Useful Element Notations Atomic Number Z The number

Useful Element Notations

Atomic Number (Z) • The number of protons in the nucleus • Determines the identity of the atom (which element it is) v. If you change the number of protons, the atom changes to another element.

Mass Number (A) • The total number of particles in the nucleus—protons and neutrons • The total number of particles that make up the mass of the atom

Isotopes: Count Those Neutrons • Atoms of an element which have various amounts of neutrons but the same number of protons v. Since the number of neutrons changes, the mass of the isotopes is different.

Isotopic Notation • It shows the mass number, atomic number, and symbol for the element.

Isotopic Notation Example: 9 4 Be 9 protons and neutrons 4 protons Therefore, there must be 5 neutrons.

Question If isotopic notation for an 40 element is 18 Ar , how many neutrons are in the nucleus? 1. 18 2. 40 3. 22 4. 58

Atomic Mass • Most elements consist of a mixture of different isotopes, which have different masses. • Therefore, the mass of the element is the average of the weights of the isotopes.

Unified Atomic Mass Units • Dalton tried to find the masses of the atom but could only find the relative masses. • We now know the exact masses.

Unified Atomic Mass Units • A unified atomic mass unit (u) is approximately the mass of a proton.

Example Calculate the average atomic mass of a chlorine atom. In nature, 75. 77% of chlorine atoms have a mass of 34. 969 u per atom (Cl-35). 24. 23% of chlorine atoms have a mass of 36. 966 u per atom (Cl-37).

Example 1. Convert the percents to decimals. 75. 77% = 0. 7577 24. 23% = 0. 2423

Example 2. Multiply the decimal percentage by the mass of each isotope to find the total mass of each isotope in the sample. Cl-35: 0. 7577 x 34. 969 u = 26. 50 u Cl-37: 0. 2423 x 36. 966 u = 8. 957 u

Example 3. Add the results to get the weighted average mass of the sample. 26. 50 u + 8. 957 u 35. 46 u The weighted average mass of a typical chlorine atom: 35. 46 u

Valence Electrons: Last But Not Least • Outer level electrons • In the s and p sublevels • Maximum # in each atom: 8 § 2 in the s § 6 in the p

Valence Electrons: Last But Not Least • Emit spectra • Participate in chemical bonds • Give elements physical properties

Electron Dot Diagrams A shorthand method of showing the valence electrons

Electron Dot Diagrams Na valence electrons: 1

Electron Dot Diagrams Mg valence electrons: 2

Electron Dot Diagrams Al valence electrons: 3

Electron Dot Diagrams Si valence electrons: 4

Electron Dot Diagrams P valence electrons: 5

Electron Dot Diagrams S valence electrons: 6

Electron Dot Diagrams Cl valence electrons: 7

Electron Dot Diagrams Ar valence electrons: 8

Ions: Charged Atoms

Changing Numbers • Changing the number of protons gives a different element. • Changing the number of neutrons gives a different isotope of the same element.

Changing Numbers • Changing the number of electrons gives an ion of that element.

Ions • Are atoms which have gained or lost electrons • Occur when an electron gains enough energy to “jump” off an atom

Ions • Are charged, rather than neutral like atoms, because the number of electrons and protons are no longer equal

Anions • Are ions that have gained extra electrons • Are negatively charged • Have gained negative charges

Cations • Are ions that have lost some of their electrons • Are positively charged • Have lost negative charges