Physical Change Across a Period

There are a few major changes in physical properties across a period. For example,
  1. the state of matter changes across the period.
  2. The size of the molecule changes across the period, too.

State of Matter


  1. The diagram above shows that the state of matter of the elements in period 2 and period 3 change from solid to gas across the period (At room temperature).
  2. In period 2, the first 4 elements are solid whereas the last 4 elements are gases. The melting point of lithium is low while the melting point of boron and carbon is very high.
  3. In period 3, the first 6 elements are solid whereas the last 2 elements are gases.

Atomic Size


  1. As shown in the diagram above, the atomic size of elements in period 2 and period 3 decreases across a period.
  2. This is because
    1. The number of proton in the nucleus increases whereas the number of shells remains unchanged across a period.
    2. As the number of proton in the nucleus increases, the positive charge of the nucleus will also increases.
    3. The negative charge of the valence electrons will also increases due to the increase of number of valence electrons across a period.
    4. Thus, the attraction force between the nucleus and the valence electrons is getting stronger and stronger across a period.
    5. This force will pull the valence electrons closer to the nucleus and thus reduces the atomic radius.
    6. Therefore the size of atom decreases across a period from left to right.
  3. Diagram below shows the change of the atomic size of the elements in all seven period. The trends can be conclude as below:
    1. The size of atom increases down the group.
    2. The size of atom decreases across a period from left to right.
    3. For transition metal, the size of atom does not have obvious change across a period.

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