Atomic and Physical Properties of Group 2 Elements
This section looks at the trends in atomic and physical properties of Group 2 elements: beryllium, magnesium, calcium, strontium, and barium. It provides clear information on key properties, such as atomic radius, first ionization energy, and electronegativity. While radium is also part of Group 2, it is less frequently discussed because it is radioactive.
Trends in Atomic Radius
The atomic radius increases from beryllium to barium in Group 2. This happens because new electron shells are added as you go down the group. Beryllium has a much smaller atomic radius than the other elements in this group.
Explaining the Increase in Atomic Radius
Atomic Volume, Atomic, and Ionic Radii:
The atomic volume increases from beryllium to radium due to the addition of electron shells, which enlarges the size of the atoms. Consequently, the atomic and ionic radii of M²⁺ ions also increase as you move down the group. The atomic radii of Group 2 elements are generally smaller than those of alkali metals in the same period because Group 2 elements have a higher nuclear charge, which pulls the electrons closer to the nucleus. As a result, alkaline earth metals are denser and have higher melting points compared to alkali metals.
Table: Some Physical Properties of Alkaline Earth Metals
| Property | Be | Mg | Ca | Sr | Ba | Ra |
|---|---|---|---|---|---|---|
| Atomic weight | 9.01 | 24.31 | 40.08 | 87.62 | 137.34 | 226 |
| Abundance (% of Earth’s crust) | 6.4×10⁻⁴ | 2.0 | 3.45 | 0.915 | 0.040 | 1.3×10⁻¹⁰ |
| Density (g/cm³) | 1.84 | 1.74 | 1.55 | 2.54 | 3.75 | 6.00 |
| Melting point (°C) | 1287 | 650 | 842 | 777 | 727 | 696 |
| Boiling point (°C) | 2471 | 1090 | 1484 | 1382 | 1897 | 1737 |
| Atomic volume (cm³/mol) | 4.90 | 13.97 | 25.9 | 34.54 | 63.7 | 38.0 |
| Atomic radius (Å) | 1.12 | 1.60 | 1.97 | 2.15 | 2.22 | — |
| Covalent radius (Å) | 0.90 | 1.36 | 1.74 | 1.91 | 1.98 | — |
| Ionic radius (Å) | 0.31 | 0.65 | 0.99 | 1.13 | 1.35 | 1.40 |
| First ionization energy (kJ/mol) | 899.4 | 737.7 | 589.7 | 547.5 | 502.9 | 509.3 |
| Second ionization energy (kJ/mol) | 1757.1 | 1450.7 | 1145.4 | 1064.3 | 965.2 | 979.06 |
| Total ionization energy (I₁ + I₂) (kJ/mol) | 2656.5 | 2188.4 | 1735.1 | 1611.8 | 1468.1 | 1488.36 |
| Oxidation state | +2 | +2 | +2 | +2 | +2 | +2 |
| Electronegativity (Pauling scale) | 1.5 | 1.2 | 1.0 | 0.9 | 0.9 | 0.9 |
| Flame coloration | None | None | Brick red | Crimson | Apple green | Red |
| Oxidation potentials (volts) | 1.70 | 1.37 | 2.87 | 2.89 | 2.90 | 2.92 |
| Heat of atomization (kJ/mol) | 327.26 | 146.89 | 181.21 | 1458.67 | 1276.42 | — |
| Heat of hydration (kJ/mol) | 2385.45 | 1925.1 | 1653.07 | |||
| Ionic potential of M²⁺ ion (charge/radius ratio) | 6.66 | 3.08 | 2.12 | 1.82 | 1.55 | 1.33 |
Trends in First Ionization Energy
First ionization energy is the energy required to remove the most loosely held electron from a gaseous atom. It generally decreases as you move down the group due to the increasing atomic radius and electron shielding effect. However, radium has a slightly higher ionization energy than barium.
Explaining the Decrease in First Ionization Energy
The first and second ionization energies decrease with the increase in atomic radius from beryllium to barium. As the atomic radius increases, the outermost electron is further from the nucleus and experiences less electrostatic attraction, making it easier to remove.
Trends in Electronegativity
Electronegativity is the measure of an atom’s ability to attract and hold onto bonding electrons. It decreases as you go down the group. This trend is due to the increasing atomic radius and electron shielding, which reduce the nucleus’s ability to attract electrons.
Trends in Melting and Boiling Points
Melting Points
In Group 2 of the periodic table, there is a general trend of decreasing melting points as you move down the group. However, magnesium is an exception to this trend. Its melting point is actually lower than that of calcium, the element directly below it. Therefore, when magnesium is included, the trend in melting points is not straightforward.
Boiling Points
There is no clear trend in the boiling points of Group 2 elements. Magnesium again serves as an anomaly which has an unusually low boiling point due to its small atomic size and high charge density, which leads to strong metallic bonds. If we disregard magnesium, we observe that the boiling points decrease from beryllium to strontium, and then increase slightly as we move to radium.
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