2. ELEMENTS Remember: Atoms of one type form an element –A bunch of gold atoms form gold…etc. Elements are the simplest form of a substance Every element is unique and has its own chemical and physical properties

3. Compounds and Molecules A molecule is formed when two or more atoms join together chemically. atoms A compound is a molecule that contains at least two different elements.elements All compounds are molecules but not all molecules are compounds.

4. Atoms, Elements, Compounds, Molecules OSCH 4 FeO 2 N 2 HH 2 OH 2 SO 4

5. Elements are arranged: Vertically into Groups Horizontally Into Periods

6. Each atom has the same number of electrons in it’s outermost shell.

7. Valence Electrons: Electrons in the outer shell

8. The number of outer or “valence” electrons in an atom effects the way an atom joins with other atoms. The way atoms join determine many properties of the element. This is why elements within a group usually have similar properties.

9. Each atom has the same number of electron holding shells.

10. The numbers represent the period as well as the amount of electron holding shells.

11. Each group has distinct properties The periodic Table is divided into several groups based on the properties of different atoms.

13. Non-reactive Gases at room temperature Very Stable Noble Gases

14. WHY STABLE? Noble gases have full valence levels!

15. Atoms whose shells are not full tend to interact with other atoms and gain, lose, or share electrons HYDROGEN (H) Atomic number = 1 CARBON (C) Atomic number = 6 NITROGEN (N) Atomic number = 7 OXYGEN (O) Atomic number = 8 Electron Outermost electron shell (can hold 8 electrons) First electron shell (can hold 2 electrons)

16. When atoms gain or lose electrons Charged atoms called ions are created –Lose electrons---positive charge – Gain electrons--- negative charge –Like charges repel and opposite charges attract Ionic bonds Na Sodium atom Cl Chlorine atom Na + Sodium ion Cl – Chloride ion Sodium chloride (NaCl) NaClNaCl + –

17. Ionic Bonds Electrons are transferred Forms between metals and nonmetals Dissolve easily in water, usually

18. Covalent Bonds Sharing of outer electrons Very strong bond If “pull” is unequal creates a polar covalent bond = a polar molecule (ex: water)