1 - PRINCIPLES OF CHEMISTRY 

Atoms - 

1 - Nucleus 

• Middle of atom 
• Protons and neutrons
• Positive charge 
• Contains most of the mass of the atom 

2 - Electrons 

• Move around nucleus in shells 
• Negatively charged 
• Cover a lot of space 
• Size of orbit depends on size of atom 
• Virtually no mass 

Protons =  heavy and positive 
Neutrons = heavy and neutral 
Electrons = small mass, negative 




Number of electrons = number of protons (in neutral atom/not isotope) - 

• Neutral atoms = no charge overall 
• Charge on electrons is same size as the charge on protons (but opposite) 
• Electrons added/removes - atom becomes a charged ion 

Atomic number and mass number - 

• Atomic number = no. of protons 
• Mass number = protons + neutrons 
• No. of neutrons = mass number - atomic number 

Molecules - 

• Groups of atoms (joined together) 
• Can be one element (H2, N2) or more than one element (H2O, CO2) 
• Held together by covalent bonds