The modern view of atomic structure
Atom consist of tiny nucleus (with diameter of about 10-13 cm) and electrons that moves about the nucleus at an average distance of about 10-8 cm from it.
Nucleus (have very small size compared to the size of atom and most of the atom mass) contains:
1- Protons: have positive charge equal in magnitude to the electron’s negative charge but much heavier.
2- Neutrons: have almost the same mass of proton but no charge.
Atomic number (Z): number of protons
Mass number (A): total number of protons and neutrons
- Atom has no net charge so number electrons equal number protons (atomic number) .
- The chemical properties of an atom determined by number and arrangement of electrons (by atomic number)
- Isotopes: atoms with the same number of protons and different number of neutrons (same atomic number but different mass number)
Writing the Symbols for Atoms
Write the symbol for the atom that has an atomic number of 9 and a mass number of 19. How many electrons and how many neutrons does this atom have?
Number of Electrons: 9 , number of neutrons: 10