Why and how are elements arranged in the 4th period? Why does the size of an atom not decrease regularly in a period? Give the trend of ionization energy in a period.
The Correct Answer and Explanation is:
Why and How Are Elements Arranged in the 4th Period?
Elements in the 4th period of the periodic table are arranged in order of increasing atomic number, which means each subsequent element has one more proton and one more electron than the previous one. These electrons fill atomic orbitals according to the Aufbau principle, starting with the 4s orbital, then 3d, and finally 4p orbitals. This gives rise to the sequence:
- K (19) to Ca (20): filling the 4s orbital
- Sc (21) to Zn (30): filling the 3d orbitals (transition metals)
- Ga (31) to Kr (36): filling the 4p orbitals
This arrangement ensures that chemical properties vary periodically and elements with similar outer electron configurations (and thus similar properties) fall into vertical groups.
Why Atomic Size Does Not Decrease Regularly Across a Period
Generally, atomic size (or radius) decreases across a period due to the increasing nuclear charge (more protons), which pulls electrons closer to the nucleus. However, in the 4th period, the size does not decrease regularly. This irregularity is primarily due to the presence of the 3d orbitals:
- As 3d electrons are added (from Sc to Zn), they do not shield the nuclear charge effectively.
- This leads to only a slight decrease or near constant atomic radii across the transition metals.
- After Zn, the size starts decreasing more noticeably again with p-block elements (Ga to Kr).
Trend of Ionization Energy in a Period
Ionization energy (IE) is the energy required to remove an electron from a neutral atom in its gaseous state.
- Trend: Ionization energy generally increases across a period from left to right.
- Reason: Increasing nuclear charge holds electrons more tightly.
- Exceptions: Slight dips occur, such as between Group 2 and Group 13 and Group 15 and Group 16, due to sublevel electron configurations and electron repulsion effects.
In the 4th period, ionization energy increases overall from K to Kr, but with minor irregularities due to electron shielding and subshell stability.
