Chemistry 4A

Chemistry 4A

1. Key Processes

   • Moving across each period, you can see that the number of occupied energy levels is the same as the period number
   • As you go across each period from left to right, an energy level gradually becomes filled with electrons. The highest occupied energy level contains just one electron on the left-hand side of the table. It is filled by the time you get to the right-hand side
   • Moving down each group, you can see that the number of electrons in the highest occupied energy level is the same as the group number
   • Each element in a group therefore has the same number of electrons in its highest occupied energy level. Group 0 is a partial exception to this rule. Although it comes after Group 7, it is not called Group 8 - and it contains helium, which has only two electrons in its outer shell
   • John Dalton developed an atomic theory in the 1800s. He did experiments, worked out some atomic weights and invented symbols for atoms and molecules. His most important conclusions are...
     • All matter is made of atoms
     • Atoms cannot be broken down into anything simpler
     • All the atoms of a particular element are identical to each other and different from the atoms of other elements
     • Atoms are rearranged in a chemical reaction
     • Compounds are formed when two or more different kinds of atoms join together
   • JJ Thompson discovered the electron in 1897. This showed that the atom contained smaller pieces, whereas Dalton had thought that atoms could not be broken down into anything simpler
   • In 1911 Ernest Rutherford used experimental evidence to show that an atom must contain a central nucleus. This was further evidence that an atom contained smaller pieces
   • Niels Bohr further developed Rutherford's nuclear atom model. He used experimental evidence to support the idea that electrons occupy particular orbits or shells around the nucleus of an atom
     • The development of the theory of atomic structure is an example of...
       • How a theory may change as new evidence is found
       • How a scientific explanation is provisional but may become more convincing when predictions based on it are confirmed later on

2. Atoms contain a nucleus (containing protons and neutrons) surrounded by electrons

3. Electrons are negatively charged, protons are positively charged and neutrons have no charge

4. Protons and neutrons have mass of 1 and electrons have mass of 0.0005 (zero)

5. Atoms have a radius of 10 m and a mass 10 g

6. Atomic number = number of protons / electrons

7. Mass number = number of protons + number neutrons

8. Isotopes are varieties of the same element – they have the same atomic number but different mass numbers