Atomic Structure Modern Atomic Theory

Modern Atomic Theory The atom consists of positive protons, negative electrons, and neutral neutrons. Protons and neutrons are located in the nucleus of the atom, which is small and massive. Electrons are located outside of the nucleus, which creates the volume of the atom.

The Modern View of Atomic Structure

Subatomic particles Actual Electron e- -1 9.11 x 10-28 Proton p+ +1 1 mass (g) Relative mass (amu) Name Symbol Charge Electron e- -1 1/1840 9.11 x 10-28 ≈ 0 Proton p+ +1 1 1.67 x 10-24 Neutron n0 1 1.67 x 10-24 amu = atomic mass unit

Atomic Number Henry Moseley discovered that each element has a unique positive charge. Atomic Number or Z Number of Protons For neutral atoms # protons = # of electrons How many protons and electrons are in Si? 14 p and 14 e What element contains 8 protons? oxygen

Ions Ion charge is written as a superscript. Most atoms are not neutral. An ion forms when an atom loses or gains electrons. Lose electrons = positive charge Gain electrons = negative charge Ion charge is written as a superscript. Charge = # protons - # electrons What is the element with 16 protons and 18 electrons? S-2 What is the element with 20 protons and 18 electrons? Ca+2

Atomic Mass Atomic mass or A is the number of protons and neutrons in an atom. Atomic Mass Unit or amu based on carbon-12, exactly 12 amu We always round atomic mass to the nearest whole number. Why? Atomic mass is the weighted average of all of isotopes for that element based on abundance.

X Isotopic Notation A Z We write symbols for elements as follows: Mass number X A Z Element symbol Atomic number

Isotopes Atoms with the same number of protons different number of neutrons, therefore different atomic masses. 1 H 2 1 H 3 1 H

Isotopes

Naming Isotopes We can also put the mass number after the name of the element. carbon- 12 carbon -14 uranium-235

Calculating averages Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results. Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu. 62.93 amu x .691 = 43.5 amu 64.93 amu x .309 = 20.1 amu Total = 43.5 + 20.1 = 63.6 amu

Na Atomic Mass 23 11 What is the atomic mass of Na? What is the atomic number of Na? Write this in isotopic notation: To find the neutrons subtract the two numbers. 23 amu 11 amu 23 Na 11 23 -11 = 12 amu

Br Symbols 80 35 Find the number of protons number of neutrons 35 p number of electrons Atomic number Mass Number 35 p 45 n 80 35 e Br 35 35 80 amu

Br Symbols 80 -1 35 Find the number of protons number of neutrons 35 p number of electrons Atomic number Mass Number 35 p 45 n 80 -1 36 e Br 35 35 80 amu

Symbols if an element has an atomic number of 34 and a mass number of 78 what is the number of protons number of neutrons number of electrons Complete symbol 34 p 44 n 34 e 78 Se 34

Symbols if an element has 91 protons and 140 neutrons what is the Atomic number Mass number number of electrons Complete symbol 91 231 amu 91 231 Pa 91

Summary Mass number Element charge +1 H 1 Atomic number Element symbol

Ba Symbols 135 +2 56 Find the number of protons number of neutrons number of electrons Atomic number Mass Number Is this an ion? Is this an isotope? 56 p 79 n 135 +2 54 e Ba 56 56 135 amu Yes Yes