What Is the Ionization Energy Trend Across Periods and Groups
Ionisation energy is the amount of energy required to remove an electron from a specific gaseous atom or ion. It applies to all the elements on the periodic table and not just the atoms that are gases at room temperature.
Trends and Periods
Looking at the periodic trend, as the students go from lithium over to neon, across the periodic table, the students can notice that there’s an increase in the ionisation energy. Lithium is positive 520 kilojoules per mole, and Beryllium’s goes up to 900 kilojoules per mole, and then again, in general, there’s an increase in ionisation energies going over to neon. That is because there’s also a relative increase in the effective nuclear charge. An ion is just an atom or a molecule with a charge, and it’ll have a charge if the protons are not equal to electrons. Neutrons are also composed of atoms but are neutral. The charge is given from protons or electrons, which is a net charge for an atom or molecule. A molecule’s just a cluster of atoms bonded together. The negative ions are more significant in the number of electrons than protons. So, for example, Hydrogen in its neutral state has one proton and one electron. Still, even if one of the electrons is taken away, then Hydrogen would have a positive charge, and essentially, it would just be, in its most common isotope, it would just be a proton by itself. And so, when it’s a positive ion where the number of protons is more than electrons, it is called Cations. Cation is just another word for positive ions. Likewise, we can have negative ions. For example, Fluorine. When fluorine gains an electron, it will have a negative charge. A negative ion is named an Anion.
With the help of Ionisation, one can ionise different elements in the periodic table and turn them into cations. However, turning the element into gas is necessary before moving onto the electron.
Metals have low ionisation energy, whereas nonmetals have high ionisation energy. Ionisation energy will increase from left to right, and it will rise from the bottom to the top on the periodic table. Therefore, the lowest ionisation energy will be Francium, and the highest ionisation energy will be Helium.
FAQs on Ionization Energy Trend in the Periodic Table Explained
1. What is ionization energy in chemistry?
The ionization energy is the minimum energy required to remove the most loosely bound electron from an isolated gaseous atom to form a positive ion. It is usually expressed in kJ mol-1 and applies to atoms in the gaseous state.
- First ionization: X(g) → X+(g) + e-
- Second ionization: X+(g) → X2+(g) + e-
2. What is the trend of ionization energy across a period?
Ionization energy generally increases from left to right across a period in the periodic table. This happens because:
- Atomic number increases, so nuclear charge increases.
- Electrons are added to the same energy level.
- Effective nuclear charge increases, pulling electrons closer.
3. What is the trend of ionization energy down a group?
Ionization energy generally decreases down a group in the periodic table. This occurs because:
- Atoms have more electron shells.
- The outer electron is farther from the nucleus.
- Shielding effect increases due to inner electrons.
4. Why does ionization energy increase across a period?
Ionization energy increases across a period because the effective nuclear charge increases while shielding remains nearly constant. Key reasons include:
- More protons are added to the nucleus.
- Electrons are added to the same principal energy level.
- The atomic radius decreases.
5. Why does ionization energy decrease down a group?
Ionization energy decreases down a group because the atomic radius increases and shielding becomes stronger. Specifically:
- Additional energy levels increase the distance between nucleus and valence electron.
- Inner-shell electrons cause greater electron shielding.
- The nuclear attraction for the outer electron weakens.
6. What is the difference between first and second ionization energy?
The first ionization energy is the energy required to remove the first electron from a neutral atom, while the second ionization energy is the energy required to remove an electron from the resulting positive ion. For example:
- First: Na(g) → Na+(g) + e-
- Second: Na+(g) → Na2+(g) + e-
7. Why is the second ionization energy always higher than the first?
The second ionization energy is always higher because an electron is removed from a positively charged ion, which holds its remaining electrons more strongly. After the first electron is removed:
- The ion has a greater effective nuclear attraction per electron.
- There is less electron–electron repulsion.
8. What factors affect ionization energy?
Ionization energy is mainly affected by atomic radius, nuclear charge, shielding effect, and electron configuration. The major factors are:
- Atomic size: Larger atoms have lower ionization energy.
- Nuclear charge: Higher charge increases ionization energy.
- Shielding: More inner electrons reduce nuclear attraction.
- Subshell stability: Half-filled and fully filled subshells are more stable.
9. What are the common exceptions to the ionization energy trend?
Common exceptions to the ionization energy trend occur between Group 2 and Group 13 and between Group 15 and Group 16. For example:
- Be > B: Boron’s outer electron is in a higher-energy p-orbital, making it easier to remove.
- N > O: Oxygen has paired electrons in a p-orbital, increasing electron repulsion.
10. How is ionization energy related to reactivity?
Ionization energy is inversely related to the reactivity of metals, meaning lower ionization energy leads to higher metallic reactivity. For example:
- Alkali metals like Li, Na, and K have low first ionization energies.
- They readily form cations such as Na+ and K+.
