Atomic Structure

•Nucleus•Electron

Cloud

Nucleus: contains the protons and neutrons

Electron cloud:

A simple atomic model

Subatomic Particleswhat’s inside an atom?

Particle Name Location Charge Mass

Proton (p+) nucleus +1 1 amu

Electron (e-) cloud -1 2000x smaller

than p+

Neutron (n0) nucleus 0 1 amu

Atoms are always neutral because the # protons = # electrons. So + equals -

The Nucleus – Pt. 1

• Very small, very dense – large mass in a very small volume

• Positive charge• Protons and neutrons (nucleons)• Center of the atom• All the atom’s mass is concentrated

in the nucleus (the nucleons)

The Nucleus – Pt. 2Nucleons = the particles inside the nucleus and

their importance

Protons (p+)

• Positive charge

• Mass = 1 amu

• Atomic number (Z): identifies each unique atom and = the protons

• Ex. 6C 92U

• Atomic mass (A): mass of atom = p+ + no

Neutron (no)

• No charge, neutral

• Mass = 1 amu (same as proton)

• Atomic mass (A): mass of atom = p+ + no

• Ex. U238

Electron Cloud• All electrons are located in the cloud; cloud has a large volume

with very tiny mass (low density)

• Cloud is negative in charge (because electrons are negative) but…

• Atoms are neutral: electrons in cloud = protons in nucleus

• Electrons = properties (ex. color, whether it’s a metal, how does it react)

• Cloud is built up in layers with each bigger layer able to hold more electrons than the inner ones. The most important layer is the ….

• VALENCE SHELL - found on the outer edge of the cloud. Holds the electrons that give the atom its properties. Every element in a FAMILY has the same number of VALENCE electrons

Who is this atom and how do you know? Write the correct symbol for it.

Electron layers within the cloud

C126

Metals Metalloids (Semimetals) Nonmetals

Lose e- Gain e-

Except:Family 8 has the OCTETand never gains e-

ISOTOPESIsotope = different forms of the same

atomHow are they the same?

o same protons

o Same atomic number (Z)

o same symbol

How are they different?o different atomic mass (A)o different number of neutrons

Isotopes are like different features on the same model car

different atomic mass

same symbol/atomic numberC12

6 C146

3 Isotopes of Hydrogen

H11 H21 H31

p+ ______ ______ ______

e- ______ ______ ______

n0 ______ ______ ______

Isotope Practice #1

W18474 I127

53

Determine the number of neutrons in each isotope

Al2713

p+ ______ ______ ______

e- ______ ______ ______

n0 ______ ______ ______

Isotope Practice #2

Using the protons and neutrons, calculate the mass and write the symbol for each isotope: Symbolmass

protons

86 protons and 136 neutrons

7 protons and 8 neutrons

27 protons and 32 neutrons

Isotope Practice #3One last way to identify an isotope is by using its name and its mass (since every isotope is different by mass)

Example: Phosphorus – 31 (or P - 31)

A. Identify these isotopes in this fashion:

12 protons and 12 neutrons 24 protons and 28 neutrons

B. How many protons and neutrons are in each isotope?

Sr – 88 I – 127 Hg - 201

Calculating Average Atomic Masses

•Use all isotopes to determine the average

•Add up all the masses and divide by the total number of isotopes present in the sample

•If there are many isotopes – sort them out first

Frequency (Mass #1) + Frequency (Mass #2) …

Total number of isotopes in sample

Frequency is how many times that isotope is counted

… means there could be more than 2 isotopes in the sample

Example: calculating average atomic mass

A sample of Ca isotopes:

40 38 39 40 40 40 40 38 41 39 40 40 40 40 40 40 40 40 39 39 40 40 40 40 38 41 40

40 40 40 40 39 40 40

24(40) + 3(38) + 5(39) + 2(41)

34

960 + 114 +195 + 82

34

1351

34

Average atomic mass = 39.74 amu

IONS – How the electron cloud changes

• Electron clouds change size when electrons are gained or lost. The atom is no longer neutral and becomes an ION which is either positive or negative

• CATIONS are Positive IONS and have lost electrons: Na+1 (Sodium has lost 1 electron)

• ANIONS are Negative IONS and have gained extra electrons: P-3 (Phosphorus has gained 3 electrons)

• Anion’s names always end in “ide” - ex. phosphide• Electrons removed from or put into the VALENCE

SHELL - the outermost part of the cloud• Just remember though, the ion’s symbol hasn’t

changed because protons haven’t changed• An ION’S charge = protons - electrons

Cations lose electrons = form positive ions = empty the valence shell of the cloud to reach the octet (8).

Sodium atoms have 11 electrons: 1 shell – 2 electrons

2 shell – 8 electrons

3 shell – 1 electron

Sodium ion loses 1 electron (it’s in Family 1) and forms a +1 ion. Notice the 3 shell is now empty, but the 2 shell has the octet (8).

Na+1 ion

Na atom

Anions gain electrons = form negative ions = finish filling the valence shell of the cloud to reach the octet (8).

Chlorine atoms have 17 electrons: 1 shell – 2 electrons

2 shell – 8 electrons

3 shell – 7 electron

Chlorine ion gains 1 electron (it’s in Family 7) and forms a -1 ion. Notice the 3 shell is full and has the octet (8).

Atomic mass

(p+ + no)

Atomic number

(protons)

XA

Z

Ion Charge(p+ - e-)

charge

Subscript (in compounds only)

subscript

Element Symbol

Comparing Atoms, Isotopes, and Ions

p+

Atoms

Isotopes Ions

RadioactivityRadioactivity• Breakup of an isotope’s large unstable nucleus into a

smaller more stable isotope nucleus (decay)• Some isotopes are radioactive, some aren’t Why is the nucleus unstable?• Neutron to proton ratio: Most stable atoms have 1:1 no

to p+ ratio. Last stable ratio is 1.51:1 no to p+ ratio (206Pb)

• Strong nuclear forces (the “glue” that holds the protons to the neutrons) can’t hold nucleus together

• Breakup of an isotope’s large unstable nucleus into a smaller more stable isotope nucleus (decay)

• Some isotopes are radioactive, some aren’t Why is the nucleus unstable?• Neutron to proton ratio: Most stable atoms have 1:1 no

to p+ ratio. Last stable ratio is 1.51:1 no to p+ ratio (206Pb)

• Strong nuclear forces (the “glue” that holds the protons to the neutrons) can’t hold nucleus together

Radioactivity – Pt. 2• Radiation: particles and energy released from a

nucleus during its decay

• Transmutation: original nucleus changes into a new nucleus with a different mass and/or identity

• Half-life: time required for half the atoms of a radioactive isotope to decay into something smaller and more stable (or less radioactive)

• To do this, the radioactive nucleus releases radiation

Types of RadiationTypes of Radiation1) Alpha Particle () Helium nucleus Massive particle; common for heavy radioactive

isotopes to release these When released changes both identity and mass Slow; easy to block with paper, Al foil, plastic Used in smoke detectors

He42

Types of RadiationTypes of Radiation2) Beta Particle (β) A high speed electron Faster than an alpha particle because it’s so tiny When released, changes the identity but not the mass Passes 1-2 cm deep into the skin and can damage it Can pass through paper but is blocked by Al foil

e01

Types of RadiationTypes of Radiation3) Gamma Rays (γ ) Not a particle but pure energy Fastest of all radiation types and most

dangerous Blocked by lead shields

Types of RadiationTypes of Radiation

Radioactive isotopes can also release:

4. Proton particles Will change mass and identity

5. Neutron particles Will change mass only

p11

on10

Transmutations, Fission, and Fusion (nuclear Equations)

• The change of one radioactive element and its nucleus into another (mutation)

• Done by releasing radiation (alpha, beta, protons, or neutron release)

• Done by absorbing radiation or other small atoms (fusion). So 2 nuclei join together to produce something bigger

• Done by collisions between large radioactive atoms and smaller atoms. Result is the large atoms breaks apart (fission) in a chain reaction

Nuclear Equations Pt. 2In all nuclear equations (radiation release, fission, and fusion), total mass (A) and total identity (Z) are conserved.

Original radioactive atoms Final radioactive atoms

)()( newXoriginalX AZ

AZ

AtePo 22285

01

22284

Ex. Radioactive Polonium-222 releases beta radiation.

Original radioactive atom

releases or equals

Beta particle radiation

New radioactive atom

Check A and Z:

A) 222 = 0 + 222

and

Z) 84 = -1 + 85

Half - Life• Half-life: time required for half the atoms of a

radioactive isotope to decay• Some isotopes have long half-lives, some are

short.• Ex. U-238 has a half life of 4.46 x 109 years and

Co-60 has a half life of 10.5 minutes • After 1 decay, half of the isotope’s atoms are still

radioactive and half are now stable (or less radioactive and not the same isotope at the start)

How to solve Half-life problems

To calculate the number of decays use:

total decay time

half-life

To calculate the half-life use:

total decay time

number of decays

Set up your grid. The number of columns changes from problem to problem

Decays 0

Half –life or time passed

0

% or amount left

Put your starting amount here

With each C-14 half-life that passes, (5760 years) the % goes down by ½, and the years go up by 5760

Decays 0 1 2 3 4 5Years passed

0 5760 11520 17280 23040 28800

% radioactive

100 50 25 12.5 6.25 3.125

Half-Life Practice Problem

Radioactive Oxygen-19 has a half-life of 27 seconds. If a laboratory has a sample of 40 atoms of Oxygen-19, how many does it have left after 108 seconds?

Step 1: Calculate the number of decays (columns) to put in your grid

Total time = 108 seconds = 4 decays

half-life 27 seconds

Step 2: Set up your grid with 4 columns and fill in the boxes.

Decays 0 1 2 3 4

Half-Life Time passed

seconds

0 54 81 108 135

Amount of

atoms40 20 10 5 2.50

Areas of Guided Notes Left Blank Earlier in Presentation• 5B Complete the VALENCE SHELL diagrams for each

of the families of Periodic Table.

• When the elements combine they can either share or exchange valence electrons, to reach the perfect number, or octet (8) just like Family Noble Gases the family that already has a complete valence shell

• 26. The electron cloud changes sizes when the electrons are lost or gained. When this happens, the atom is no longer NEUTRAL and is called an ION.

• 27. Ions form when atoms want to have a complete valence shell. A valence shell is complete when it has 8 electrons, or an octet. Family 8 (Noble Gases) is the only family on the periodic table with a complete valance shell

• 33. The charge of an ion is always shown in the exponent to the right of the ion’s symbol.

34. METALSform cations (+ ions) and are found on the LEFT side of the periodic table.

35. In general, the LARGER the atom and its cloud, the easier it is for it to lose electrons and form a cation (+ ion).

36. NON METALS form anions (- ions) and are found on the RIGHT side of the periodic table.

37. In general, the SMALLER the atom and its cloud the easier it is for the cloud to gain electrons and form and anion (- ion).

38.Family 8 (Noble Gases) doesn’t ever form ions because it already possesses a full valence shell or 8 valence electrons. This means this family never forms compounds.

Eample: Radioactive Radon 222 releases alpha radiation

Rn He + Po

H + H He + n

Number of Decays

0 1 2 3 4 5 6

Half Life 0 8 minutes 16 24 32 40 48

% Remaining 100 50 25 12.5 6.25 3.125 1.5625

Number of Decays

0 1 2 3 4

Half Life 0 4.7 years 9.4 14.1 18.8

% Remaining 100 50 25 12.5 6.25