Chapter 4 Atomic Structure Defining the Atom Atomic Theory Atomic Models Structure of the Nuclear Atom Distinguishing among Atoms Isotopes

Early models of the Atom An Atom is the smallest particle of an element that retains its identity in a chemical reaction. Democritus reasoned that atoms were indivisible and indestructible. Dalton turned the ideas into the Atomic Theory using experimental methods: see p 213 1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same element are identical. Atoms of different elements are different. 3. Atoms of different elements can physically mix together or can chemically combine in simple-number ratios to form compounds. 4. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combination but never changed themselves.

Atomic Sizes Atoms are so small they can only be seen with instruments such as the scanning electron microscope. Atomic radii are between 5 x 10-11 to 2 x 10-10 m. A copper penny contains about 2 x 1022 atoms.

Structure of the Atom Subatomic Particles: Electron discovered by JJ Thomson using a cathode ray tube. Milliken continued the experiments to determine the charge and mass of an atom. Negative 1/1840 mass of a hydrogen atom

Subatomic particles (cont) E. Goldstein proved the presence of protons using the same cathode ray tube Positively charged Mass of 1 amu (1.67 x 10-24 g) J. Chadwick confirmed the existence of the neutron. No charge Mass of 1 amu (1.67 x 10-24 g) Both particles are considered to be comprised or tinier particles called quarks

The Atomic Nucleus Up to this point, the model of the atom was known as the ‘plum-pudding’ model because of the electrons scattered throughout a positive charge. E.Rutherford conducted an experiment (p222) that placed the protons in an area known as the nucleus with the electrons outside that area. Dense nucleus with protons and neutrons Electrons outside nucleus taking up most of the atom’s space “If an atom were the size of a football stadium, the nucleus would be about the size of a marble.”

4.3Distinguishing among Atoms Atomic Number: the number of protons in the nucleus an element. No two elements have the same atomic number. Examples: If atoms are neutral, the atomic number also indicated the number of electrons an atom has. Elements are on the periodic table in order of atomic number. Examples: Mass Number: a whole number that indicates the total number of protons and neutrons in an atom. Number of neutrons = mass number – atomic number

Section 3 (cont) Isotopes: because not all atoms of the same element have the same number of neutrons, the mass number can differ. These variations of atoms are called isotopes. examples Atomic Mass: the weighted average of the atoms in a naturally occurring element sample. On the periodic table. The atomic mass can be determined based on the relative abundance of each isotope present See example problems in the book p240-242