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The Modern View Of Atomic Structure: An Introduction
The way to imagine an atom:
- It contains a tiny nucleus that itself contains all of the positive charge and the vast majority of the mass of the atom. All common nuclei are on the order of 10 -12^ cm.
- The electrons which move around the nucleus at a distance of ~10- cm contain all of the negative charge and a small amount of mass. The chemistry mainly results from the electrons! Nucleus contains:
1) Protons – Have a positive charge (equal but opposite in
magnitude to the charge of an electron ( e
- )).
2) Neutrons – Have no charge ( neutral ) but are almost the same
mass as a proton. (no) (nuclear glue) See table 2.2 **
About the nucleus: It is very dense! If you have a solid particle that was made up of just nucleus (with no electrons) the size of a pea, it would have a mass of 250,000,000 tons! 1 ton = 2000 lbs. It is very small as compared to the whole atom. Remember the previous numbers. So you may be asking if this is how simple things are then why all of the different molecules, chemistry, physical properties, etc? The difference comes from the e-. Most of the volume of the atoms come from the electron cloud and therefore this is the part that “bumps” into or “touches” the nearest neighbor atoms. But do not discount the importance of the nucleus, for one reason: because it is the why there is a given number of e- ‘s in the atomic cloud. Atoms of different elements have different chemical behavior. Another way to say this is: atoms with different numbers of protons and electrons have different chemical properties.
Atoms – have no net charge. Therefore for a given atom the number of
protons must equal the number of electrons.
Elements – whether charged or not are always identified by the number
of protons! An Example:
Calcium
Atoms of Calcium (Ca) have 20 protons ( p + ) in the nucleus, it must therefore have 20 electrons around the nucleus. Calcium atoms always have 20 p + and 20 e
- in the atom. There are also
neutrons in the nucleus. But the number of neutrons in the
nucleus of calcium atoms can differ!
Examples:
CO 2 , SiO 2 , CH 4 , C 2 H 6 , O 2 , H 2 O, NH 3 , P 2 O 5 , etc.
b) Structural formula – Individual bonds between atoms are shown (as a line) The structural formula does not necessarily indicate the shape of the molecule but it may. Each line that represents a bond also represents the two electrons that make up that bond. The line can be a solid, dashed or wedged line! Examples: N H H H H O H C C H H H H H H
c) Ball-and-stick model – shows the atoms and their relative
orientation to one another in the molecule
d) Space filling model – shows the relative size of atoms and
their relative orientation in a molecule.
- Ionic bond is the second type of chemical bond that we will discuss.
Ion – An atom or group of atoms that has a net positive or negative
charge. Examples: If you lose one electron from a potassium atom then you get:
K → K
+
+ e
-
So there is now 19 protons but only 18 electrons, this leads to a net positive charge of 1. This is now called a potassium ion (or potassium cation).
Cation – Positively charged ion.
Anion – Negatively charged ion.
Example of an anion:
Cl + e
-
→ Cl
- Called a chloride ion (or chloride anion).
Notice the location for these elements on the periodic
table!
Ionic Bonding – Forces of attraction between oppositely charged ions.
These forces are usually much stronger than a covalent bond. Example:
2 Na(s) + Cl2(g) → 2 NaCl(s)
Sodium (Na) metal is a soft metallic solid. Chlorine (Cl) is a greenish gas. Sodium chloride is a clear crystalline ionic solid.
Ionic solid (salt) – solid consisting of oppositely charged ions.
They can be simple ions like:
Cl
-
, Br
-
, Na
+
and K
+ Names? or, The can be polyatomic (meaning many atoms) ions like:
