Consider S, Cl, and K and their most common ions. (a) List the atoms in order of increasing size.

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Identify the most common ions for sulfur (S), chlorine (Cl), and potassium (K). Sulfur commonly forms S^2-, chlorine forms Cl^-, and potassium forms K^+.

Recall that cations (positively charged ions) are generally smaller than their neutral atoms due to loss of electrons which reduces electron-electron repulsion and allows the nucleus to pull the remaining electrons closer. Anions (negatively charged ions) are generally larger than their neutral atoms because the addition of electrons increases electron-electron repulsion, causing the electron cloud to expand.

Compare the charges of the ions: K^+ (a cation), Cl^- (an anion), and S^2- (an anion with a higher negative charge than Cl^-).

Understand that the size of ions increases as the magnitude of the negative charge increases. Therefore, among anions, those with higher negative charges are larger because they have more electrons for the same number of protons in the nucleus.

Arrange the ions in order of increasing size based on the above understanding: K^+ (smallest), Cl^-, S^2- (largest).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Atomic Radius

The atomic radius is a measure of the size of an atom, typically defined as the distance from the nucleus to the outermost electron shell. Atomic size generally increases down a group in the periodic table due to the addition of electron shells, while it decreases across a period from left to right due to increased nuclear charge, which pulls electrons closer to the nucleus.

Ionic Size

Ionic size refers to the size of an ion, which can differ significantly from the neutral atom. Cations (positively charged ions) are smaller than their parent atoms because they lose one or more electrons, reducing electron-electron repulsion and allowing the remaining electrons to be pulled closer to the nucleus. Conversely, anions (negatively charged ions) are larger than their parent atoms due to the addition of electrons, which increases repulsion among electrons.

Periodic Trends

Periodic trends are patterns observed in the periodic table that describe how certain properties of elements change across periods and down groups. For atomic and ionic sizes, the trend shows that as you move from left to right across a period, the size decreases, while moving down a group increases the size. Understanding these trends is essential for predicting the relative sizes of atoms and their ions.

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