Atoms and Compounds

Atoms and Compounds

1. 117 elements

   • All substances are made of atoms
     • Atoms consist of a small nucleus surrounded by electrons
   • when elements react, their atoms join with atoms of other elements
     • Compounds are formed (2 or more elements)

2. Nucleus contains protons and neutrons(particles)

   • Protons have a positive charge and neutrons have no charge
     • Atomic number(proton number)- number of protons in an atom of an element
       • Elements arranged in atomic no. in the periodic table
     • Atomic number also the number of electrons

3. Electrons are tiny negatively charged particles

   • move around the nucleus
   • Number of protons is equal to the number of electrons
     • No overall charge
     • Charges are equal and opposite(proton+1 electron-1)

4. Mass number- total no. of particles in the nucleus of an atom

   • Number of neutrons + number of protons

5. Atoms of the unreactive noble gases(group 0) all have very stable arrangement of electrons

   • Electrons are in energy levels that are represented as shells
     • the elements in the same group have the same no. of electrons on their outer shell
     • Occupy lowest shell first

6. Compounds of metals bonded to non-metals have ionic bonds

   • compounds of non-metals have covalent bonds