AS-Level The Periodic Table

  • Atomic radii decrease across a period in the periodic table.

  • Atomic radii increase down a group in the periodic table.

  • Electrons in inner sub-shells block or shield the positive charge of the nucleus from the electrons in the outer sub-shells; this is called inner electron shielding.

  • The greater the degree of electron shielding, the less tightly outer electrons are pulled into the nucleus so the larger the atomic radii.

  • Across a period, the number of protons in the nucleus of an element increases but the level of shielding stays the same; this pulls the outer electrons in tighter and decreases the atomic radii.


Atomic Radii


Elements with a different number of outer electrons behave differently. This means that as you move across a period in the periodic table, elements will exhibit different characteristics. These differences are repeated for each period; we describe these patterns as ‘periodic trends’.


As you go across a period, atomic radius decreases.














Moving across a period, the atomic number of the elements increases. Each time you move across the period by one element, one extra proton and electron are added. The positive charge of the nucleus increases by one each time, and the added electrons go into the same shell as all the elements in the period. This increases the electrostatic attraction between the nucleus and the outermost electrons. As the attraction is greater, the electrons are ‘pulled’ in tighter to the nucleus, effectively decreasing the atomic radius.





As you go down a group, atomic radius increases.


Moving down the group, the number of electron sub-shells increases. This means that the outermost electrons have more inner sub-shells between them and the nucleus. These extra inner sub-shells effectively dilute the positive charge from the nucleus that reaches the outermost electrons; this is called shielding. As the inner electrons experience a greater positive charge than the outer electrons, the outer electrons are not ‘pulled’ in as tightly to the nucleus, increasing the atomic radius.