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.

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 np

pnn

n

Modern Model of the Atom:“Electron Cloud Model”

Subatomic particles;

Name Symbol ChargeRelative mass

Actual mass (g)

Electron e- -1 1/1840 9.11 x 10-28

Proton p+ +1 1 1.67 x 10-24

Neutron n0 0 1 1.67 x 10-24

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.

Ca MgS

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.

SymbolsSymbols What is the… What is the…

–Net chargeNet charge

–number of protonsnumber of protons

–number of electronsnumber of electrons

–Atomic numberAtomic numberBr 1-

SymbolsSymbols What is the… What is the…

–Net chargeNet charge

–number of protonsnumber of protons

–number of electronsnumber of electrons

–Atomic numberAtomic numberCa 2+

SymbolsSymbols What is the… What is the…

–Net chargeNet charge

–number of protonsnumber of protons

–number of electronsnumber of electrons

–Atomic numberAtomic numberO 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 n0 are what make up the mass of atoms; the

mass of electrons is insignificant.

IsotopesAtoms of the same element can have differing

masses due to differing numbers of neutrons.

Example: Carbon-12 ( 12C ) Carbon-13 ( 13C ) 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 n0 )

Two ways of writing isotopes:Contain the symbol of the element, the mass number and the atomic number:

X Massnumber

Atomicnumber

K39

19

1.)

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

F19 9

Mass Number – Atomic Number = # of Neutrons

SymbolsSymbols Find the Find the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Atomic numberAtomic number

–Mass NumberMass Number

Br80 35

SymbolsSymbols if an element has an atomic if an element has an atomic

number of 34 and a mass number number of 34 and a mass number of 78 what is the of 78 what is the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Complete symbolComplete symbol

SymbolsSymbols if an element has 91 protons and if an element has 91 protons and

140 neutrons what is the 140 neutrons what is the

–Atomic numberAtomic number

–Mass numberMass number

–number of electronsnumber of electrons

–Complete symbolComplete symbol

SymbolsSymbols if an element has 78 electrons and if an element has 78 electrons and

117 neutrons what is the 117 neutrons what is the

–Atomic numberAtomic number

–Mass numberMass number

–number of protonsnumber of protons

–Complete symbolComplete 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 atomis

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 35.453

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 8 + 10 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.

1st shell (n = 1) gets 2 e-

2nd shell (n = 2) gets 8 e-

Inner shells fill first;

3rd shell (n = 3) gets 8 e-

…before 4th 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)