1. Chapter 4 Atoms and Elements
Atoms - indestructible building blocks from
which are substances are constructed.
2006, Prentice Hall

2. CHAPTER OUTLINE Atoms, Elements and Symbols
Periodic Table of the Elements
Properties of Metals and Non-Metals
The Atomic Theory
The Modern Atom
Atomic Structure
Ions
Isotopes and Atomic Mass

3. Experiencing Atoms
atoms are incredibly small, yet they compose everything
atoms are the pieces of elements
properties of the atoms determine the properties of the elements
If every atom within a pebble were the size of the pebble itself, then the pebble would be larger than Mt. Everest (~29,000 ft)

4. Experiencing Atoms there are about 117 elements found in nature
and 23 have been made in laboratories
each has its own, unique kind of atom
they have different structures
therefore they have different properties
If you keep dividing matter into smaller and smaller pieces you end up with an atom.

5. EARLY CONCEPTS OF THE ATOM
The smallest particle of matter that still retains its properties is called an atom.
In the fifth century B.C., the Greek philosopher Democritus proposed that matter is composed of a finite number of discrete particles, named atomos (meaning un-cuttable or indivisible)

6. DALTON’S ATOMIC THEORY
Dalton’s model represented the atom as a featureless ball of uniform density.
This model is referred to as the “soccer ball” model.
In 1808, John Dalton, built on ideas of Democritus, and formulated a precise definition of the building blocks of matter.

7. DALTON’S ATOMIC THEORY
explains the difference between an element and a compound.
explains two scientific laws, and
predicts a new scientific law.

8. DALTON’S ATOMIC THEORY
Postulate
Deduction
Each element consists of indivisible, small particles called atoms. 1
All the atoms of an element are identical to one another, but different from others.
Gives a more precise definition for an element. 2

9. DALTON’S ATOMIC THEORY
Atoms of oxygen are different from atoms of hydrogen
All atoms of oxygen are identical to one another
All atoms of hydrogen are identical to one another
Atoms of each element are identical to one another, but different from others.
Atoms consists of indivisible, small particles.
5.1

10. DALTON’S ATOMIC THEORY
Postulate
Deduction
Atoms combine chemically in definite whole-number ratios to form compounds.
Supports Law of Definite Composition; predicts Law of Multiple Proportions. 3
Atoms can neither be created nor destroyed in chemical reactions.
Supports Law of Conservation of Mass. 4

11. LAW OF DEFINITE COMPOSITION
As a result compounds always contain elements in the same proportions by mass.
Atoms combine in definite whole-number ratios to form compounds.

12. LAW OF MULTIPLE PROPORTIONS
Two or more elements may combine in different ratios to form more than one compound.

13. Sizes of Atoms
using compositions of compounds and assumed formulas, Dalton was able to determine the relative masses of the atoms
Dalton based his scale on H = 1 amu
we now base it on C-12 = 12 amu exactly
unit = atomic mass unit = amu
another term for amu is ?
absolute sizes of atoms
mass of H atom= 1.67 x 10-24g
volume of H atom = 2.1 x 10-25cm3
dalton

14. Some Notes on Charges Two Kinds of Charge called + and –
Opposite Charges Attract
Like Charges Repel
To be Neutral, something
must have no charge or equal amounts of opposite charges

15. The Atom is Divisible!
Work done by J.J. Thomson and others proved that the atom had pieces called electrons
Thomson found that electrons are much smaller than atoms and carry a negative charge
the mass of the electron is 1/1836th the mass of a hydrogen atom
the charge on the electron is the fundamental unit of charge which we will call –1 charge units

16. DISCOVERY OF THE ELECTRON
Negatively charged particles from cathode were pulled towards positively charged plate, anode, and allowed to pass through and be detected on a fluorescent screen.
Smaller particles than the atom also exist and are called subatomic particles.
In 1897, J.J. Thomson performed experiments with a cathode ray tube.

17. DISCOVERY OF THE ELECTRON
These observations indicated that the cathode rays were negatively charged.
These rays were later named electrons.
In absence of a magnetic field, the cathode rays were not deflected.
In presence of a magnetic and electric fields, the cathode rays were deflected towards the positive plate.

18. ATOMIC MODEL
Based on these findings, Thomson proposed an atomic model, composed of negatively charged electrons embedded in a uniform positively charged sphere.
This model is called the “plum pudding” model.

19. DISCOVERY OF THE NUCLEUS
In these experiments he bombarded a thin sheet of gold foil with -particles (large, positively charged) emitted from a radioactive source.
In 1910, Ernest Rutherford carried out a number of experiments to further probe the nature of the atom.

20. DISCOVERY OF THE NUCLEUS
Few of the particles were observed to be turned back towards the direction they came from.
The majority of the particles were observed to pass through un-deflected or slightly deflected.
Some of the particles were observed to be deflected at large angles.

21. NUCLEAR MODEL OF THE ATOM
The scatterings were caused by glancing collision of -particles with the nucleus.
The deflections were caused by head-on collision of -particles with the nucleus.
Based on these observations, Rutherford proposed a model of the atom consisting of a small, massive positive center (nucleus), surrounded by electrons in mostly empty space.
Scattering
Deflection
5.5

22. Structure of the Atom
Rutherford proposed that the nucleus had a particle that had the same amount of charge as an electron but opposite sign
based on measurements of the nuclear charge of the elements
these particles are called protons
protons have a charge of +1 and a mass of 1 amu
since protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons

23. Some Problems
How could beryllium have 4 protons stuck together in the nucleus?
shouldn’t they repel each other?
If a beryllium atom has 4 protons, then it should weigh 4 amu; but it actually weighs 9.01 amu! Where is the extra mass coming from?
each proton weighs 1 amu
remember, the electron’s mass is only about amu and Be has only 4 electrons – it can’t account for the extra 5 amu of mass

24. The Must Be Something Else There!
to answer these questions, Rutherford proposed that there was another particle in the nucleus – it is called a neutron
neutrons have no charge and a mass of 1 amu
the masses of the proton and neutron are both approximately 1 amu

25. THE MODERN ATOM
The electrons (e-) move rapidly through the atomic volume, held by the attractive forces to the nucleus.
The current model of the atom describes it as a neutral spherical entity, composed of a positively charged nucleus surrounded by negatively charged electrons.
The nucleus consists of positively charged protons (p+) and neutrally charged neutrons (n0).

26. ATOMIC STRUCTURE
The number of protons in an atom determines its identity, and is called atomic number (Z).
In a neutral atom, the number of protons (+) are equal to the number of electrons (–).
Almost all the mass of the atom rests in the nucleus.
Therefore the number of protons and neutrons in an atom is called the mass number (A).
The modern atom consists of 3 subatomic particles:
Particle
Charge
Relative
Mass
Proton +1
~1800
Neutron
Electron –1 1

27. Elements each element has a unique number of protons in its nucleus
the number of protons in the nucleus of an atom is called the atomic number
the elements are arranged on the Periodic Table in order of their atomic numbers
each element has a unique name and symbol
symbol either one or two letters
one capital letter or one capital letter + one lower case

28. The Periodic Table of Elements

29. Elements: Origins of the Names of the Elements
Most chemical symbols are based on the English name of the element.
Some symbols are based on Latin names.
The symbol for potassium is K, from the Latin kalium, and the symbol for sodium is Na, from the Latin natrium.
Additional elements with symbols based on their Greek or Latin names include the following:
lead Pb  plumbum
mercury Hg  hydrargyrum
iron Fe  ferrum
silver Ag  argentum
tin Sn  stannum
copper Cu  cuprum

30. Elements: Origins of the Names of the Elements
Early scientists gave newly discovered elements names that reflected their properties:
Argon, from the Greek argos, means “inactive,” referring to argon’s chemical inertness.
Other elements were named after countries:
Polonium after Poland
Francium after France
Americium after the United States of America.
Other elements were named after scientists.
Every element’s name, symbol, and atomic number are included in the periodic table (inside the front cover) and in an alphabetical listing (inside the back cover) in this book.

31. Review What is the atomic number of boron, B?
What is the atomic mass of silicon, Si?
How many protons does a chlorine atom have?
How many electrons does a neutral neon atom have?
Will an atom with 6 protons, 6 neutrons and 6 electrons be electrically neutral?
Will an atom with 27 protons, 32 neutrons and 27 electrons be electrically neutral?
Will a Na atom with 10 electrons be electrically neutral?