Atoms and the periodic table

1. Atoms and the periodic table How did atomic theory develop?

2. The idea of the atom dates back to 430BC. • The atom is defined as the smallest particle that can be considered an element • An Englishman, John Dalton, described the atom as an invisible particle that makes up an element. • JJ Thomson’s discovery of negatively charged particles called electrons disproved Dalton’s invisible atom idea.

3. Rutherford, a former student of Thomson, proposed the existence of nucleus, a small positively charged region of an atom containing positively charged particles called protons. • Then a student of Rutherford, Niels Bohr, revised the model to propose that electrons were found only in specific orbits around the nucleus. • The modern cloud model proposes that an electron’s movement is related to its energy level and electrons move rapidly within a cloudlike region around the nucleus.

4. What is the modern model of the atom? • With the discovery of the neutrally charged neutron, the modern model of the atom emerged. • At the center of the atom, is a tiny dense nucleus containing protons and neutrons. Surrounding the nucleus is a cloud like region of moving electrons. • Scientists use units known as atomic mass units(amu). • Protons have a charge of 1 and a mass of 1 amu. • Neutrons have no charge and a mass of 1 amu • Electrons have a negative charge and a mass of 1/2000 amu.

5. All the mass of an atom is concentrated in the nucleus. • Number of protons equals the number of electrons and this is called the atomic number. • Definition of an element is based on its atomic number. • Atoms of elements that have the same number of protons but a different number of neutrons are called isotopes. • An isotope is identified by its mass number which is the sum of the protons and neutrons in the atom

6. Summarizing chapter 9,lessons 1-4 • Mendeleev noticed a pattern of properties in elements arranged by increasing atomic mass • The periodic table includes each element’s atomic number, symbol, name and atomic mass. • The properties of an element can be predicted from its location in the periodic table.

7. Physical properties of metals include luster, malleability, ductility, and conductivity. • Metals are classified as alkali metals, alkaline earth metals, transition metals, metals in mixed groups, lanthanides and actinides. • Most nonmetals are poor conductors. Solid nonmetals tend to be dull and brittle. • The families containing nonmetals include the carbon family, the nitrogen family, the oxygen family, the halogen family, the noble gases, and hydrogen.

8. Atomic theory grew as a series of models that developed from experimental evidence • At the center of the atom is a tiny dense nucleus containing protons and neutrons. Surrounding the nucleus is a cloudlike region of moving electrons.