Basic structure of atoms Basic Structure of an Atom • Nucleus and electron cloud • Nucleus: very small, dense core which consists of protons and neutrons (nucleons) Nucleons • Protons: – Positive charge +1 – Mass= 1.763 x10-24 grams – Symbols include +, p, 1H1 • Neutrons: – Neutral charge 0 – Mass= 1.765 x10-24 grams – Symbols include n, 0n1 Electron cloud • Electrons move very rapidly in complicated paths called orbitals. • Because of this motion, they appear to form a cloud. – Negative charge -1 – Mass: 9.1 x10-28 grams – Symbols include e-, -1e0 • Normally, atoms are electrically neutral. – This means that for each proton in the nucleus, there is one electron on the outer surface • Atoms which don’t have this balance are called ions – Atoms with extra electrons are negatively charged and called anions – Atoms with a deficiency of electrons are positively charged and called cations Atomic Number (Z) • Number of protons which an atom contains – Always a whole number – Determines which type of element an atom is Atomic Mass Number (A) • Found by adding mass of protons and neutrons – Whole number Determining the number of neutrons • Atomic mass number= neutrons + protons • Atomic mass- atomic number = neutrons 40.078 Ca 20 How many protons are in … 7 Li 3 35 207 Cl Pb 82 ? Isotopes • Atoms of the same element that have different numbers of neutrons • Isotopes of an element do not exhibit chemical differences • They do exhibit physical differences (i.e. mass) Atomic Weight • Represents the average weight of an element taking into account the different weights of any naturally occurring isotopes for that element Calculating Atomic Weights • Example: Chlorine has 2 naturally occurring isotopes- Cl-35 and Cl-37 – 75% Cl-35 (.75)(35) + (.25)(37)= 35.5 Chromium occurs in 4 isotopic forms with the following approximate distribution: 6% Cr-50, 85% Cr-52, 8% Cr-53, and 1% Cr54. Calculate the atomic weight.