What are Chemical bonds and Molecules
- If two neutral atoms bond to each other (share electrons) the bond is called a covalent bond. A covalent bond may have 2, 4 or 6 electrons, with each atom contributing 1, 2 or 3 electrons respectively.
- If two charged atoms bond to each other (one positively charged and one negatively) the bond between them is called an ionic bond. In this bond, one atom assumes a net negative charge, one a net positive charge and they are held together by the attractive forces of the charges (much as two magnets will attract each other if you put the + end of one and the – end of the other close to each other).
- A group of two or more atoms, bonded together is called a molecule. Molecules may also be referred to as compounds.
- An element is any chemical that is comprised of only one type of atom. This may be single atoms, or molecules. The oxygen we breathe is an element, and exists as two covalently bound oxygen atoms.
- In larger molecules some sections may have an overall charge, while other sections are neutral. This is called polarity.
- Bonds between two atoms or ions may be stronger or weaker depending on how much charge (or how many electrons) are involved.
The Periodic Table
The periodic table lists all the currently known elements. It also lists them in a precise order, which allows us to get additional information at a glance. Elements are listed from the top left to the bottom right in order of increasing atomic number. The atomic number tells us the number of protons in the atoms making up the element. While atoms can lose electrons, they do not lose protons, so an atom of oxygen always has the same number of protons, whether it is in a compound, as a diatomic element (O2) or carrying a charge. The atomic numbers are for the individual atoms, not the overall element.
The columns of the table are called groups. The group of an element gives us information about the electrons and how readily an element will donate or accept electrons. The table also includes the atomic mass of the elements.
Parts of a Molecule
Atoms group together in small OR very large numbers to make chemical compounds.
There are however sub groups within compounds. Take water for example:
Water is H2O and by its correct chemical name is hydrogen dioxide. (di = 2). So, there are two atoms of hydrogen and one of water (so it is NOT an element). Looking at the periodic table we find that hydrogen has only one electron (we can tell this from the atomic number – hydrogen’s atomic number is 1, that means it has one proton, to be neutral it has to have one electron). Oxygen on the other hand, has 8 electrons. So, it can only ever be bound to one other atom.
This means that the chemical configuration of water is:
H – O – H (with oxygen bound two both the hydrogen atoms)
And not, for example:
H – H – O
Using Lewis dot drawings we can better visualise the bonds:
H : O : H (Each atom contributes 1 electron to a bond, making both bonds single)
In actual fact, one of these two bonds tends to be stronger than the other, and the molecule breaks up producing a hydrogen ion with a positive charge (H+, also referred to as a proton as it is one proton with no orbiting electrons), and a hydroxide ion (ie. OH-). This is the basis of acidity and alkalinity. In pure water, there are as many OH- ions as H+ and the resulting solution is neutral. If there are more H+ ions in a solution the solution will be acidic (pH will be less than 7) and if there are more OH- ions the solution will be alkaline (or basic) and the pH will be more than 7.
When a molecule breaks up like this, but there is still some attraction between the atoms or molecules that made it up, it is called dissociation. Why does water dissociate? Because the bonds formed between the hydrogen atoms and the oxygen atom because the electrons aren’t shared 50/50, with the large oxygen atom taking more than its share. This means each hydrogen atom has a partial positive charge and the oxygen atom has a partial negative charge. This makes the individual water molecules attract each other like little magnets. In this situation, the bonds are neither covalent nor ionic, but are called hydrogen bonds and they are much less stable, and therefore weaker than the other bond types.
Water is made up of a hydroxide ion where the bond between the hydrogen and oxygen is very strong, AND a hydrogen ion joined to the hydroxide by a weaker and more easily disrupted bond.
Similarly, other chemicals, even very complex chemicals are made up of various groups (within which the chemical bonds are very strong), joined together by bonds which are weaker.
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