Problem 1 - Chemistry in 30 days

Question

Of the following elements, which possesses the highest electron affinity ? (a) As (b) \(\mathrm{O}\) (c) \(\mathrm{S}\) (d) Se

Step-by-Step Solution

Answer

Oxygen (O) has the highest electron affinity among the given elements.

Step 1: Understanding Electron Affinity

Electron affinity refers to the amount of energy released when an atom in the gas phase accepts an electron to form an anion. Elements with higher electron affinity values tend to gain electrons more easily due to their higher effective nuclear charge and smaller atomic radii.

Step 2: List of Elements and Trends

The elements given are Arsenic (As), Oxygen (O), Sulfur (S), and Selenium (Se). Electron affinity generally increases across a period and decreases down a group in the periodic table. Thus, moving from left to right in a period (from elements such as As to O), the electron affinity increases.

Step 3: Comparing Electron Affinities

Oxygen (\(\mathrm{O}\)) is located in Period 2, Group 16, while the other elements are in Group 15 and 16 but in Period 3 and above. Oxygen tends to have a higher electron affinity than Sulfur, Selenium, or Arsenic due to its smaller size and thus greater ability to attract additional electrons.

Key Concepts

Periodic Table Trends

The periodic table is not just a list of elements; it's a map that tells us how elements behave based on their position. One of the key trends you will encounter is electron affinity, which generally increases as you move across a period from left to right. This means that elements on the right side of the periodic table are more likely to accept electrons compared to those on the left. Conversely, as you move down a group, electron affinity tends to decrease. This is because elements further down have more electron shells, which makes it harder for the nucleus to attract additional electrons. Understanding these trends is crucial in predicting how different elements will interact chemically.

Atomic Size

Atomic size, or atomic radius, is another important factor that influences electron affinity. As you move down the periodic table, atoms increase in size because more electron shells are added. This increased distance from the nucleus can reduce the atom's ability to attract additional electrons. Thus, smaller atoms, such as Oxygen, have higher electron affinities due to their larger effective nuclear charge, making them more efficient at pulling in electrons. In contrast, larger atoms like Selenium and Arsenic find it more challenging to gain additional electrons.

Effective Nuclear Charge

The effective nuclear charge (ENC) is the net positive charge experienced by an electron in an atom. It's a concept that helps explain why electron affinity varies among elements. ENC increases across a period because the number of protons in the nucleus increases, thereby increasing the attractive force felt by electrons.

• More protons mean a stronger pull on electrons, enhancing electron affinity.
• It decreases slightly down a group because additional electron shells are added, which shield outer electrons from the nucleus.

Understanding ENC is key to explaining why Oxygen has a higher electron affinity than Sulfur or Selenium.

Anion Formation

When an atom gains an electron, it forms a negatively charged ion known as an anion. The energy change during this process is measured as electron affinity. Elements with a high electron affinity are more likely to form anions. For instance, Oxygen, with its high electron affinity, readily gains an electron to become O⁻.

• Formation of anions involves a shift in electron configuration.
• High ENC and small atomic size favor the formation of stable anions.
• Elements with high electron affinities are often found in the upper right of the periodic table.

The ability to form anions efficiently is a defining feature of nonmetals like Oxygen.