Question

Each of the following properties shows a regular trend in Group \(1 \mathrm{~A}(1) .\) Predict whether each increases or decreases up the group: (a) Melting point (b) \(\mathrm{E}-\mathrm{E}\) bond length (c) Hardness (d) Molar volume (e) Lattice energy of EBr

Short answer

Melting point increases, E-E bond length decreases, hardness increases, molar volume decreases, lattice energy of EBr increases up the group.

Step-by-step solution

- Understand Group Trends in the Periodic Table

Group trends refer to patterns or regular changes that occur down a group (column) in the periodic table. For Group 1A (Alkali metals: Li, Na, K, Rb, Cs, Fr), atomic properties change in predictable ways as you move up or down the group. This is influenced by factors such as atomic size, electron shielding, and effective nuclear charge.

- Predict the Trend for Melting Point

In Group 1A, melting points decrease as you move down the group. This is because the atoms become larger, and the metallic bonds become weaker. Therefore, as you move up the group, the melting point increases.

- Predict the Trend for E-E Bond Length

The atomic size of elements increases as you move down the group. Consequently, the bond length between two similar atoms (E-E bond) increases. Therefore, as you move up the group, the E-E bond length decreases.

- Predict the Trend for Hardness

Hardness in Group 1A elements decreases as you move down the group because the metallic bonding becomes weaker due to the increased atomic size and distance between atomic nuclei. Therefore, as you move up the group, the hardness increases.

- Predict the Trend for Molar Volume

Molar volume is related to the size of atoms and the distance between them in the solid state. As atomic size increases down the group, the molar volume increases. Therefore, as you move up the group, the molar volume decreases.

- Predict the Trend for Lattice Energy of EBr

Lattice energy is influenced by the charge and size of the ions. For alkali metals reacting with bromine (forming EBr), larger ions down the group result in lower lattice energy. Therefore, as you move up the group, the lattice energy of EBr increases.

Key concepts

melting point trends

In chemistry, the melting point is the temperature at which a solid turns into a liquid. For Group 1A elements, which include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr), there is a clear trend in melting points as you move up or down the group. The melting points of these elements decrease as you move down the group. This happens because the atoms get larger, and the distance between the nuclei and the bonding electrons becomes greater. This weakens the metallic bonds, which makes it easier for the atoms to move past one another and melt.
As a result, to summarize this trend, if you move up the group from Cs to Li, the melting points increase. This means that lithium has the highest melting point, while francium has the lowest.

bond length trends

Bond length refers to the distance between the nuclei of two bonded atoms. In Group 1A, the bond length between two similar atoms (e.g., Li-Li or Na-Na) shows a predictable trend. As you move down the group, the atomic radius increases, which leads to longer bond lengths. Larger atoms have their nuclei further apart when bonded, resulting in longer bonds.
Conversely, as you move up the group from francium (Fr) to lithium (Li), the atomic radius decreases, and so does the bond length. Therefore, lithium will have the shortest bond lengths among Group 1A elements, while francium will have the longest.

hardness trends

Hardness is a measure of how resistant a material is to deformation, such as scratching or indentation. In Group 1A elements, hardness decreases as you move down the group. This is because of the weaker metallic bonds that form as atomic size increases. Larger atoms have more shielding and less effective nuclear charge per electron, which means that the metallic bonds in elements like cesium (Cs) and francium (Fr) are much weaker than those in lithium (Li) and sodium (Na).
Therefore, if you move up the group from Cs to Li, the hardness increases. Lithium is the hardest alkali metal, while francium is the softest.

molar volume trends

Molar volume is the volume occupied by one mole of a substance. It is influenced by atomic size and the space between atoms in the solid state. In Group 1A elements, the molar volume increases as you move down the group. This is because larger atoms take up more space, leading to a greater volume.
As you move up the group from Fr to Li, the molar volume decreases. This means that lithium, with its smaller atomic size, has the smallest molar volume, while francium has the largest molar volume.

lattice energy trends

Lattice energy is the energy required to separate one mole of an ionic solid into its gaseous ions. For Group 1A elements reacting with bromine to form EBr (where E is an alkali metal), there is a trend in lattice energy based on the size of the ions. As you move down the group, the ions become larger. Larger ions have lower lattice energy because the distance between the oppositely charged ions increases, reducing the electrostatic attraction.
Thus, as you move up the group from CsBr to LiBr, the lattice energy increases. Lithium bromide (LiBr) has the highest lattice energy due to the smaller size of the lithium ion, while cesium bromide (CsBr) has the lowest lattice energy.