What are Periodic Trends?
Periodic trends are observable patterns in the properties of an element that are dependent on its position in the Periodic Table. These trends have allowed scientists in the past to predict certain characteristics of unknown elements. This is due to the structural similarities’ elements have within a period or family that allows for these trends to take place. We discuss trends for properties such as electronegativity, atomic radius, first ionization energy, reactivity and electron affinity.
Which properties can be identified using periodic trends?
Electronegativity is a property that measures the tendency of an atom to attract electrons to form a bond. The scale that was formed in order to measure this property is the Pauling scale. It was created by measuring the bond energy of the different elements joined by covalent bonding. This scale ranges from 0.7 to 4.0, where cesium is the least electronegative element, and with fluorine being the most electronegative element. This is observed through Periodic Trends, as the further right of a period, and higher up a group are observed to have increased values in electronegativity. Note that the noble gases do not have an electronegativity.
The minimum amount of energy required to remove an electron from a neutral atom’s outermost electron shell in the gaseous phase is called the ionization energy.
This property is describing the difficulty of removing an electron. The Ionization energy increases further right down the period, and higher up a group as more energy is required to remove a tightly bounded electron from the atom. An example of high ionization energy is in helium, as it has a very small atomic radius with +2 charge present in the nucleus. This allows for electrons to be retained as a result of these attractive forces.
Reactivity describes the ability of a molecule or atom to undergo a chemical reaction, followed by a release in energy. This property is dependent on characteristics such as electronegativity and ionization energy. These are factors that affect the interactions of electrons that chemical reactions undergo. Reactivity is dependent on the classification of an element (metals and non-metals), as they both have differing periodic trends. The reactivity of metals increases further left along a period, and further down a group. On the other hand, reactivity in non-metals increase further right down a period, and further up a group. The most highly reactive element to be observed is cesium, as it spontaneously reacts with air and water!
Electron Affinity is a property describing the tendency of a neutral atom to gain an electron, in order to form a negative ion. In this process, energy is released, also known as an exothermic process (excluding noble gases). A trend is observed, as elements further right a period and higher up a group have an increased electron affinity. This is seen as the electrons that form the negative ions are added to the outer electron shell, causing an increased attraction between the electrons and the nucleus. An exception to this rule is with the element with the highest electron affinity, chlorine. This is due to chlorine having more space for electrons than fluorine in its outermost orbital. The larger space in chlorine’s orbital causes its electrons to be more susceptible to share the space with an extra electron, increasing the electron affinity.
Atomic Radius is a term describing the distance between an atom’s nucleus, and its outermost electron shell. Several factors affect this distance; including the number of an element, and the number of electron shells. Through Periodic trends, the atomic radius increases in size further left of a period, and lower down a group. Following this pattern, cesium is shown to have the largest atomic radius.