Why does the atomic size decreases when going across Period 2?

Period 2

When going across Period 2, the positive charge of the nucleus of the elements increases, atoms of elements in Period 2 have two shells occupied with electrons. Hence, the increasing positive charge of the nucleus pulls the two shells of electrons closer to the nucleus when going across Period 2. This causes the decreases in the atomic size.  

STPM Chemistry Syllabus 2nd Term

Chapter 7: Chemical Energetics
7.1 Enthalpy changes of reaction, ΔH
7.2 Hess’ law
7.3 Born-Haber cycle
7.4 The solubility of solids in liquids

Chapter 8: Electrochemistry
8.1 Half-cell and redox equations
8.2 Standard electrode potential
8.3 Non-standard cell potentials
8.4 Fuel cells
8.5 Electrolysis
8.6 Applications of electrochemistry

Chapter 9: Periodic Table: Periodicity
9.1 Physical properties of elements of Period 2 and Period 3
9.2 Reactions of Period 3 elements with oxygen and water
9.3 Acidic and basic properties of oxides and hydrolysis of oxides

Chapter 10: Group 2
10.1 Selected Group 2 elements and their compounds
10.2 Anomalous behavior of beryllium
10.3 Uses of Group 2

Chapter 11: Group 14
11.1 Physical properties of Group 14 elements
11.2 Tetrachlorides and oxides of Group 14 elements
11.3 Relative stability of +2 and +4 oxidation states of Group 14 elements
11.4 Silicon, silicone and silicates
11.5 Tin alloys

Chapter 12: Group 17
12.1 Physical properties of selected Group 17 elements
12.2 Reactions of selected Group 17 elements
12.3 Reactions of selected halide ions
12.4 Industrial applications of halogens and their compounds

Chapter 13: Transition elements
13.1 Physical properties of first row transition elements
13.2 Chemical properties of first row transition elements
13.3 Nomenclature and bonding of complexes
13.4 Uses of first row transition elements and their compounds