Question

(a) Why is calcium generally more reactive than beryllium? (b) Why is calcium generally less reactive than rubidium?

Short answer

(a) Calcium is more reactive than beryllium because it has a larger atomic radius (1.97 Å) and a lower ionization energy (590 kJ/mol), making it easier for calcium to lose electrons. (b) Calcium is less reactive than rubidium because rubidium has a larger atomic radius (2.50 Å) and a lower ionization energy (403 kJ/mol), allowing rubidium to lose electrons more easily.

Step-by-step solution

a) Comparing Reactivity of Calcium and Beryllium

Calcium (Ca) belongs to Group 2 (alkaline earth metals) and Period 4, while Beryllium (Be) also belongs to Group 2 and Period 2. As we go down a group in the periodic table, the atomic radius increases due to the addition of electron shells. Calcium has an atomic radius of 1.97 Å, while beryllium has a smaller atomic radius of 1.11 Å. A larger atomic radius means that the valence electrons are farther away from the nucleus, making it easier for them to be lost. This makes calcium more reactive than beryllium. In addition, ionization energy (the energy required to remove an electron) decreases as we go down a group in the periodic table. Calcium has a lower ionization energy (590 kJ/mol) than beryllium (899 kJ/mol), so calcium will lose electrons more easily. This is another reason why calcium is more reactive than beryllium.

b) Comparing Reactivity of Calcium and Rubidium

Calcium and Rubidium (Rb) both belong to Period 4 in the periodic table, but rubidium is in Group 1 (alkali metals), while calcium is in Group 2. As we move from left to right within a period, the atomic radius decreases, and ionization energy generally increases. Rubidium has an atomic radius of 2.50 Å and an ionization energy of 403 kJ/mol, while calcium has an atomic radius of 1.97 Å and an ionization energy of 590 kJ/mol. Rubidium's larger atomic radius and lower ionization energy mean that it loses electrons more easily than calcium. Since reactivity of metals is related to their ability to lose electrons easily, rubidium is more reactive than calcium.

Key concepts

Periodic Table

The periodic table is a structured arrangement of all known elements, organized in a way that showcases patterns in their properties. Each element is placed based on its atomic number, which is the number of protons in its nucleus. Elements are grouped into periods (rows) and groups or families (columns) that share similar chemical properties.

When analyzing reactivity, particularly in metals, the position of an element on the periodic table is significant. Metals located in groups 1 and 2, known respectively as alkali metals and alkaline earth metals, are more reactive than those located in groups further to the right of the table. As we move down the elements in a given group, generally, their reactivity increases. This is due to changes in atomic size and ionization energy among other factors.

Atomic Radius

Atomic radius is essentially the size of an atom, defined as the distance from the nucleus to the outer boundary of the electron cloud. It gives insight into how an atom will behave in chemical reactions, especially how easily it may lose electrons.

As you go down a group in the periodic table, additional electron shells are added. This increases the atomic radius. For example, calcium (Ca) and beryllium (Be) belong to the same group, but calcium has a larger atomic radius because it is lower in the group. A larger atomic radius generally means that the valence electrons are further from the nucleus and held less tightly, making them more easily lost. Hence, calcium is more reactive than beryllium.

• Bigger atomic radius: Easier to lose electrons.
• Smaller atomic radius: Harder to lose electrons.

Ionization Energy

Ionization energy is the amount of energy required to remove an electron from an atom. It is a crucial factor in determining how reactive a metal can be.

When moving down a group in the periodic table, ionization energy decreases because the outermost electrons are further away from the nucleus. This makes them easier to remove. Hence, metals like calcium, which have a lower ionization energy (compared to beryllium, for example), tend to lose electrons more readily, making them more reactive.

• Low ionization energy: Easier electron loss, more reactive.
• High ionization energy: Harder electron loss, less reactive.

Alkaline Earth Metals

Alkaline earth metals, found in group 2 of the periodic table, include beryllium, magnesium, calcium, strontium, barium, and radium. These metals share characteristics such as having two valence electrons and typically forming +2 cations.

As with all metals, reactivity is linked to their ability to lose electrons. Within this group, reactivity increases as you move down the group. This is due to increased atomic radius and decreased ionization energy. Calcium is more reactive than beryllium because it has a larger atomic radius and lower ionization energy, making it easier for calcium to lose electrons.

Alkali Metals

Alkali metals are highly reactive metals located in group 1 of the periodic table, and they include elements like lithium, sodium, and rubidium. They are characterized by having a single electron in their outermost shell, which they lose easily to form +1 cations.

These metals are extremely reactive, more so than the alkaline earth metals in group 2, because they only need to lose a single electron. Rubidium, for example, is more reactive than calcium because it has a significantly larger atomic radius and much lower ionization energy. This makes the outer electron easier to remove, thus increasing its reactivity.

• Single valence electron: Very reactive.
• Larger atomic radius and lower ionization energy compared to alkaline earth metals.