Atoms and Molecules

1. Atoms and Molecules

2. Atoms • Atoms are the smallest whole particles of an element. • Atoms are made of protons (+) and neutrons (0) in the nucleus and electrons (-) buzzing around the nucleus. • Atoms are mostly empty space but the electrons move so fast atoms act like a solid, like the blades of a fan when it’s on.

3. Atomic Theory • The theories about atoms have changed a lot since Democritus in 400 B.C. He said atoms of a substance have the properties of that substance. Like cheese particles that are squishy and yummy. • In 1803 A.D. John Dalton developed what we now call atomic theory. That all matter is made of atoms, atoms cannot be created or destroyed in a chemical reaction, all atoms of a certain element are exactly the same and compounds are made from multiple atoms of different elements. • What caused the theory of atomic structure to change?

4. Atomic Theory • Thomson used a vacuum tube or cathode ray tube, to discover electrons that he called corpuscles. He said that electrons are distributed in a positive mass like blueberries in a muffin. This was called the plum pudding model.

5. Atomic Theory • A few years later Ernest Rutherford did the gold foil experiment. • In this experiment he shot positive alpha particles at a thin sheet of gold atoms. He expected that they would all just pass through in straight line if Thomson was right. What he discovered was that some particles bounced back or in other directions. That must mean there is a positive nucleus that is tightly packed in the center of an atom.

6. Atomic Theory • Neils Bohr worked with Rutherford and developed the explanation that electrons travel in rings like the layer of an onion and electrons in these rings have different energy levels. • Several years later the cloud model of the atoms was developed using the newly discovered principles of quantum physics. This states that electrons can be in one place or the other but never in between

7. Atomic Theory • In 1932 James Chadwick discovered another particle in the nucleus called the neutron (it was called neutron because it is neutral, meaning it has no electrical charge). • What is the name of the model we now consider to be closest to the truth about how atoms look?

8. Bonding • As we’ve discovered by building and presenting our atoms, many atoms do not have full outside rings. Usually it is missing a few electrons out of the possible electrons on the outside ring. • These lonely electrons on the outside ring are called valence electrons. • For example Carbon has 6 electrons total, so there are 2 on the inside ring and 4 on the second ring. Those 4 electrons are the valence electrons. • What are valence electrons?

9. Bonding • Sometimes when atoms bond together they give up or gain these electrons and sort of pretend that they have what they need to be stable when they do that. (Atoms don’t think about this and wonder who to give electrons to like a person would). • Once an atom is bonded to another atom it is called a molecule. When the atoms that are bonded together are different types of atoms that molecule is referred to as a compound.

10. Bonding • Atoms give up an electron (or more if needed) when they have 3 or less electrons. When an atom gives up electrons it gains a positive charge because it now has more protons than it does electrons. • When an atoms has 5 or more electrons it usually gains electrons from other atoms and becomes negative because it now has more negative electrons than it does positive protons

11. Bonding • When an atom has a negative or positive charge it is called an ion. If an atom has to give up electrons to be stable it is a positive ion, when it gains electrons to be stable it is a negative ion. • Usually negative and positive ions come together to make neutral molecules and compounds. But occasionally groups of atoms come together and still have a negative or positive charge (these are called polyatomic ions). Poly means many. • When is an atom called an ion?

12. Bonding • Polyatomic ions combine with other ions or polyatomic ions to became stable compounds. • When atoms in the form of an ion give up electrons to become molecules and compounds the bond they have is called an ionic bond because the atoms have become ions in order to form a bond to make molecules and compounds. • What is a polyatomic ion?

13. Chemical Formulas • Chemical formulas tell us what types of atoms and how many of each atom is involved in a molecule or compound. • For example MgCl2 has only 1 Mg (magnesium) and 2 Cl (chlorine). This is known because the Mg has no number after it which means there’s only 1 and Cl has a #2 after it which tells us there’s 2 Chlorine atoms.

14. Chemical Formulas • So how many carbon atoms are in C6H12O6? • How many oxygen atoms are in H2O? • When you hear the name of an ionic compound such as sodium chloride (table salt) you can know that the sodium gave up electrons because the name of the positive ion comes first and the negative ion (the one that gained electrons) comes second. • What would you call MgCl2 that we talked about earlier?

15. Properties of Compounds • Ionic compounds usually form crystals like salt and they usually have a very high melting points. • When ionic compounds dissolve in water it causes the water to conduct electricity. Because water itself is a stable molecule, without ions water does not conduct electricity.

16. Covalent Bonds • These bonds are formed when atoms share electrons and neither become positive or negative atoms. • These bonds are usually between non metals like oxygen, nitrogen, phosphorus etc. • In covalent bonds no electrons are given away or gained so the atoms stay neutral.

17. Covalent Bonds • How are covalent bonds different from ionic bonds? • For every electron atoms share there is one bond. • For example hydrogen’s outside ring can only hold two but actually only has one. This means for hydrogen’s outside ring to be full it needs to share one electron with another atom. This means hydrogen can only form one bond.

18. Covalent Bonds • While oxygen has 6 valence electrons on its outside ring it needs 2 more electrons to be stable. • Why do you think water is H2O instead of HO? • Some atoms can share so many electrons that they form double or triple bonds with the element it is sharing with. The only atoms that can form quadruple bonds are the atoms in the same column as Carbon. This doesn’t happen that often.

19. Molecular Compounds • Molecular Compounds form when molecules are bonded together with covalent bonds instead of ionic bonds. • Molecular compounds usually have lower melting and boiling points and won’t conduct electricity if dissolved in water. • What are two differences between Ionic Compounds and Molecular Compounds?

20. Unequal Sharing • Sometimes in a molecular compound one atom pulls harder on the electrons than the other. This give the atoms a SLIGHT electrical charge, but not like the larger charge of an ion. • When one atom pulls harder on the shared electrons it makes a polar molecule. This is then called a polar bond.

21. Unequal Sharing • Polar molecules often have an attraction to the other polar molecules near it. So the oxygen end of one water molecule is attracted to the hydrogen end of the water molecule next to it even though each molecule has its own hydrogen or oxygen atoms. • What unique property of water do you think this attraction causes? Think back to the penny lab and the soap.