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?
Answer:
Number
of Electrons: 9 , number of neutrons: 10