Atoms and Elements Chapter 2

Atomic Structure Electrons Protons Neutrons Atoms have no net charge. The number of electrons outside the nucleus equals the number of protons within the nucleus.

2.2 Atomic Number and Atomic Mass All atoms of a given element have the same number of protons in the nucleus. Number of protons in the nucleus is its atomic number. One atomic mass unit (1 u) is one twelfth of the mass of an atom of carbon with six protons and six neutrons. 1 u = 1.661x g

Mass Number protons and neutrons have mass of approximately 1 u (electrons are about 1/2000 of this mass) sum of protons and neutrons is defined as the mass number symbolized by an “A” can be expressed as sodium-23 or 23 Na

2.3 Isotopes Data from mass spectroscopy demonstrate evidence that Dalton’s model is incorrect; these data then require a modification of that model. These data demonstrate direct evidence of different isotopes from the same element.

Isotopes atoms with same atomic number but different mass numbers are called isotopes, named by mass number isotopes of hydrogen each have separate names protium = hydrogen with one proton deuterium = 2 H or heavy hydrogen (1 neutron) tritium = 3 H radioactive hydrogen (2 neutrons)

Isotope Abundance percent abundance is the number of atoms of a given isotope divided by the total number of all isotopes of that element multiplied by 100 the atomic mass of an element is affected by the percent abundance of each isotope

2.4 Atomic Weight the atomic weight is the average mass of a representative sample of atoms usually closer to the mass of the most abundant isotope(s) Atomic mass is the mass of an atom at rest. Relative atomic mass (atomic weight) is the average of the atomic masses of all the element’s isotopes.

2.5 The Periodic Table Mendeleev discovered that elements with similar properties appeared in a regular pattern when organized by increasing atomic mass. law of chemical periodicity – the properties of the elements are periodic functions of atomic number

The Periodic Table vertical columns of groups or families have similar characteristics A groups are main group elements B groups are the transition elements horizontal rows are called periods metals, nonmetals, metalloids allotropes are different forms of the same element (diamond, graphite, bucky balls)

Overview of Elements Group 1A – alkali metals Group 2A – alkaline earth metals Group 3A – B, Al, Ga, In, Tl Group 4A – C, Si, Ge, Sn, Pb Group 5A – N, P, As, Sb, Bi Group 6A – O, S, Se, Te, Po S, Se, Te are called the chalcogens Group 7A – halogens Group 8A – noble gases

Overview of Elements The Transition Elements B group elements; all metals reactivity varies commercial uses (structures, paints, vehicles, coins, batteries) lanthanides and actinides are below the table

Homework for Chapter 2 After reading sections 2.1 – 2.5, you should be able to do the following problems… P. 105 (85-94)