Trends across a period and down a group: atomic size, valency and metallic character
Across a period atoms get smaller and less metallic; down a group they get larger and more metallic. Valency rises then falls across, but stays fixed down.
Two causes behind every trend
- Across a period, the number of shells stays the same, but the nuclear charge (number of protons) rises by one at each step. So the nucleus pulls the outer electrons more strongly.
- Down a group, a new shell is added at each step. The outer electrons are farther from the nucleus, so its pull on them is weaker.
Atomic radii across period 2 and down group 1 (values from your NIOS lesson)
| Period 2 element | Radius (pm) | Group 1 element | Radius (pm) |
|---|---|---|---|
| Li | 134 | Li | 134 |
| Be | 90 | Na | 154 |
| B | 82 | K | 196 |
| C | 77 | Rb | 211 |
| N | 75 | Cs | 225 |
| O | 73 | - | - |
| F | 72 | - | - |
Why atomic size changes
From lithium to fluorine, each atom has one more proton and one more electron. But the new electrons all go into the same L shell. The stronger positive pull draws that shell inwards, so the radius falls from 134 pm to 72 pm. Down group 1, each element has one more shell than the one above. So its outer electron sits farther out, and the radius grows from 134 pm for lithium to 225 pm for caesium. Other books give slightly different radii, but the trends are the same.
Metallic character across period 3 and down group 14
| Across period 3 | Nature | Down group 14 | Nature |
|---|---|---|---|
| Na | Metal | C | Non-metal |
| Mg | Metal | Si | Metalloid |
| Al | Metal | Ge | Metalloid |
| Si | Metalloid | Sn | Metal |
| P | Non-metal | Pb | Metal |
| S | Non-metal | - | - |
| Cl | Non-metal | - | - |
Metallic character and valency
- Metallic character is the ease of losing electrons to form positive ions. The alkali metals lose electrons most easily.
- Down a group it increases: bigger atoms hold their outer electrons loosely. So potassium is more metallic than sodium.
- Across a period it decreases: the stronger nuclear pull makes electrons harder to remove. Atoms on the right tend to gain electrons instead, which is non-metallic character.
- Valency across period 3: Na 1, Mg 2, Al 3, Si 4, P 3, S 2, Cl 1, Ar 0. It rises to 4, then falls to 0.
- Valency down a group stays the same, because the number of valence electrons is the same. All of group 1 have valency 1, and all of group 2 have valency 2.
Summary of the trends
| Property | Across a period (left to right) | Down a group (top to bottom) |
|---|---|---|
| Atomic number | Increases by 1 at each step | Increases |
| Number of shells | Stays the same | Increases by 1 at each step |
| Valence electrons | Increase from 1 to 8 | Stay the same |
| Atomic size | Decreases | Increases |
| Metallic character | Decreases | Increases |
| Non-metallic character | Increases | Decreases |
| Valency | Rises from 1 to 4, then falls to 0 | Stays the same |
Sodium and potassium are stored under kerosene because they react with air and water, and potassium reacts even more violently than sodium. That is 'more metallic down a group' in action.
2 marks: “Which is bigger, a sodium atom or a chlorine atom? Give a reason.” Sodium. Both have three shells, but chlorine has 17 protons against sodium's 11. Its stronger nucleus pulls the electrons closer, so the chlorine atom is smaller.
3 marks: “Explain how (a) atomic size and (b) metallic character change down group 1, with reasons.” (a) Size increases, because a new shell is added at each step and the outer electron is farther away (Li 134 pm to Cs 225 pm). (b) Metallic character increases, because the outer electron is held more weakly and is lost more easily; so potassium is more metallic than sodium.
1 marks: “Fill in the blank: Metallic character ______ from left to right across a period.” decreases.
Across: shrink and lose metal. Down: swell and gain metal. Valency across period 3: 1, 2, 3, 4, 3, 2, 1, 0.