Chap. 5 – Atomic Structure and the Periodic Table

5.1 ATOMS Democritus of Abdera– the Greek guy; a philosopher; fourth centrury B.C. Dalton – the English guy; started teaching at age 12; 1766-1844. His atomic theory stated

All elements are composed of submicroscopic indivisible particles called atoms Atoms of the same element are identical. The atoms of any one element are different from those of any other element

Atoms of different elements can physically mix together or can chemically combine with one another in simple whole number ratios to form compounds Chemical reactions occur when atoms are separated, joined or rearranged. However, atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

The atom is the smallest particle of an element that retains the properties of that element Atoms are unbelievably small! One copper penny contains 2.4  1022 atoms. If you were to line up 100,000 atoms of copper, you would get a line about 1 cm long.

5.2 Structure of the Nuclear Atom Electrons are negatively charged subatomic particles located in the electron cloud surrounding the nucleus of an atom. They have relatively negligible mass.       

Thomson discovered electrons in 1897 Thomson discovered electrons in 1897. Thomson passed an electric current through contained gases. A glowing beam called a cathode ray formed between the electrodes. Thomson discovered that the cathode ray was attracted to the positive pole of a magnet; therefore, must be negatively charged. Thomson concluded that electrons must be part of all elements because the cathode ray behaved the same no matter what the gas.

Millikan improved earlier estimates of the charge on the electron using his oil drop experiment. A proton is a positively charged subatomic particles found in the nucleus of the atom. In 1896, Goldstein modified Thomson’s cathode ray tubes and found oppositely charged particles flowing the opposite direction of the cathode rays. These canal rays contained positively charged particles.

A neutron is a neutral subatomic particles found in the nucleus of the atom. In 1932, Chadwick confirmed the existence of neutrons

Notice that nearly all the mass of the atom is located in the nucleus. Relative masses in amus (atomic mass units) Electron 1/1840 amu Proton 1 amu Neutron 1 amu Notice that nearly all the mass of the atom is located in the nucleus.

5.2 Atomic Nucleus Rutherford’s gold foil experiment proved that most of the atom is empty space with almost all the mass and all the positive charge concentrated in a small region at the center of the atom (the nucleus). (p. 111-112) 

If a nucleus were the size of a pea, it would have a mass of 2 If a nucleus were the size of a pea, it would have a mass of 2.3  105 kg. (230,000 kg)(1 kg = 2.204 lb) If the atom were a size of a professional football stadium, the nucleus would be the size of a marble suspended in the center of the stadium

5.3 Distinguishing Between Atoms ATOMIC NUMBER The atomic number of an element is the number of protons in the nucleus. Elements are defines by the atomic no. All atoms are electrically neutral – the number of p+ must equal the number of e-. Example: The atomic number of sulfur is 16.

The mass number is the sum of protons and neutrons in one atom. MN = p+ + n0 Example: The mass number of a Be atom which contains 4 p+ and 5 n0 is 9.

ISOTOPES   Isotopes are atoms that have the same number of protons but different numbers of neutrons. (Different mass numbers.) Example: Three naturally occurring isotopes of carbon exist; carbon-12, carbon-13, and carbon-14. Each isotope has 6 protons and 6, 7, or 8 neutrons, respectively.

ATOMIC MASS A  Atomic mass is the weighted average of all naturally occurring isotopes reported in number of grams per 1 mole. Example: The atomic mass of Ti is 47.90 grams/mole.

CALCULATING ATOMIC MASS To calculate atomic mass you must know the number of isotopes of the element, the mass of each isotope, the natural percent abundance of each isotope.

Example:. Boron has two naturally occurring isotopes Example: Boron has two naturally occurring isotopes. The isotope with mass 10.012 amu has a relative abundance of 19.91%. The isotope with mass 11.009 has a relative abundance of 80.09%.

Multiply the mass of each isotope by its percent abundance Multiply the mass of each isotope by its percent abundance. Add the results. (10.012)(0.1991) = 1.992 amu (11.009)(0.8009) = 8.817 amu 10.81 amu (10.81 g/mol = the atomic mass)

5.4 The Periodic Table Dmitri Mendeleev, a Russian chemist, organized the first periodic table in the mid 1800’s. His periodic table was organized by increasing atomic mass and similarity of properties. He left several “holes” in the periodic table because he felt sure that elements would be discovered that would fit correctly into those holes. He was correct

In 1913, Mosely, a British physicist, determined the atomic number of elements and rearranged the periodic table by atomic number rather than by atomic mass. The modern periodic table (the one you have)is arranged in seven horizontal rows called periods.

Properties of elements change as we move across the periodic table Properties of elements change as we move across the periodic table. Elements directly above or below one another possess similar properties. The periodic law states that when elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties.

Each vertical column is called a group or family. All groups are represented by a number and the letter A or B, the first group is 1A; the last group is 8A. All A group elements are representative elements because they exhibit a wide range of both physical and chemical properties.

Representative metals can be separated into three classes High electrical conductivity Ductile (can be drawn into wire) Malleable ( can be hammered into thin sheets) Except for hydrogen, the elements left of the zigzag lines are metals.

1A: Alkali metals 2A: Alkaline earth metals The B elements make up the transition metals (center block) and the inner transition metals (the lower block) The inner transition metals are also known as the rare-earth elements Approximately 80% of elements are metals.

NONMETALS Upper right corner of the periodic table. Nonlustrous Poor electrical conductors Brittle Several are gases at room temp, one is a liquid.

Group 18 (8A): Noble Gases. Sometimes referred to as inert gases because they are very unreactive Group 7A: Halogens

METALLOIDS (SEMI-METALS) Share two sides with the zigzag line Exception – aluminum, which is classified as a metal. Properties between those of metals and nonmetals.