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Effective Nuclear Charge and Influence on Atomic Radius

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by

Ryan Correa

on 10 February 2014

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Transcript of Effective Nuclear Charge and Influence on Atomic Radius

Effective Nuclear Charge (Zeff)
- Nucleus is positive and electrons are negative
- The nucleus attracts the electrons which thus creates a pull or a force
- The core electrons moving in between the valence electrons and nucleus reduce that pull or force
- The farther away the shell the more energy the electrons have
- That pull between the nucleus and electrons with the disrupting core electrons is known as the effective nuclear charge


Core Electrons

- Not a part of the valence shell
- Not involved in the bonding of atoms
- When finding the noble gas configuration of an element, the noble gas that you end up with make up the core electrons in your element
Lithium(Li) has a noble gas configuration of [He] 2s1 which means if you took away the 1 valence electron on the 2s orbital, all that would be left in the Li electron configuration is the make up of the Noble Gas He

- Core electrons are tightly bound to the nucleus

Effective Nuclear Charge and Influence on Atomic Radius
By: Robby Smithline, Paige
Deschapelles, Ryan Correa,
Nick Couch and Tom Bewkes
The Shielding Effect

- Describes the balance between the pull of the protons on (valence) electrons and the repulsion forces from inner electrons
- Electrons can shield each other from the pull of the nucleus
- Describes the decrease in attraction between an electron and the nucleus in any atom with more than one electron shell
- More electron shells greater shielding effect


- The process in which core/shielding electrons block the valence electrons from the charge created by the nucleus
Definition of Atomic Radius

Atomic Radius has two separate definitions
1. The measure from the nucleus of an atom to the valence electrons in the last orbital
2. The distance between two bonded nuclei
No specific size -- differs between all atoms
Affected by how positively charged the nucleus is
Stronger the force the smaller the atom
Weaker the force the larger the atom




Zeff Influencing Atomic Radius
The magnitude of the attraction between the electron and the nucleus is proportional to the magnitude of the charge of the nucleus
Electrons that lie between the nucleus and outer electrons are said to shield/screen resulting in a less forceful charge
The farther away the electrons shells are the less of a pull the electrons face resulting in a larger atomic radius

What do we need this equation for?
- To determine how positively charged the nucleus is resulting in how large or small the atomic radius is
Equation: (Zeff)=Z-S
-Z=Number of protons (Atomic number)
-S=Number of core/shielding electrons
-All electrons but the valence (outermost) electrons

Example Problem:
Iron (Fe): 26th element
Step One:
find Number of Protons in Iron (atomic Number) which is 26
Step Two:
do the electron configuration of Iron (1s22s2 2p63s23p64s23d6)
Step Three:
Find the amount of valence electrons (In this example there are 2 valence electrons )
Step Four:
take those 2 total valence electrons and subtract it from the atomic number (in this case 26) so it is 26-2=24
- This step tells us our “S” in the equation (24)
Step Five:
Plug in all the variables (Z=26, S=24) to the equation, Zeff=Z-S, so Zeff=26-24
Step Six:
Zeff of Fe=2

RS
Finding Zeff
Bibliography
Practice Problems
Trends on the Periodic Table
Periods
Atomic Radius
decreases
when moving left to right
When moving across a period the elements add protons and electrons -- creates more of a pull towards the nucleus
Energy of the Atom remains neutral
Groups
Atomic Radius
increases
when moving down a group
Electrons shells get added to the elements so electrons are farther away from the nucleus/more energy
Shielding Electrons
Find the Zeff of Chromium (Symbol:Cr/ #24 on the periodic table)
Barrier created by inner electrons and shells that are closest to the nucleus
Reduces the pull on valence electrons
Think of it as hearing - the farther away you get from the noise the less you can hear, but the closer you are the louder it is

Shielding electrons and core electrons are the same!
Find the Zeff of Tungsten (Symbol: W/ #74 on the periodic table)
Find the Zeff of Lead ( Symbol: Pb/ #82 on the periodic table)
RS
RC
NC
TB
PD
NC
PD

"Atomic Radii." - Chemwiki. N.p., n.d. Web. 29 Jan. 2014. <http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Table_of_the_Elements/Atomic_Radii>.

"Atomic Radii." - Chemwiki. N.p., n.d. Web. 29 Jan. 2014. <http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Table_of_the_Elements/Atomic_Radii>.

"Atomic radius." Princeton University. N.p., n.d. Web. 29 Jan. 2014. <http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Atomic_radius.html>.

"Atomic radius." Princeton University. N.p., n.d. Web. 29 Jan. 2014. <http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Atomic_radius.html>.

"Electron Configurations continued." Electron Configurations continued. N.p., n.d. Web. 30 Jan. 2013. <http://www.csus.edu/indiv/m/mackj/eit/elcconfig.PDF>.

"Shielding vs. Deshielding." Shielding vs. Deshielding. N.p., n.d. Web. 30 Jan. 2013. <http://www.chem.ucla.edu/harding/ec_tutorials/tutorial51.pdf>.

"Size of Atoms." Size of Atoms. N.p., n.d. Web. 30 Jan. 2014. <http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch7/size.html#cov>.

"Size of Atoms." Size of Atoms. N.p., n.d. Web. 30 Jan. 2014. <http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch7/size.html#cov>.

"The Shielding Effect and Effective Nuclear Charge." The Shielding Effect and Effective Nuclear Charge. N.p., n.d. Web. 30 Jan. 2013. <https://www.boundless.com/che mistry/periodic-properties/electron-configuration/the-shielding-effect-and-effective-nuclear-charge/>.

"Valence and Core Electrons." - Chemwiki. N.p., n.d. Web. 30 Jan. 2014. <http://chemwiki.ucdavis.edu/Wikitexts/UC_Davis/UCD_Chem_124A%3A_Kauzlarich/ChemWiki_Module_Topics/Valence_and_Core_Electrons>.

"atomic and ionic radius." atomic and ionic radius. N.p., n.d. Web. 27 Jan. 2014. <http://www.chemguide.co.uk/atoms/properties/atradius.html>.

"atomic and ionic radius." atomic and ionic radius. N.p., n.d. Web. 27 Jan. 2014. <http://www.chemguide.co.uk/atoms/properties/atradius.html>.

"atomic and ionic radius." atomic and ionic radius. N.p., n.d. Web. 27 Jan. 2014. <http://www.chemguide.co.uk/atoms/properties/atradius.html>.


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