List These Atoms Or Ions In Order Of Decreasing Size

The size of atoms and ions is a fundamental property in chemistry that dictates many of their interactions and behaviors. Understanding how to arrange atoms and ions in order of decreasing size requires a grasp of periodic trends, effective nuclear charge, and the effects of gaining or losing electrons.

Factors Affecting Atomic and Ionic Size

Several factors influence the size of atoms and ions. These include:

• Principal Quantum Number (n): This number represents the energy level or shell of an electron. As n increases, the electrons are, on average, farther from the nucleus, leading to larger atomic size.
• Nuclear Charge (Z): This refers to the number of protons in the nucleus. A greater nuclear charge exerts a stronger pull on the electrons, causing the atom to shrink.
• Effective Nuclear Charge (Zeff): This is the net positive charge experienced by an electron in a multi-electron atom. It takes into account the shielding effect of inner electrons, which reduces the full nuclear charge felt by the outer electrons.
• Number of Electrons: Adding electrons increases electron-electron repulsion, causing the electron cloud to expand. Conversely, removing electrons reduces repulsion and causes the electron cloud to contract.
• Electron Configuration: The distribution of electrons in different orbitals affects how effectively they shield each other from the nuclear charge.

Periodic Trends in Atomic Size

Atomic size generally follows predictable trends in the periodic table:

• Across a Period (Left to Right): Atomic size decreases. This is because, as you move across a period, the number of protons in the nucleus increases, leading to a greater effective nuclear charge (Zeff). This increased attraction pulls the electrons closer to the nucleus, resulting in a smaller atomic radius. The principal quantum number remains constant across a period, so the effect of increased nuclear charge dominates.
• Down a Group (Top to Bottom): Atomic size increases. As you move down a group, electrons are added to higher energy levels (larger n values). This means the outermost electrons are, on average, farther from the nucleus. While the nuclear charge also increases, the effect of adding new energy levels is more significant, causing the atoms to become larger. Shielding by inner electrons also plays a role, reducing the effective nuclear charge felt by the outer electrons.

Ionic Size

Ionic size is the radius of an ion. Ions are formed when atoms gain or lose electrons. When an atom loses electrons to form a cation (positive ion), the remaining electrons are more strongly attracted to the nucleus, causing the ion to shrink compared to the neutral atom. Conversely, when an atom gains electrons to form an anion (negative ion), the increased electron-electron repulsion causes the electron cloud to expand, making the ion larger than the neutral atom.

• Cations: Generally smaller than their corresponding neutral atoms. The positive charge attracts the remaining electrons more strongly.
• Anions: Generally larger than their corresponding neutral atoms. The added electrons increase repulsion.

Isoelectronic Series

An isoelectronic series is a group of ions or atoms that have the same number of electrons. When comparing the sizes of ions in an isoelectronic series, the ion with the greater nuclear charge will be smaller because the electrons are more strongly attracted to the nucleus. For example, consider the isoelectronic series: O2-, F-, Na+, Mg2+, and Al3+. All these species have 10 electrons, but they have different numbers of protons. In this series, Al3+ (13 protons) is the smallest, and O2- (8 protons) is the largest.

Steps to Order Atoms or Ions by Size

To list atoms or ions in order of decreasing size, follow these steps:

1. Identify the Atoms and Ions: Determine which elements and ions are involved.
2. Determine the Number of Protons and Electrons: Find the number of protons (atomic number) and electrons for each species. For neutral atoms, the number of protons equals the number of electrons. For ions, adjust the number of electrons based on the charge (add electrons for anions, subtract for cations).
3. Consider Periodic Trends: Use the periodic table to get a general idea of the relative sizes of the neutral atoms. Remember that size decreases across a period and increases down a group.
4. Account for Ionic Charge: Cations are smaller than their parent atoms, and anions are larger. The greater the positive charge, the smaller the cation. The greater the negative charge, the larger the anion.
5. Compare Isoelectronic Species: If there are isoelectronic species, the ion with the largest nuclear charge (most protons) will be the smallest.
6. Arrange in Order of Decreasing Size: Based on the above considerations, list the atoms and ions from largest to smallest.

Examples

Let's walk through some examples to illustrate the process of ordering atoms and ions by size.

Example 1: Ordering Neutral Atoms

List the following atoms in order of decreasing size: Na, Cl, Mg, K.

1. Identify the Atoms: Na (Sodium), Cl (Chlorine), Mg (Magnesium), K (Potassium).
2. Determine Protons and Electrons:
   • Na: 11 protons, 11 electrons
   • Cl: 17 protons, 17 electrons
   • Mg: 12 protons, 12 electrons
   • K: 19 protons, 19 electrons
3. Consider Periodic Trends:
   • Na and Mg are in the same period (Period 3), with Na to the left of Mg. So, Na > Mg.
   • K is directly below Na in Group 1, so K > Na.
   • Cl is in Period 3, far to the right of Na and Mg, so Cl is the smallest among these.
4. Arrange in Order: K > Na > Mg > Cl

Example 2: Ordering Ions

List the following ions in order of decreasing size: S2-, Cl-, K+, Ca2+.

1. Identify the Ions: S2- (Sulfide), Cl- (Chloride), K+ (Potassium), Ca2+ (Calcium).
2. Determine Protons and Electrons:
   • S2-: 16 protons, 18 electrons
   • Cl-: 17 protons, 18 electrons
   • K+: 19 protons, 18 electrons
   • Ca2+: 20 protons, 18 electrons
3. Recognize Isoelectronic Series: All these ions have 18 electrons.
4. Compare Isoelectronic Species: In an isoelectronic series, the ion with the most protons is the smallest. Therefore:
   • Ca2+ < K+ < Cl- < S2-
5. Arrange in Order: S2- > Cl- > K+ > Ca2+

Example 3: Ordering Atoms and Ions

List the following species in order of decreasing size: O, O-, O2-.

1. Identify the Species: O (Oxygen atom), O- (Oxygen anion with one extra electron), O2- (Oxygen anion with two extra electrons).
2. Determine Protons and Electrons:
   • O: 8 protons, 8 electrons
   • O-: 8 protons, 9 electrons
   • O2-: 8 protons, 10 electrons
3. Account for Ionic Charge: Adding electrons increases the size due to increased electron-electron repulsion.
4. Arrange in Order: O2- > O- > O

Example 4: A More Complex Example

List the following species in order of decreasing size: Na+, Mg2+, Al3+, Ne, F-.

1. Identify the Species: Na+ (Sodium ion), Mg2+ (Magnesium ion), Al3+ (Aluminum ion), Ne (Neon atom), F- (Fluoride ion).
2. Determine Protons and Electrons:
   • Na+: 11 protons, 10 electrons
   • Mg2+: 12 protons, 10 electrons
   • Al3+: 13 protons, 10 electrons
   • Ne: 10 protons, 10 electrons
   • F-: 9 protons, 10 electrons
3. Recognize Isoelectronic Series: Na+, Mg2+, Al3+, Ne, and F- are all isoelectronic, with 10 electrons each.
4. Compare Isoelectronic Species: Within an isoelectronic series, the species with the most protons is the smallest.
5. Arrange in Order: F- > Ne > Na+ > Mg2+ > Al3+

Common Pitfalls

When ordering atoms and ions by size, watch out for these common mistakes:

• Ignoring the Effect of Ionic Charge: Forgetting that cations are smaller than their neutral atoms and anions are larger.
• Not Recognizing Isoelectronic Species: Failing to notice when ions have the same number of electrons, which simplifies comparison.
• Overlooking Periodic Trends: Not using the periodic table to get a general sense of the relative sizes of neutral atoms.
• Miscounting Protons or Electrons: Making errors when determining the number of protons and electrons for each species.

Significance of Atomic and Ionic Size

The size of atoms and ions has significant implications in chemistry and materials science:

• Chemical Reactivity: Atomic and ionic size influences the reactivity of elements. Smaller atoms or ions with a high charge density can exert a strong polarizing effect on nearby molecules or ions, affecting reaction rates and mechanisms.
• Lattice Energy: The lattice energy of ionic compounds depends on the size and charge of the ions. Smaller ions with higher charges result in greater lattice energies, leading to more stable compounds.
• Coordination Chemistry: The size of metal ions influences the number and type of ligands they can coordinate with. Larger ions can accommodate more ligands, while smaller ions tend to form complexes with fewer ligands.
• Solid-State Properties: The size of atoms and ions affects the crystal structure, density, and other physical properties of solids. For example, the packing efficiency in a crystal lattice depends on the relative sizes of the constituent atoms or ions.
• Biological Systems: Ionic size plays a crucial role in biological systems, such as ion channels in cell membranes. The selectivity of these channels for specific ions depends on the size and charge of the ions.

Conclusion

Arranging atoms and ions in order of decreasing size requires a comprehensive understanding of periodic trends, effective nuclear charge, and the effects of gaining or losing electrons. By systematically considering these factors, you can accurately predict the relative sizes of atoms and ions and understand how size influences their chemical and physical properties. Paying attention to the number of protons and electrons, recognizing isoelectronic species, and utilizing the periodic table as a guide are crucial steps in this process. Mastering these concepts is essential for success in chemistry and related fields.