There are a few major changes in physical properties across a period. For example,
- the state of matter changes across the period.
- The size of the molecule changes across the period, too.
State of Matter
- 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).
- 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.
- In period 3, the first 6 elements are solid whereas the last 2 elements are gases.
- As shown in the diagram above, the atomic size of elements in period 2 and period 3 decreases across a period.
- This is because
- The number of proton in the nucleus increases whereas the number of shells remains unchanged across a period.
- As the number of proton in the nucleus increases, the positive charge of the nucleus will also increase.
- The negative charge of the valence electrons will also increase due to the increase in the number of valence electrons across a period.
- Thus, the attraction force between the nucleus and the valence electrons is getting stronger and stronger across a period.
- This force will pull the valence electrons closer to the nucleus and thus reduces the atomic radius.
- Therefore the size of the atom decreases across a period from left to right.
- The diagram below shows the change in the atomic size of the elements in all seven periods. The trends can be concluded as below:
- The size of the atom increases down the group.
- The size of the atom decreases across a period from left to right.
- For transition metal, the size of the atom does not have obvious change across a period.