Element in Period 3

1. On going down from left to right across a period, there are is an increase of one proton in each element.
2. The number of electron-filled shells is the same but the number of valence electrons increases by one when going from left to right across the Period Table.
3. The elements in period 3 are sodium(Na), magnesium(Mg), aluminium(Al), silicon(Si), phosphorus(P), sulfur(S), chlorine(Cl), argon(Ag).

   Elements in Period 3	Na	Mg	Al	Si	P	S	Cl	Ar
   Proton number	11	12	13	14	15	16	17	18
   No. of positive charges in nucleus	+11	+12	+13	+14	+15	+16	+17	+18
   Electron arrangement	2.8.1	2.8.2	2.8.3	2.8.4	2.8.5	2.8.6	2.8.7	2.8.8

Changes in the Properties of Elements across Period 3
1.When going down across period 3, there is a decrease in atomic size.The atoms become smaller.This is because

• The number of positive charge in nucleus increases.
• The attractive force between the positive nucleus and electrons in the electron shells become stronger.
• The electrons in each shell in the atom are more strongly attracted to the nucleus and move further inwards the nucleus.

2.When going down across period 3, there is an increase in electronegativity,the elements become less metallic and changes from metals (sodium,magnesium and aluminium) to semi-metal (silicon), and then to non-metal (phosphorus,sulphur,chlorine and argon). The electronegativity increases across period 3. This is because

• The atomic size become smaller and the distance between the nucleus and the valence electrons decreases.
• The positive charges in the nucleus increases.
• The attractive forces between the nucleus and the electrons in the electron shells become stronger.
• The atom have higher tendency to attract electrons to form negative ions in a chemical reaction.

3.The melting point of the elements very across period 3

• The melting point increases form sodium to aluminium because the metallic bonds in these metal get stronger.
• The melting point decreases form silicon to argon because the Van der Waals force of attraction between the molecules get weaker.
• The melting point of silicon is the highest because of strong three dimensional covalent bonds

4.The elements in period 3 have varies physical states.

Element in Period 3	Physical state at room temperature and pressure
Sodium	Shiny grey and soft solid
Magnesium	Silvery-white solid
Aluminium	Silvery-white and light solid
Silicon	Black solid
Phosphorus	3 forms: White, red and black solid
Sulphur	Yellow solid
Chlorine	Green-yellow gas
Argon	Colourless gas