2. The Atom cont.

3. Key to the Periodic Table n Elements are organized on the table according to their atomic number, usually found near the top of the square. –The atomic number refers to how many protons an atom of that element has. –For instance, hydrogen has 1 proton, so it’s atomic number is 1. –The atomic number is unique to that element. No two elements have the same atomic number.

4. What’s in a square? Atomic Number Chemical Symbol Atomic Mass Name State of Matter “Color of symbol” Metal, nonmetal or metalloid “color of box”

5. Protons?

6. When is an atom neutral? n An atom is neutral when it has an equal number of electrons and protons. Example: n Helium has 2 protons & 2 electrons n That means it has 2 + charges & 2 - charges. n They cancel to create no charge.

7. How Many Electrons?

8. n Atomic number also equals the number of electrons.

9. How do you find an atom’s mass? n What are the only two parts of an atom that have mass? n Protons have a mass of 1 AMU n Neutrons have a mass of 1 AMU n Electrons are so teeny they don’t weigh anything.

10. Atomic Mass/Mass # n Mass Number = number of protons + number of neutrons n You Try

11. Check answers with your teacher!

13. 3 7 3 3 4 Boron 116

14. Think About This! 1. What is heavier, an atom’s nucleus or the orbitals? Nucleus 2. What happens to an atom that has an extra neutron? It becomes a heavy Isotope 3. What happens to an atom that is missing an electron? It becomes a charged ion

15. Questions... n What particles are in the nucleus? n Explain why atoms are neutral. n Why do scientists say that most of the mass of an atom is located in the nucleus?

16. How Do Atoms of Different Elements Differ? n Lets compare a Hydrogen atom and a Helium atom…

17. Hydrogen... n Draw one proton. n Draw a circle around the proton. n Place one electron on the circle. n You have just created a hydrogen atom! n Hydrogen has no neutrons.

18. DOT & CROSS DIAGRAMS With Dot & Cross diagrams elements and compounds are represented by Dots or Crosses to show electrons, and circles to show the shells. For example; Hydrogen H 1 1 Electron (-) -

19. Helium n Draw n Draw two protons and two neutrons all together. an electron orbit around them. n Place n Place two electrons on the orbit. n You n You have just created a helium atom!!!

20. Helium + + N N - - He 2 4 Electron (-)

21. HELIUM ATOM + N N + - - proton electron neutron Shell What do these particles consist of?

22. Bohr’s Atom electrons in orbits nucleus

23. What is different? n The two different elements have different numbers of electrons, protons, and neutrons. n OK...

24. Are all atoms of an element the same? n Draw another Hydrogen atom, but this time add a neutron. n It is still Hydrogen because it has only one proton, but it has more mass in the nucleus. n It is called an isotope.

25. What is an Isotope? n Isotope - atoms that have the same number of protons but have different numbers of neutrons. n Some isotopes are radioactive. n Otherwise they are mostly the same as the regular atom.

26. How can you tell one isotope from another? n By their mass numbers. n mass number = protons + neutrons. n Different isotopes will have different mass numbers because they have different numbers of neutrons.

27. Let’s see... n Hydrogen-1 has 1 proton and 0 neutrons. n Mass Number = 1 n Hydrogen-2 has 1 proton and 1 neutron. n Mass Number = 2

28. Questions… n List the charge, location, and mass of each particle. n Determine the number of protons, electrons, and neutrons in aluminum-27.