Trends of Period 3 Elements: Atomic Radius (AQA A Level Chemistry): Revision Note

Trend: Atomic Radius

• Elements in the periodic table are arranged in order of increasing atomic number and placed in vertical columns (groups) and horizontal rows (periods)

• The elements across the periods show repeating patterns in chemical and physical properties

• This is called periodicity

All elements are arranged in the order of increasing atomic number from left to right

Atomic radius

• The atomic radius is the distance between the nucleus and the outermost electron of an atom

• The atomic radius is measured by taking two atoms of the same element, measuring the distance between their nuclei and then halving this distance

• In metals this is also called the metallic radius and in non-metals, the covalent radius

Atomic radii of period 3 elements

• You can see a clear trend across the period:

The graph shows a decrease in atomic radii of period 3 elements across the period

• Across the period, the atomic radii decrease

• This is because the number of protons (the nuclear charge) and the number of electrons increases by one every time you go an element to the right

• The elements in a period all have the same number of shells (so the shielding effect is the same)

• This means that as you go across the period the nucleus attracts the electrons more strongly pulling them closer to the nucleus

• Because of this, the atomic radius (and thus the size of the atoms) decreases across the period

Ionic radius

• The ionic radius is the distance between the nucleus and the outermost electron of an ion

• Metals produce positively charged ions (cations) whereas nonmetals produce negatively charged ions (anions)

• The cations have lost their valence electrons which causes them to be much smaller than their parent atoms

• Because there are fewer electrons, this also means that there is less shielding of the outer electrons

• Going across the period from Na⁺ to Si⁴⁺ the ions get smaller due to the increasing nuclear charge attracting the outer electrons in the second principal quantum shell nucleus (which has an increasing atomic number)

• The anions are larger than their original parent atoms because each atom has gained one or more electrons in their third principal quantum shell

• This increases the repulsion between electrons, while the nuclear charge is still the same, causing the electron cloud to spread out

• Going across P^3- to Cl^- the ionic radii decreases as the nuclear charge increases across the period and fewer electrons are gained by the atoms (P gains 3 electrons, S 2 electrons and Cl 1 electron)

Ionic radii of ions of period 3 elements

• The pattern is ionic radius is less clear cut:

Ions of period 3 elements with increasing positive charge (metals) and increasing of outer electrons across the period