CHEMICAL INTERACTIONS

1. CHEMICAL INTERACTIONS *The only constant in nature is change.

2. INSIDE AN ATOM • Matter – the “stuff” that makes up everything in the universe. Matter is made up of elements. • Element – a substance that cannot be broken down into other substances bychemical or physical means. • Atom – the smallest particle of an element.

3. Structure and Composition of an Atom • An atom consists of a nucleus surrounded by one or more electrons. • Nucleus – the tiny, central core of an atom that contains even smaller particles called protons and neutrons. • Proton – has a positive electric charge (p+) • Neutron – has no charge (neutral) (n0)

4. Electron – extremely small particles that move rapidly in the spaceoutsidethe nucleus • Electrons have a negative charge (e-). • Atoms are neutral because they contain equal numbers of protons and electrons(positive and negative charges balance and cancel each other out).

5. Atomic Number • Every atom of a particular element contains the same number of protons. • Atomic Number– the number of protons in the nucleus of an atom. • The atomic number identifies the element.

6. Atomic Mass • Atomic Mass – the average mass of one atom of an element. • Atomic Mass Unit(amu) – unit of atomic mass. • Protons and neutrons = 1 amu • Electron = 1/2000 amu • Most of the mass of an atom is in the nucleus.

7. The number of neutrons in an element may vary, therefore the mass of an element may vary. • Atoms of a particular element have most of the same chemical properties despite their different masses.

8. The Role of Electrons • Electrons move around the nucleus so fast that it is impossible to know exactly where any electron is at a particular time. • The space around the nucleus is a spherical cloud of negatively-charged electrons (electron cloud).

9. Most of an atom’s mass is in its nucleus. Most of an atom’s volume is the electroncloud. • How far away are the electrons from the nucleus? [page 24] • The diameter of an atom is about 100,000 times the diameter of the nucleus!

10. Valence Electrons • Valence Electron(s) – the electron(s) that are the farthest away from and most loosely held by the nucleus of an atom. • Many properties of the atom, and therefore the element, are determined by the number of valence electrons. • Valence electrons form chemical bonds.

11. Valence Number – the number of valenceelectrons in an atom. • The valence number can vary from 1 to 8.

12. Electron Dot Diagrams • Electron dot diagrams are used to show chemical bonding. • Electron dot diagrams use the symbol of the element surrounded by dots which represent the valence electrons. There are no more than two dots per side.

13. When an atom forms a chemical bond, one of two things happen: 1) The number of valence electrons are shared in order to increase to a total of 8 for each atom (2 for hydrogen). Example: H2O H O H

14. 2) All the valence electrons are given up or transferred to another atom. Example: NaCl Na Cl

15. When atoms end up with 8 or 0 valence electrons, the atom becomes more stable (less reactive) than they were before. The exception is Hydrogen which is stable with a valence number of 2. • Helium has 2 electrons and is therefore stable and unreactive.

16. Sodium gives up its 1 valence electron to have 0 valence electrons (stable). Chlorine accepts the electron from sodium to give it 8 valence electrons (also stable). • By giving up 1 electron, sodium takes on a charge of +1. By accepting an extra electron, chlorine takes on a charge of -1. Opposite charges attract and the atoms are held together by this attraction. Na+1 Cl-1

17. Models of Atoms [Pages 26-27] • 1808 - Dalton Model (John Dalton) – tiny, solid balls • 1897 - Thomson Model (J.J. Thomson) – plum pudding model • 1904 - Nagaoka Model (HantaroNagaoka ) – solar system model • 1911 – Rutherford Model (Ernest Rutherford) – atom is mostly empty space, electrons orbit randomly • 1913 – Bohr Model (Niels Bohr) – electrons are found in shells or energy levels • 1932 – Chadwick Model (James Chadwick) – discovered that neutrons were in the nucleus of atoms • Modern Model – electron cloud model

18. THE PERIODIC TABLE • Physical property – can be observed without changing the substance into something else (examples: density, melting point) • Chemical property – observed when a substance interacts (reacts) with another substance to form one or more new substances (example: flammability)

19. Dimitri Mendeleev developed the first useful periodic table in 1869. • Mendeleev’s periodic table was arranged by increasing atomic mass. Patterns appeared when the elements were arranged this way. • Atomic mass – the average mass of one atom of an element. • The modern periodic table is arranged by increasing atomic number.

20. The word periodic means “a regular, repeated pattern.” The properties of the elements repeat in each period of the table. • The periodic table is one of the most useful tools to help people study chemistry. • Scientists organize the periodic table according to their physical and chemical properties. • Periodic Table of the Elements – pages 32-33.

21. Reading the Periodic Table • Each square of the periodic table usually includes the element’s symbol, its name, its atomic number, and its atomic mass. • Atomic numbers increase from left to right and top to bottom.

22. Groups and Periods • Group (family) – the elements in a column • There are 18 groups in the table. Most groups have a name. All elements in a group have the same number of valence electrons. • Periods – the horizontal rows of the table • There are 7 periods. Some of the periods have different numbers of elements.

23. Properties of Elements • An element’s physical and chemical properties can be predicted from its location in the periodic table. • Metals – found on the left hand side of the periodic table. Metals usually give up their valence electrons during a chemical reaction. • The properties of metals include: hard, shiny, malleable, ductile, good conductors of heat and electricity.

24. Nonmetals – found on the right side of the periodic table • Nonmetals usually gain or share valence electrons in a chemical reaction • The properties of nonmetals are the opposite of metals: dull, crumbly (soft and not malleable or ductile), and do not conduct heat or electricity. • Hydrogen is a nonmetal found on the metal side of the table.

25. Metalloids– have some properties of metals and some of nonmetals. • Metalloids are found along the bold zig-zag, or stair step line that separates the metals and nonmetals. • Metalloids have varying ability to conduct electricity making them excellent semiconductors. • Semiconductors are used in computers, transistors, and lasers (examples: silicon, germanium).

26. There are many patterns or trends across and down the periodic table. • The periodic table works because it is based on the structures of atoms, especially the valence electrons.

27. Chemical Reactions Chemical reaction – a change in matter that produces one or more new substances with properties that are different from those of the starting substances.

28. Evidence for Chemical Reactions • Color change • Formation of a precipitate • Production of a gas • A change in temperature • Changes in properties

29. Precipitate – a solid that forms from solution during a chemical reaction. Endothermic reaction – a chemical reaction that absorbs energy (ex. – chemical cold pack) Exothermic reaction – a reaction that releases energy in the form of heat ( ex. – burning charcoal).

30. Molecule – a combination of two or more atoms that are bonded together. Compound – a substance made of two or more elements that have been chemically combined. *Molecules are the smallest particle that makes up a compound.

31. Chemical Equations Chemical equation – a short, easy way to show a chemical reaction, using symbols instead of words. Chemical formula – a combination of symbols that show the ratio of the elements in a compound (ex. – H2O, CO2, C3H8, C6H12O6)

32. Subscripts – numbers in a chemical formula that tell the number of atoms in a molecule or the ratio of elements in a compound. *If a letter symbol in a formula doesn’t have a subscript, the number 1 is understood (never actually write a subscript of 1).

33. A chemical equation uses symbols and formulas to show the reactants and the products of a chemical reaction. reactant + reactant  product + product

34. Law of Conservation of Mass - matter is neither created nor destroyed in a chemical reaction. • The total mass of the reactants must equal the total mass of the products. • A chemical equation must show the same number of each type of atom on both sides of the equation.

35. Balancing Chemical Equations How to balance a chemical equation: • Examine the equation and do a quick inventory to see what is out of balance (if anything) • Use whole number coefficients only to balance the equation. Never alter a subscript. • Focus on one element at a time. Balance that element and then move on to another element. • Save any elements or diatomic molecules to balance last. • Check your work.