Atomic Structure

Two regions of every atom: Nucleus -is made of protons and neutrons -is small and dense Electron cloud -is a region where you might find an electron -is made up of many “shells” p n p p n n n Modern Model of the Atom: “Electron Cloud Model”

Subatomic particles; NameSymbolCharge Relative mass Actual mass (g) Electrone-e- 1/ x Protonp+p x Neutronn0n x

Opposite charges attract each other ( + and - ) Alike (similar) charges repel each other ( + and + ) ( - and - )

= the # of protons in the nucleus Atomic Number (“Z”)

Counting Charge: Ions “Atomic Number” Z is the number of protons. determines what element an atom is. ( Z )– (# of e - ) = net charge of the atom Ion: An atom with a net charge Cation: positive ion Anion: negative ion

Ions: Mini-Lecture

Counting Charge: Ions Fe ( Z )– (# of e - ) = net charge of the atom If you see an element symbol alone, assume it has an equal number of p + and e -, and therefore no net charge. CaMgS Ti 3+ Cl 1- O 2- Fe 2+ Cations and anions are indicated by writing the net charge of the ions in superscript to the right of the element symbol.

Symbols n What n What is the… –Net –Net charge –number –number of protons of electrons –Atomic –Atomic number Br 1-

Symbols n What n What is the… –Net –Net charge –number –number of protons of electrons –Atomic –Atomic number Ca 2+

Symbols n What n What is the… –Net –Net charge –number –number of protons of electrons –Atomic –Atomic number O 2-

Practice

Counting Nucleons; Isotopes “Atomic Number” Z is the number of protons. determines what element an atom is. if the atom is neutral, Z = # of e - “Mass Number” the number of protons + neutrons. called the mass number, because p + and n 0 are what make up the mass of atoms; the mass of electrons is insignificant.

Isotopes Atoms of the same element can have differing masses due to differing numbers of neutrons. Example:Carbon-12 ( 12 C ) Carbon-13 ( 13 C ) are isotopes of Carbon. Isotopes: Atoms that share the same atomic number (same element; same # of p + ) but have a different mass number. (different isotopes of that element: different # of n 0 )

Two ways of writing isotopes: Contain the symbol of the element, the mass number and the atomic number: X Mass number Atomic number K ) 2.) Put the mass number after the element’s name: carbon- 12 carbon –14 uranium-235

Symbols Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass Number F 19 9 Mass Number – Atomic Number = # of Neutrons

Symbols n Find n Find the –number –number of protons of neutrons of electrons –Atomic –Atomic number –Mass –Mass Number Br 80 35

Symbols n 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

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

Symbols n if an element has 78 electrons and 117 neutrons what is the –Atomic number –Mass number –number of protons –Complete symbol

Study Guide

Atomic Mass How heavy is an atom of oxygen? It depends on number of protons and neutrons: –Remember: the mass of an electron is negligible. There are different isotopes of oxygen. So, we take an average based on how common each isotope of oxygen is and calculate the… Average Atomic Mass

Measuring Atomic Mass When measuring the mass of atoms, we do not use grams because the numbers would be too small; instead we use the 1 amu = 1 / 12 the mass of a carbon-12 atom. ( official definition) = basically, it is the mass of one proton or one neutron Atomic Mass Unit (amu)

Why use amu? Because an amu is basically defined as the mass of a proton or a neutron, The mass number of an atom is the mass of the atom in amu. A Carbon-12 atom has a mass of 12 amu. A Potassium-40 atom has a mass of 40 amu. A Uranium-235 atom has a mass of 235 amu.

Average Atomic Mass: A Summary Definition: the weighted average of all naturally occurring isotopes of an element. Units of atomic mass are the amu. Average atomic masses are not whole numbers because they are averages. Sneak Preview: The average atomic mass of an element (in amu) is also numerically equal to the mass of one ‘mole’ of that element in grams. (More on this later…)

A Block on the Periodic Table symbol Cl name Chlorine atomic number 17 average atomic mass You will find these differently arranged in different Periodic tables. The Atomic Number gives the elements position in the table.

Introduction: Bohr Diagrams

Modern View The atom is mostly empty space Two regions Nucleus- protons and neutrons Electron cloud- has many shells (energy levels)

Electrons are found in ‘shells’ = ‘energy levels’ can hold electrons can hold 8 electrons can hold 2 electrons

Bohr said: Electrons can only be at certain, specific energy levels (distances from the nucleus) “Electron energy is quantized” n = 1 n = 2 n = 3

Bohr Model Diagrams …show energy levels (shells) and the electrons in them. nucleus is not shown. 1 st shell (n = 1) gets 2 e - 2 nd shell (n = 2) gets 8 e - Inner shells fill first; 3 rd shell (n = 3) gets 8 e - …before 4 th shell starts to fill

Valence Shell: The outermost shell that contains electrons. Valence Electron: An electron in an atom’s valence shell

Modern View The atom is mostly empty space Two regions Nucleus- protons and neutrons Electron cloud- has many shells (energy levels)