Periodic Classification of Elements • Topic 3 of 3

Trends in the Modern Periodic Table

The real beauty of the periodic table is that an element's position lets you predict its properties. As you move across a period or down a group, properties change in regular, repeating patterns called periodic trends. Four of them matter most at this level.

Valency

Valency is the combining capacity of an element, decided by the number of valence electrons. Across a period, valency first increases from 1 to 4 and then decreases to 0 (for the noble gas). For period 3: Na = 1, Mg = 2, Al = 3, Si = 4, P = 3, S = 2, Cl = 1, Ar = 0. Down a group, valency stays the same because the number of valence electrons does not change.

Atomic size (atomic radius)

Atomic size is the distance from the centre of the nucleus to the outermost shell.

  • Across a period (left to right): atomic size decreases. The number of shells stays the same, but the nuclear charge (number of protons) increases, pulling the electrons in more strongly. So the atom shrinks.
  • Down a group: atomic size increases. A new shell is added at each step, so the outermost electrons are farther from the nucleus, despite the rising nuclear charge.

Metallic and non-metallic character

Metals tend to lose electrons (they are electropositive); non-metals tend to gain electrons (they are electronegative).

  • Across a period: metallic character decreases and non-metallic character increases. As atomic size falls and nuclear pull rises, atoms hold their electrons more tightly, so losing electrons becomes harder and gaining them becomes easier.
  • Down a group: metallic character increases and non-metallic character decreases. As atomic size grows, the outer electrons are loosely held and easily lost, so elements become more metallic.

This is why metals lie on the left of the table, non-metals on the right, and a staircase of metalloids (such as silicon and germanium) runs between them.

Why the trends exist — the two competing factors

Almost every trend is a tug-of-war between two effects: the increasing nuclear charge (which pulls electrons in) and the addition of new shells (which pushes the outer electrons out). Across a period, nuclear charge wins because no new shell is added. Down a group, the new shells win because the extra distance and shielding outweigh the rising charge.

Atomic size and metallic character: decrease across a period, increase down a groupAcross a period (left -- right)Atomic size decreases, metallic character decreasesDown a groupAtomic size increasesMetallic character increasesNaMgSiClsize shrinks across period
Solved Examples
1
Worked Example
Arrange the elements Na, Mg and Al (all in period 3) in increasing order of atomic size.
Solution
  1. All three are in the same period, so atomic size decreases from left to right.
  2. The order across the period is Na, then Mg, then Al.
  3. So Al is smallest and Na is largest; increasing order reverses this.

Answer: Al < Mg < Na (increasing size).

2
Worked Example
Why does atomic size decrease as we move from left to right across a period?
Solution
  1. Across a period the number of shells remains the same.
  2. The atomic number (nuclear charge) increases by one at each step.
  3. The greater nuclear charge pulls the electrons closer, so the atom becomes smaller.

Answer: The nuclear charge increases while the number of shells stays the same, pulling electrons inward, so the size decreases.

3
Worked Example
An element X has configuration 2, 8, 7. Predict its valency.
Solution
  1. The number of valence electrons is 7.
  2. It needs only 1 more electron to complete its octet.
  3. So its valency is 8 - 7 = 1.

Answer: Valency of X is 1 (it gains 1 electron; X is chlorine).

4
Worked Example
Among Li, Na and K (all in Group 1), which is the most metallic and why?
Solution
  1. Down a group, metallic character increases because atomic size grows.
  2. K is at the bottom, so it has the largest size of the three.
  3. Its outer electron is loosely held and most easily lost, making it the most metallic.

Answer: Potassium (K) is the most metallic, as its large size means its valence electron is lost most easily.

5
Worked Example
An element is in Period 3 and Group 2. Identify it, write its configuration and state its valency.
Solution
  1. Period 3 means 3 shells; Group 2 means 2 valence electrons.
  2. Configuration 2, 8, 2 corresponds to magnesium (Z = 12).
  3. With 2 valence electrons, it loses 2 to form Mg2+, so its valency is 2.

Answer: Magnesium (2, 8, 2); valency = 2.

6
Worked Example
Compare the non-metallic character of fluorine and chlorine, both in Group 17.
Solution
  1. Down a group, non-metallic character decreases as atomic size increases.
  2. Fluorine is at the top of Group 17, so it is smaller than chlorine.
  3. Its small size lets it attract electrons most strongly, making it more non-metallic.

Answer: Fluorine is more non-metallic than chlorine because its smaller size gives it a stronger pull on electrons.

Key Points

  • Valency increases (1 to 4) then decreases (4 to 0) across a period, but stays the same down a group.
  • Atomic size decreases across a period (nuclear charge rises, shells fixed) and increases down a group (new shells added).
  • Metallic character decreases across a period and increases down a group; non-metallic character does the opposite.
  • Metals lie on the left, non-metals on the right, with metalloids (Si, Ge) forming a staircase between them.
  • Every trend is a tug-of-war between increasing nuclear charge (pulls electrons in) and added shells (push them out).
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.As we move from left to right across a period, the atomic size:
Explanation: Nuclear charge increases while shells stay the same, so electrons are pulled in and size decreases.
Q2.Down a group, the metallic character of elements:
Explanation: Atomic size grows down a group, so outer electrons are lost more easily and metallic character increases.
Q3.The valency of an element with configuration 2, 8, 8, 1 is:
Explanation: It has 1 valence electron, which it loses, so its valency is 1 (it is potassium).
Q4.Which of the following has the largest atomic size?
Explanation: All are in period 3; size decreases left to right, so Na (leftmost) is the largest.
Practice more MCQs → 📝 Assignment · 30 marks