Physical Properties of the Period 3 Elements (CIE A Level Chemistry)

• 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

Arrangement of elements in the periodic table

Elements are arranged by 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 radius 

The atomic radius gives a measure of the size of atoms

Atomic radii of Period 3 elements table

Graph of atomic radii across Period 3

There is 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

Explaining why atomic radius decreases across Period 3

The diagram shows that across Period 3, the elements gain extra electrons in the same principal quantum shell

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
  • This is because there are less electrons, which 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 decrease 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 table

Graph of ionic radii across Period 3 ions

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

Explaining why ionic radius changes across Period 3

The diagram shows the electron configuration of the ions of Period 3 elements and their relative sizes

Melting point

Melting points of the elements across Period 3 table

Graph of melting points across Period 3

There is a general increase in melting point from Na to Si, followed by a sharp drop to the lower melting points of P to Ar

• A general increase in melting point for the Period 3 elements up to silicon is observed
• Silicon has the highest melting point
• After the Si element, the melting points of the elements decrease significantly

Electrical conductivity

• Electrical conductivity refers to how well a substance can conduct electricity
• Unlike the melting points, the electrical conductivity of the Period 3 elements shows a clear trend
• Going across the period, the electrical conductivity of the elements decreases significantly
  • Initially there is an increase in the electrical conductivity from Na to Al and then this decreases across the remaining elements

Trends in electrical conductivity across Period 3 table