Predict the physical properties and relative reactivity of
Ba and Sr
Cs and Br
( elements from periodic table )
To predict the physical properties and relative reactivity of elements like Ba and Sr, and Cs and Br, we need to analyze their positions in the periodic table.
1. Ba (Barium) and Sr (Strontium):
These elements belong to Group 2 of the periodic table, also known as the alkaline earth metals. As we go down Group 2, the atomic radius increases due to the addition of more electron shells. Consequently, the outermost electrons are located further away from the positively charged nucleus, leading to a decrease in attraction between the nucleus and the valence electrons. This increase in atomic size affects several physical properties:
- Atomic radius: Both Ba and Sr have larger atomic radii than many other elements, due to the increasing number of electron shells as we move down the group.
- Melting and boiling points: Generally, melting and boiling points decrease down Group 2, since the attraction between the valence electrons and nucleus becomes weaker with larger atomic size.
- Reactivity: Ba and Sr are highly reactive metals. They readily lose their two valence electrons to form +2 ions. However, Sr is slightly more reactive than Ba due to its smaller atomic size and greater effective nuclear charge, making it easier for Sr to lose its electrons.
2. Cs (Cesium) and Br (Bromine):
Cesium belongs to Group 1 (Alkali metals) and Bromine belongs to Group 17 (Halogens) of the periodic table. These groups are highly reactive due to their easily gained or lost valence electrons.
- Atomic radius: Cs has the largest atomic radius among all elements, while Br has a relatively small atomic radius.
- Melting and boiling points: Cs has a lower melting and boiling point compared to Br. Alkali metals generally have low melting and boiling points due to weak metallic bonding.
- Reactivity: Cs is one of the most reactive elements because it has one loosely held valence electron that is easily lost to form a +1 ion. Br is also reactive but tends to gain one electron to form a -1 ion. Overall, Cs is more reactive than Br due to the ease of losing electrons compared to gaining them.
In summary, within each pair, the element with the larger atomic size tends to have lower melting and boiling points, higher reactivity, and vice versa.