Chapter 4 Atomic Structure

Defining the Atom *An atom is the smallest particle of an element that retains its identity in a chemical reaction. The word atom comes from the Greek word atomos, which means indivisible. The Greek philosopher Democritus believed that atoms were indivisible and indestructible. His theory did not explain chemical behavior. His theory was not based on the scientific method. English Chemist, John Dalton used experimental methods to transform Democritus’s ideas on atoms into a scientific theory.

Dalton’s atomic theory has four points: 1. All elements are composed of atoms. 2. Atoms of the same element are identical. Atoms of one element are different from atoms of another element.

3. Atoms of different elements can physically mix together and chemically combine to form compounds in simple whole number ratios. 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into atoms of other elements. Despite their small size, individual atoms are observable with instruments such as scanning tunneling microscopes.

4.2 Structure of Nuclear Atom Three kinds of subatomic particles are electrons, protons, and neutrons. English physicist, J.J. Thomson discovered the electron in 1897. pg. 104/105 *Electrons are negatively charged particles. U.S. physicist, Robert Millikin calculated the charge-to-mass ratio of an electron in 1916.

*Protons are positively charged particles that have a mass 1840 times larger then that of an electron. . James Chadwick discovered the neutron in 1932. The neutron has a mass nearly equal to that of the proton. *Nucleus is the tiny central core of an atom and is composed of protons and neutrons.

In 1911, Ernest Rutherford conducted his gold foil experiment, which led to the discovery of the nucleus. pg. 107 In the nuclear atom, protons and neutrons are located in the nucleus with electrons distributed around the nucleus. Electrons occupy almost all of the atoms volume.

4.3 Distinguishing Among Atoms Elements are different because they contain different number of protons. *The atomic number indicates the number of protons found in the nucleus of an atom of the element. *Mass number indicates the total number of protons and neutrons in the nucleus of an atom of the element. The number of neutrons in an atom is the difference between the mass number and the atomic number.

*Isotopes are atoms of the same element with the same number of protons but different number of neutrons. They also have different mass numbers. *Atomic mass unit is defined as 1/12 of the mass of a carbon-12 atom. The atomic mass of an element is a weighed average mass of the atoms in a naturally occurring sample of the element. To calculate the atomic mass of an element multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.

*A periodic table allows you to easily compare the properties of one element (or group of elements to another element (or group of elements). *Period is a horizontal row of the periodic table. The properties of each element vary in a period. *A group is a vertical column of the periodic table also known as a family. Elements have similar properties in a group.