1. CHEMISTRY 112 Atomic Structure

2. Early Models of the Atom An atom is the smallest particle of matter that retains its identity in a chemical reaction Democritus was an early Greek philosopher who suggested the existence of atoms He believed they were indivisible and indestructible

3. John Dalton and Atomic Theory From experimentation, Dalton was able to devise a set of ideas supporting the idea of the atom Elements are composed of tiny indivisible particles called atoms Atoms of the same elements are identical and one element’s atom differ from another Atoms of different elements can come together to form compounds Atoms of one element cannot change into atoms of another

4. A Problem with Dalton We now know that the atom is divisible It is composed of subatomic particles Electrons are negatively charged particles and they are found outside the nucleus in regions called orbitals Protons are positively charged and found in the nucleus of an atom with neutrons, which have no charge There are even smaller particles but we do not study them in introductory chemistry

5. The Size and Shape of an Atom The nucleus is found at the core of an atom It is very dense and positively charged The region around the nucleus is large and mostly empty space Although all three subatomic particles are small, the electron is 1/1840 the size of a proton/neutron Table 4.1, page 108

6. Atomic Number Elements are different from one another because they contain different numbers of protons The atomic number = number of protons In atoms, the number of proton = number of electron Therefore, an atom is electrically neutral

7. Mass Number The mass number is the number of protons added to the number of neutrons The number of neutrons can vary from one atom to the next within one sample of an element Such atoms are called isotopes of an element and have differing mass numbers – they are still, however, chemically alike because their p+ and e- count are equivalent to one another

8. Isotopes There is a certain method used for naming and writing isotopes