrary to electronegativity, electron affinity is a purely quantitative measurement of the energy shift that occurs when an electron is added to the nucleus of an atom of neutral gas. The more the electron affinity value is negative, the greater the association of an atom with electrons.

• Within a period, the electron affinity increases from left to right. This is caused by a decrease in the atomic radius of the atom.

• The electron affinity of atoms reduces from the top to the bottom of a group. The increase in atomic radius is the reason for this phenomenon.

• Melting Point Trends

The melting points of substances are defined as the amount of energy required to break a bond(s) to transform that substance from solid to liquid. The stronger a bond is between the atoms of an element, the greater the amount of energy required to break that bond. Because temperature is directly proportional to energy, a high bond dissociation energy corresponds to a high temperature. Melting points are variable and do not usually follow a discernible pattern across the periodic table elements. Metals have high melting points, while nonmetals have low melting points. Carbon, although a nonmetal, possesses the highest melting point of all elements.

• Ionisation Energy Trends

It can be defined as the energy required to eliminate an electron from a neutral atom in its gaseous state. The lower the atom’s energy, the greater the likelihood that it will transform into a cation. So, if this energy is significant, an atom is less likely to transform into a cation.

• If we move across a period, the ionisation energy of the elements grows in a typical left to right fashion. This is because the valence shell is stable.

• If, on the other hand, we move down a group, the ionisation energy decreases from top to bottom, primarily because of electron shielding.

• Because of their full valence shells, the noble gases have extremely high ionisation energies, making them extremely unreactive. It is important to note that helium has the highest ionisation energy among all the elements.

• Oxidation and Reduction Potential Trends

• Oxidation Potential Trends

Oxidation is a reaction characterised by the loss of electrons. Oxidation potential follows the same trends as that of ionisation energy. The smaller the ionisation energy of an atom, the easier it becomes to remove an electron.

• Reduction Potential Trends

Reduction is a reaction characterised by the gain of electrons. Reduction potential follows the same trend as that of electron affinity. The more negative electron affinity is, the easier it is to give an electron.  

Conclusion

Trends in physical and chemical properties, as a concept, are extremely important and have a wide range of use in chemistry. Atomic radius, ionisation energy, oxidation and reduction potential, melting point, electron affinity, metallic character, and electronegativity are major periodic trends. The shielding effect, the number of protons in the nucleus, and the number of energy levels are primary factors that play a role in these periodic trends.