Chapter 4 Atoms and Elements

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

2. CHAPTER OUTLINE

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

7. DALTON’SATOMIC THEORY 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 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’SATOMIC 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’SATOMIC THEORY 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 Atoms combine in definite whole-number ratios to form compounds. As a result compounds always contain elements in the same proportions by mass.

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

17. DISCOVERY OFTHE ELECTRON • 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. • These observations indicated that the cathode rays were negatively charged. • These rays were later named electrons.

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

20. DISCOVERY OFTHE NUCLEUS • 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. • Few of the particles were observed to be turned back towards the direction they came from.

21. Deflection Scattering NUCLEAR MODELOF THE ATOM • 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. • The deflections were caused by head-on collision of -particles with the nucleus. • The scatterings were caused by glancing collision of -particles with the nucleus. 5.5

22. Structure of the Atom • Rutherford proposed that the nucleus had a particle that had the same amount of charge as an electronbut 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 0.00055 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 current model of the atom describes it as a neutral spherical entity, composed of a positively charged nucleus surrounded by negatively charged electrons. • The electrons (e-) move rapidly through the atomic volume, held by the attractive forces to the nucleus. • The nucleus consists of positively charged protons (p+) and neutrally charged neutrons (n0).

26. ATOMICSTRUCTURE • The modern atom consists of 3 subatomic particles: • 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).

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?