BONDS - 01

OPENING QUESTION: How do atoms come together to form chemical bonds?

OBJECTIVE:  I will be able to sketch how positive and negative ions form ionic compounds after today's class.

WORD O' THE DAY:

    1. ionic bond ("Positive/Negative ions")
    2. cation ("positive ion")
    3. anion ("negative ion")

WORK O' THE DAY:

Let's get out our periodic tables and look once again at the charges that each atom tends to want to make.

Recall that there are obvious and important trends as we move across the period table:

Column 1 & 2 atoms -- Give up electrons very, very easily to form positive ions (cations)

Column 16 (at least those to the right of the STAIRS) & 17 atoms -- Want to steal electrons to form negative ions (anions)

Column 18 atoms are Couch Potatoes--- They don't want to play with anyone. They don't react with anyone. They are stable with full outer shells.

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Let's take a moment to see how ionic compounds are made.

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A neutral atom that loses one or more electrons is called a cation.

Cations are very simple to name: We use the name of the atom and add the word ion.

Therefore when sodium loses an electron out of its outermost shell we say that it is a +1 ion and we name it "sodium ion"

When magnesium loses 2 electrons from its outermost shell we say that it is a +2 ion and we name it "magnesium ion"

Let's spend a bit of time discussing *why* an atom might want to lose an electron and why another atom might want to gain an electron (This is KEY)

There are two general concepts that help us understand *how* an atom gains or loses an electron (HOLD ON TO YOUR HATS...this is a bit advanced!):

  1. Valence electron distance to the nucleus:

    • The farther the valence electrons are from the nucleus, the less force they feel from the positively pulling protons in the nucleus.

    • The less positively pulling proton force those valence electons feel, the more vulnerable they are.

    • The more vulnerable those electrons are, the easier it is to lose them.

    • The easier it is to lose an electron, the more likely that electron will be lost and the atom will form a positive ion.

  2. Number of positively pulling protons in the nucleus:

    • The more positively pulling protons there are in the nucleus, the more force is exerted pulling the valence electrons towards the nucleus.

    • The more positively pulling proton force exerted by protons on electrons, the more likely those atoms are to reach out and snag an electron to form a negative ion.

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A neutral atom that gains one or more electrons is called an anion.

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So -- atoms to the left of the stairs want to form positive ions (cations). Atoms to the right of the stairs want to form negative ions (anions).  So it follows that positive ions and negative ions just might want to get together.... and they DO.

For example:

Na + Cl→ NaCl

Now here's where it gets *REALLY* interesting. Ionic compounds like our good friend NaCl are characterized by a fairly large difference in their electronegativities... That means that they act somewhat like magnets with North (positive) and South (negative) attractions. The bigger the difference between the electronegativities, the STRONGER the attraction.

Now..... recall that when we have sodium and chlorine combining to form NaCl, it almost never happens between just 2 atoms. There are trillions x trillions of sodium atoms combining with chlorine atoms to form a small teaspoon full of table salt...

THAT means there is an awful lot of positively pulling and negatively pulling charge present.... so....

WE DON'T GET A SINGLE NaCl forming...

The positively pulling atoms attract nearby negative atoms and vicey versa.. and we get...

THIS:

Work with your group to DESIGN AND BUILD a simplified model of a sodium chloride crystal and we'll discuss further

 

CLOSING QUESTION: Exit ticket: write the chemical formula scandium chloride