Question

State whether each of the following properties of the main group elements generally increases or decreases (a) from left to right across a period and (b) from top to bottom within a group: metallic character, atomic size, ionization energy, acidity of oxides.

Short answer

Metallic character decreases across a period, increases down a group. Atomic size decreases across a period, increases down a group. Ionization energy increases across a period, decreases down a group. Acidity of oxides increases across a period, decreases down a group.

Step-by-step solution

Analyzing Trends Across a Period

For metallic character, as you move from left to right across a period in the periodic table, elements become less metallic. This is because they have more electrons in their outer shell, making it harder to lose electrons easily. For atomic size, atomic radii generally decrease from left to right across a period. This happens as the positive nuclear charge increases, pulling electrons closer to the nucleus. Ionization energy usually increases across a period. As atoms hold on more tightly to their electrons due to increased nuclear charge, more energy is required to remove an electron. For acidity of oxides, non-metal oxides, which are more acidic, become more common as you move from left to right across a period because elements transition from metallic to non-metallic character. Thus, acidity of oxides generally increases.

Analyzing Trends Down a Group

For metallic character, as you move down a group in the periodic table, elements become more metallic. This happens because the outer electrons are further from the nucleus and more easily lost. Atomic size increases down a group. Additional electron shells are added, increasing the distance between the outer electrons and the nucleus. Ionization energy decreases down a group. As atomic size increases, the outer electrons are further from the nucleus and are less tightly bound, requiring less energy to remove. For acidity of oxides, when moving down a group, metallic character increases, and oxides tend to become more basic. Hence, the acidity of oxides decreases.

Key concepts

Metallic Character

The metallic character of an element describes how easily it can lose electrons to form positive ions, often referred to as cations. This property is closely tied to an element's position on the periodic table.

As you move from left to right across a period, the metallic character tends to decrease. This is because elements increase their number of valence electrons and their nuclear charge without gaining new electron shells, making it more difficult to lose electrons. Thus, elements on the left side of the periodic table, like alkali metals, are richer in metallic character than those on the right side, like the noble gases.

Moving down a group in the periodic table, the metallic character increases. Elements further down a group have additional electron shells, which means their valence electrons are further from the nucleus and are more loosely held. This makes them more likely to lose electrons easily and exhibit stronger metallic characteristics.

Ionization Energy

Ionization energy is the energy required to remove an electron from an atom. It provides insights into how strongly atoms hold onto their electrons.

Across a period from left to right, ionization energy generally increases. This is because the increasing nuclear charge attracts the electrons more strongly, making them harder to remove. For instance, oxygen and nitrogen have higher ionization energies than lithium or beryllium.

Conversely, as you move down a group in the periodic table, ionization energy decreases. The added electron shells result in larger atomic sizes, causing the outermost electrons to be further from the nucleus, which reduces the energy needed to remove them. Hence, for elements in the same group, such as the alkali metals, ionization energy is lower for those at the bottom, like cesium, compared to those at the top, like lithium.

Atomic Size

Atomic size, or atomic radius, refers to the distance from the nucleus of an atom to its outermost electron shell. This property is crucial for understanding an element's behavior and reactivity.

When moving from left to right across a period, atomic size generally decreases. The increase in nuclear charge without new electron shells means electrons are pulled closer to the nucleus, resulting in a smaller atomic radius. For example, in the second period, the atomic radius of lithium is larger than that of fluorine.

As you go down a group, atomic size increases. Elements have additional electron shells which take up more space. This results in a larger atomic radius, even though the nuclear charge also increases. For instance, in the halogen group, iodine has a larger atomic radius than chlorine.

Acidity of Oxides

The acidity of oxides examines how oxides react with water to form acids, and is a good indicator of the non-metallic character of elements.

Across a period, the acidity of oxides generally increases. As you move from left to right, the elements shift from metallic to non-metallic in character. Non-metal oxides, like those of sulfur and nitrogen, are typically acidic when dissolved in water because they can form acidic solutions, such as sulfuric acid or nitric acid.

In contrast, when you proceed down a group, the acidity of oxides decreases as elements become more metallic. Metallic oxides, such as those from the alkali metal group, tend to form basic solutions instead. Therefore, the oxides of heavier elements in a group, like sodium oxide, are more basic compared to the more acidic oxides found higher in the group, such as those of oxygen.