Chapter 4

1. Chapter 4 Atomic Structure

2. Democritus • Greek Philosopher • First to suggest the idea of atoms. • Believed atoms were indivisible & indestructible

3. John Dalton • English chemist & school teacher. • Started the modern idea of the atom.

4. Dalton’s Atomic Theory • Dalton’s Atomic Theory: • All elements are composed of tiny particles called atoms • Atoms of the same element are identical; atoms of one element are different than other elements. • Atoms of different elements can physically mix or chemically combine in whole-number ratios to form compounds. • Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of 1 element are never changed into another element.

5. Dalton’s Atomic Theory

6. Atoms • Atom—smallest particle of an element that retains the properties of that element. • Atoms can be detected using a scanning tunneling microscope. The Elements: Forged in Stars The Origin of the Elements

7. Sec 2: Nuclear Atom • Atoms are now known to be divisible. • Atoms are composed of electrons, protons, and neutrons. • Electrons—negatively charged subatomic particles. • Electrons were discovered by J.J. Thomson using a cathode ray. Negative electrons are attracted to positively charged metal plates.

8. Nucleus • Nucleus—the central core of an atom composed of protons & neutrons. • The nucleus was discovered by Ernest Rutherford using the gold foil experiment.

9. Protons & Neutrons • Protons—positively charged subatomic particles. • Neutrons—subatomic particles with no charge. • Discovered by Chadwick 1932 • Mass of neutron is about equal to proton (1 amu) • Atoms are neutral; they have no overall charge. • Positive protons cancel out negative electrons

10. Isotopes • Isotopes—Atoms that have the same number of protons, but different numbers of neutrons.

11. Atomic Number • Atomic Number—the number of protons in an atom of an element. • Every element has a different number of protons.

12. Mass Number • Mass number—the total number of protons and neutrons in an atom • Mass # = protons + neutrons • Example: a helium atom has 2 protons and 2 neutrons. What is the mass number? • 4 • # of neutrons = mass # – atomic # • Shorthand notation: • You can use the mass number to name specific atoms. Example: Helium-4

13. Sec. 3: Atomic Mass • The mass of a proton or neutron is 1.67 x 10-24 g. • The mass of an electron is 9.11 x 10-28 g. • This is negligible in comparison. • Atomic mass unit (amu)—1/12 the mass of a Carbon-12 atom. • The approx. mass of a proton or neutron.

14. Atomic Mass • Atomic mass—a weighted average mass of the atoms in a naturally occurring sample of the element. • The atomic mass is closest to the most abundant isotope of that element.

15. Atomic Mass • To calculate atomic mass, you need to know • The mass of each isotope • The natural abundance of each isotope. (in decimal form) • Mass number of each isotope x its abundance then add the results. • (Mass #1 x abundance1) + (Mass #2 x abundance2) …

16. Example Element B has 2 isotopes. One isotope has a mass of 10.012 amu with an abundance of 19.92%. The other isotope has a mass of 11.009 amu with an abundance of 80.09%. What is the atomic mass?

17.  Done  with Section 3  Calculating Atomic Mass • 10.012 amu x 0.1991 = 1.993 amu • 11.009 amu x 0.8009 = +8.817 amu atomic mass = 10.810 amu

18. Sec. 4 The Periodic Table • Development of the Periodic Table: • About 70 elements were discovered before the 1880s • Dmitri Mendeleev first listed elements in a systematic, organized way. • He arranged atoms by their properties and in order of atomic mass. • Mendeleev constructed the first periodic table. • He left blank places for unknown elements. • Henry Moseley determined atomic # and arranged elements by this instead of atomic mass.

19. The Modern Periodic Table • Each element is represented by its symbol and atomic number placed in a square • The horizontal arrangement of elements are called periods. • Periodic Law—when the elements are arranged in order of increasing atomic number, there is a repeated pattern of physical and chemical properties.

20. The Modern Periodic Table • Group—a vertical arrangement of atoms. • The elements in any group have similar chemical and physical properties. • Each group is identified by a number and letter A or B.

21. The Modern Periodic Table • Group A elements are known as representative elements. • Representative elements are broken into 3 classes. • Metals—high electrical conductivity and luster • Alkali metals—Group 1A • Alkaline Earth Metals—Group 2A elements • Most non-representative elements are also metals

22. The Modern Periodic Table • Nonmetals—occupy upper right corner of the periodic table. • Nonlustrous and poor conductors of electricity. • Some are gases, some are solids • 2 nonmetal groups have special names • Halogens—group 7A (ex. chlorine & bromine) • Noble gases—group 8A (or group 0) • Inert gases—they do not undergo chemical reactions • Ex. Neon gas is used in neon lights.

23. The Modern Periodic Table • Metalloids—elements with properties that are intermediate between metals and nonmetals. • A heavy line divides metals from nonmetals. • Most elements that border this line are metalloids.