Question

The fourth-period element with the largest atom is (a) \(\mathrm{K} ;\) (b) \(\mathrm{Br} ;\) (c) \(\mathrm{Pb} ;\) (d) \(\mathrm{Kr}\).

Short answer

The fourth-period element with the largest atom is \( \mathrm{K} \) (Potassium).

Step-by-step solution

Disregard the Incorrect Element

Disregard the element Pb (Lead) as it is not part of the fourth period. It belongs to the sixth period.

Apply the Rule of Atomic Size

According to the rule of atomic size, size decreases across a period from left to right. Since K (Potassium) is the first element in the fourth period and Kr (Krypton) is the last, K will have the largest atomic radius, among these three.

Identifying the Largest Atom

Hence, among K, Br, and Kr, the element K (Potassium) will have the largest atom.

Key concepts

Atomic Radius

The atomic radius is a measure of the size of an atom. It is generally the distance from the nucleus of an atom to the outermost electrons surrounding that nucleus. The atomic radius is a key concept in chemistry because it influences many properties of the elements, such as ionization energy, electronegativity, and chemical bonding.

When you move across a period in the Periodic Table, from left to right, the atomic radius tends to decrease. This happens because the number of protons in the nucleus increases, which attracts the electrons more strongly. As a result, the electrons are drawn closer to the nucleus, leading to a smaller atomic size. Conversely, when moving down a group in the Periodic Table, the atomic radius increases. This is because a new electron shell is added, making the atom larger.

Understanding atomic radius trends helps us predict and explain the chemical behavior and properties of elements. For example, elements with larger atomic radii may lose electrons more easily, forming positive ions, while smaller atoms might attract additional electrons to form negative ions.

Fourth Period Elements

The fourth period elements in the Periodic Table include the elements from potassium (K) to krypton (Kr), spanning atomic numbers 19 to 36. This period contains a mixture of metals, metalloids, and non-metals, demonstrating a wide variety of chemical properties.

Some characteristics of fourth period elements include:

• Transition Metals: Elements such as scandium (Sc) and iron (Fe) are called transition metals. They are known for their ability to form variable oxidation states and for being good conductors of electricity.


• Variable Reactivity: Potassium (K) is highly reactive, especially with water, while elements like krypton (Kr) are much less reactive due to their noble gas configuration.


• Increasing Electronegativity: As you move across the period from left to right, you will generally notice an increase in electronegativity.

The diversity in chemical behavior is closely linked to the electron configurations of these elements and their atomic structures.

Trends in Periodic Table

The Periodic Table is organized in such a way that it reveals trends and patterns among the elements. Understanding these trends allows scientists and students to predict the chemical behavior of elements and the nature of their compounds.

Here are some critical trends observable in the Periodic Table:

• Atomic Size: As mentioned earlier, atomic size decreases across a period and increases down a group.


• Ionization Energy: This is the energy required to remove an electron from an atom. Ionization energy generally increases across a period and decreases down a group.


• Electronegativity: A measure of an atom's ability to attract and bind with electrons. Electronegativity increases across a period and decreases down a group.


• Metallic Character: Typically decreases as you move across a period and increases as you move down a group.

By understanding these trends, you can understand why potassium has a larger atomic radius than bromine and krypton. Potassium is at the beginning of the period and therefore has fewer protons pulling on its electrons compared to elements found later in the period.