What is the trend in atomic size in the series of Group IIA elements Be through Ra?
Size increases from Be to Ra.
To understand the trend in atomic size in the series of Group IIA elements from Be (beryllium) through Ra (radium), we need to consider a few factors.
1. Atomic Radius: The atomic radius is defined as the distance from the nucleus to the outermost electron orbit. Generally, as you move down a group on the periodic table, the atomic size increases due to the addition of new electron shells. This is because each subsequent element has one additional energy level, which leads to an increase in the size of the atom.
2. Effective Nuclear Charge: The effective nuclear charge is the positive charge experienced by the outermost electrons from the nucleus. It determines the attractive force between the electrons and the nucleus. As you move from left to right across a period, the effective nuclear charge increases because the number of protons in the nucleus increases. This increased charge pulls the outer electrons closer to the nucleus, resulting in a smaller atomic size.
Applying these factors to the Group IIA elements from Be to Ra:
- Beryllium (Be): Beryllium has a smaller atomic radius than the subsequent elements because it has only two electron shells. The increased effective nuclear charge experienced by the outer electrons also contributes to its small size.
- Magnesium (Mg): Moving down the group, the atomic radius increases. Magnesium has an additional electron shell compared to beryllium, leading to a larger atomic size.
- Calcium (Ca): Calcium's atomic radius is larger than that of magnesium because it has one more electron shell. The trend of increasing atomic size continues.
- Strontium (Sr): Strontium is larger than calcium due to the addition of another electron shell.
- Barium (Ba): Barium is larger than strontium as it has an additional electron shell.
- Radium (Ra): Radium has the largest atomic size among the Group IIA elements because it has one more electron shell than barium.
In summary, as you move down the Group IIA elements from Be to Ra, the trend is an increase in atomic size due to the addition of electron shells, despite the increasing effective nuclear charge.