8 th Grade Science-Atoms Unit

1. 8th Grade Science-Atoms Unit Properties of Atoms and the Periodic Table

2. Structure of the Atom • Chemical Symbols- consist of one capital letter or a capital letter plus one or two small letters ex: table 1—pg. 544

3. Atomic Components • Atoms—the smallest piece of matter that still retains the property of the element • Atoms: Contain protons and neutrons in the nucleus • Electrons—contained in electron cloud

4. Quarks—smaller particles that comprise protons and neutrons • Scientists have confirmed the existence of six uniquely different quarks • The search for the composition of protons and neutrons is an ongoing effort

5. Atomic Theory-Past Models of the Atom • Democritus—around 400 BCE composed of tiny, solid particles that could not be subdivided/cut apart further • Called these particles”atomos” • (did not conduct experiments as proof)

6. Dalton—solid sphere model (pool ball)-- • 1808--Dalton proposed an ATOMIC THEORY with 3 main points-- • All matter is made up of atoms • Atoms cannot be created or destroyed • All atoms of a certain element are identical, of they are different from atoms of all other elements

7. JJ Thomson—(1897)conducted experiments providing evidence that atoms are made up of even smaller particles with a negative charge—ELECTRONS -known as “plum pudding” or cookie dough model w/electrons embedded in positively charged sphere

8. Ernest Rutherford—(1909) -conducted the “gold foil” experiment -suggested atoms have a nucleus—positive center surrounded by moving electrons -he called the positively charged particles in the nucleus--PROTONS

9. Niels Bohr—(1913) --Hypothesized about electrons traveling in fixed orbits -The Bohr model shows electrons moving around the nucleus in circular paths a certain distance from the nucleus -This model helped to predict properties of elements -(This is the model we see drawn most often)

10. Electron Cloud Model • (1926)—current model (due to discovery of neutrons in nucleus as well) • Electron cloud—area around the nucleus of an atom where its electrons are most likely found (cannot be predicted) • Energy levels are areas of the cloud where electrons are more likely to befound

11. Masses of Atoms • AMU—unit of measurement used for atomic particles • Mass of 1 proton or 1 neutron is almost 1 amu. • Atomic number—the number of protons in an atom • Mass number—the sum of the number of protons and the number of neutrons in the nucleus

12. Atomic mass = decimal number under the symbol on the periodic table • NOT THE SAME THING AS ATOMIC NUMBER or MASS NUMBER • It is the average weight of all atoms of the element (including isotopes)

13. Calculating neutron number • If you know the mass number and atomic number of an atom, you can find the number of neutrons as well,. • Neutron #= mass # - atomic #

14. Isotopes • Carbon-12 is the most common form of carbon • However, Carbon-14 is a radioactive form of carbon • WHY? • Not all the atoms of an element have the same number of neutrons

15. ISOTOPES • Isotopes—atoms of the same element that have different numbers of neutrons • Average atomic mass—the weighted average mass of an element’s mixture of isotopes (used because most elements have more than one isotope)

16. The Periodic Table • Dimitri Mendeleev-arranged all the elements known in order of increasing atomic masses and discovered a pattern • Today’s PeriodicTable—elements are arranged by increasing atomic number and by changes in physical and chemical properties

17. Mendeleev-left blank spaces to keep elements in line according to chemical properties • He predicted the existence of two elements not yet discovered

18. Groups • The vertical columns in the periodic table-also called families • Periodic Table—pg. 556 and 557

19. Electron Cloud Structure • In neutral atoms, the electron number = the proton number • SO: Atomic number tells how many protons AND how many electrons the element has

20. Elements in the same group have the same number of electrons in their outer energy level-called VALENCE ELECTRONS • This number determines properties of the elements

21. Number of Atoms in Energy Levels • Energy Level 1—holds 2 e • Energy Level 2—holds 8 e • Energy Level 3—holds 18 e • Energy Level 4—holds 32 e • Octet Rule—explains that atoms are most stable with an outer valence holding 8 electrons

22. Periods/Rows • Horizontal groups across the periodic table • Each row ends with an element with a full outer valence (8 electrons) • Periods increase by one proton and one electron going L to R

23. Metals • Metals are on the left-hand side of the periodic table • Most are shiny, ductile, malleable and are good conductors • Ductile—drawn into wire • Malleable-can be hammered into sheets

24. Non-metals • Non-metals are on the right side of the periodic table • Most are gases, brittle, and poor conductors

25. Metalloids • Metalloids-run along the middle zig-zag line • Metalloids have some of the properties of both metals and non-metals

26. Carbon-14 has 6 protons and 8 neutrons Isotopes—atoms of the same element that have different numbers of neutrons Radioactive isotopes—those isotopes that are unstable and become radioactive

27. Identifying Isotopes • Average atomic mass—the weighted-average mass of the mixture of its isotopes • Ex: 4 out of 5 atoms of B are boron-11 and 1 out of five is boron-10 • Weighted average = 4/5 (11) + 1/5(10) = 10.8 AMU

28. Periodic Table websites: • www.chemicool.com • www.Ptable.com • www.periodictable.com • www.webelement.com