Question

Among the alkali metals, cesium is the most reactive because: (a) It has a single electron in the valence shell. (b) Its incomplete shell is nearest to the nucleus. (c) The outermost electron is more loosely bound than the outermost electron to the other alkali metals. (d) It is the heaviest alkali metal.

Short answer

Option (c) is correct: Cesium's outer electron is more loosely bound.

Step-by-step solution

Understand the Concept of Reactivity in Alkali Metals

Alkali metals are highly reactive elements due to their single electron in the outermost shell, which they tend to lose easily. As we move down the group in the periodic table from lithium to cesium, the reactivity increases because the outermost electron is further from the nucleus, making it more loosely bound to the atom.

Examine Each Option

Review each option to determine which best explains why cesium is the most reactive: - Option (a): All alkali metals have a single electron in their valence shell, so this does not specifically explain cesium's reactivity. - Option (b): This option contradicts the fact that as you move down the group, the outer electron is farther from the nucleus. - Option (c): This aligns with the concept that cesium's outer electron is less tightly bound, enhancing its reactivity. - Option (d): While it's true that cesium is the heaviest, reactivity is more influenced by electron binding than atomic mass.

Select the Correct Option

Based on the analysis, option (c) clearly explains that cesium's higher reactivity is due to its outermost electron being more loosely bound than in other alkali metals. This results from increased atomic size and electron shielding effects as we move down the group.

Key concepts

periodic table trends

In the periodic table, elements are arranged in rows called periods and columns known as groups based on their atomic number. Core characteristics of elements, such as reactivity, can often be predicted based on their position in the periodic table. For example, alkali metals belong to Group 1 and include elements like lithium, sodium, and cesium. As you move from top to bottom down the group, several trends can be observed.

• The size of atoms increases because each successive element has an additional electron shell.
• This leads to increased shielding, where inner electrons block the attraction between the nucleus and the outermost electron.
• As the electrons are further away and less tightly bound, reactivity increases as it becomes easier for the atom to lose its outer electron.

Understanding these trends is crucial when studying chemical reactivity, especially in predicting how reactive an element like cesium might be compared to others in its group.

electron binding energy

Electron binding energy refers to the amount of energy needed to remove an electron from an atom. In alkali metals, the outermost electron is only loosely bound to the atom. The energy required to detach this electron is relatively low, which is why these elements are known for their high reactivity. As you move down the group, the binding energy decreases. For cesium, the outer electron experiences less electrostatic pull from the nucleus since it is farther away compared to lithium or sodium. This lower electron binding energy is why cesium can easily lose its valence electron. With less energy needed to remove this electron, cesium reactivity is notably higher as it tends to readily engage in chemical reactions, sometimes even explosively so. Therefore, understanding electron binding energy is key when evaluating why some alkali metals are more reactive than others.

atomic size and shielding

The atomic size of an element is determined by the distance of the outermost electron shell from the nucleus. Within a column of the periodic table, atomic size grows from top to bottom, adding a new layer of electron cloud with each subsequent element. This phenomenon is crucial to understanding elements like cesium. As atomic size increases, so does the effect of electron shielding. This occurs because inner shell electrons partially shield the attractive force of the positively charged nucleus from the electron being evaluated, usually the valence electron. This shielding effect allows the outer electron to become increasingly detached, further influencing the element's reactivity by making it easier for the electron to escape. Cesium's larger atomic size and enhanced electron shielding combine to make it exceptionally reactive, as it readily loses its outermost electron to form bonds.

cesium's properties

Cesium is an alkali metal with unique and intriguing properties, making it the most reactive among its group. With an atomic number of 55, cesium is heavier and larger than other alkali metals, contributing to its position at the bottom of Group 1 in the periodic table. Key properties of cesium include:

• It has a very low melting point of 28.5°C, near room temperature, so it's one of the few metals that can be liquid.
• The metal's golden color makes it visually distinct among other alkali metals, which are typically silver.
• Its larger atomic radius means its outer electron is more easily lost, enhancing its reactivity.
• Cesium’s high reactivity allows it to explode upon contact with water, releasing hydrogen gas and heat.

Cesium’s remarkable reactivity is primarily driven by its electron configuration. Understanding this helps in appreciating not only how cesium stands out in its group but also why it is indispensable in applications such as atomic clocks and photoelectric cells.