1. Atoms and Elements
Chapter 2

2. Learning Objectives Students understand:
Composition and structure of atoms and the terms atomic number and mass number
The various representations of masses of atoms
Composition of isotopes
Importance of Coulomb’s law
Mole concept and molar mass application
How mass spectrometry is used

3. Learning Objectives Students will be able to
Carry out calculations relating atomic weights, isotopic masses, and isotopic abundances
Carry out calculations involving masses, moles, and numbers of particles
Interpret, predict, and write formulas for ionic and molecular compounds
Determine empirical and molecular formulas and percent composition

4. 2.1 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.

5. 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

6. 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 23Na

7. 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.

8. 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 = 2H or heavy hydrogen (1 neutron)
tritium = 3H radioactive hydrogen (2 neutrons)

9. 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

10. 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.

11. Practice Problems
Review and Check for Section 2-4
P. 52

12. 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

13. 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)

14. 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

15. 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

16. Homework for Chapter 2
After reading sections 2.1 – 2.5, you should be able to do the following problems…
P. 95a-b (10-11, 18, 23-24, 32)
Be familiar with the key experiments involved in the development of the current atomic model (p.54-55)