Atoms and Elements

Chapter 2

Atomic Structure

ElectronsProtonsNeutronsAtoms 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 = 1.661x10-24 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 numbersymbolized by an “A”can be expressed as sodium-23 or 23Na

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 numberisotopes of hydrogen each have separate names

protium = hydrogen with one protondeuterium = 2H or heavy hydrogen (1 neutron)tritium = 3H 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 100the 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 atomsusually 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 elementsB groups are the transition elements

horizontal rows are called periodsmetals, nonmetals, metalloids

allotropes are different forms of the same element (diamond, graphite, bucky balls)

Overview of Elements

Group 1A – alkali metalsGroup 2A – alkaline earth metalsGroup 3A – B, Al, Ga, In, TlGroup 4A – C, Si, Ge, Sn, PbGroup 5A – N, P, As, Sb, BiGroup 6A – O, S, Se, Te, Po

S, Se, Te are called the chalcogensGroup 7A – halogensGroup 8A – noble gases

Overview of Elements

The Transition ElementsB group elements; all metalsreactivity variescommercial 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)