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Molecules Compounds Bonds

What are molecules and compounds?
How are elements related to molecules? To compounds?
How are molecules, or compounds, held together?

Element: pure substance consisting of only one type of atom.

Metalloid elements

Images from metalloids site:http://geraldrhymes21.weebly.com/

Images from metalloids site:http://geraldrhymes21.weebly.com/

metal elements

Google image search metals site:http://geraldrhymes21.weebly.com/

Google image search
metals site:http://geraldrhymes21.weebly.com/

images of non-metals: http://geraldrhymes21.weebly.com/-non-metals.html

Molecule = 2 or more atoms bonded together


Compound = molecule made of atoms from different elements. text

Therefore: All compounds are molecules, yet not all molecules are compounds.

In the image below we see:
* an atom:
* a molecule (2 different types of atoms, so it’s a compound)
* another molecule (3 different types of atoms, so it’s a compound)
* another molecule (only 1 type of atom, so it’s not a compound)

* a compound

If we have different molecules mixed – but not bonded – then they are a mixture.

{ image from http://chemistry.stackexchange.com/questions/2879/what-is-the-definition-of-of-compound-mixture-element-and-molecule }

atom molecule compound mixture

Hydrogen gas (H2) is a molecule, but not a compound because it is made of only one element.

Water (H2O) is a molecule, and a compound : made of hydrogen (H) and oxygen (O) atoms.

There are two main types of chemical bonds that hold atoms together:

Covalent bonds and ionic bonds.

Atoms that share electrons have covalent bonds.

An oxygen molecule (O2) has a covalent bond.

Atoms that donate electrons to their neighbor, and then hold each other with “static electricity, have ionic bonds.


Ionic bonds occur when electrons are donated from one atom to another.

Table salt (NaCl) is a common example of a compound with an ionic bond.

Links ionic bonds

Metallic bonds occur between metal atoms.

{ http://www.kentchemistry.com/links/bonding/metallic.htm }

A metallic bond is a lattice of positive ions, immersed in a sea of electrons.

The ions are actually entire atoms: nucleus, and many electrons

The ‘sea’ around them are the valence electrons, which are not tightly bound to their home atom, and so become free to flow through the solid metal, as if they are a gas.


Metallic bonds moving GIF

* Malleable
* ductile.
* delocalized electrons in the ‘sea’ of electrons enable the metal atoms to roll over each other,
when a stress is applied.

Metallic bonds moving malleable GIF

Links metallic bonds

Ways of representing a molecule

It should be clear that there is no single “best” way to draw the structure of a molecule; the method you use depends on which aspect of the structure you want to emphasize and how much time and effort you want to spend.

Figure 2.4 “Different Ways of Representing the Structure of a Molecule” shows some of the different ways to portray the structure of a slightly more complex molecule: methanol. These representations differ greatly in their information content.

For example, the molecular formula for methanol, part a, gives only the number of each kind of atom; writing methanol as CH4O tells nothing about its structure.

The structural formula, part b, indicates how the atoms are connected, but it makes methanol look as if it is planar (which it is not).

Both the ball-and-stick model, part (c), and the perspective drawing, part (d), show the three-dimensional structure of the molecule.

The latter (also called a wedge-and-dash representation) is the easiest way to sketch the structure of a molecule in three dimensions. It shows which atoms are above and below the plane of the paper by using wedges and dashes, respectively; the central atom is always assumed to be in the plane of the paper.

The space-filling model, part e, illustrates the approximate relative sizes of the atoms in the molecule, but it does not show the bonds between the atoms. Also, in a space-filling model, atoms at the “front” of the molecule may obscure atoms at the “back.”

Source: “Molecules, Ions, and Chemical Formulas”, chapter 2, Principles of General Chemistry (v. 1.0). Creative Commons licensed, freely downloadable


Online textbook:

Chap 3 Classifying matter, sharing electrons, Chemical Compounds, Dot Structures



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