Chapter 2 - The Chemistry of Life

The matter that makes up every object consists of one or more elements that organize into atoms and molecules.

Element - a substance that cannot be broken down by chemical means into other substances

Living things are mostly composed of:

* carbon
* hydrogen
* oxygen
* nitrogen
* phosphorus
* sulfur

## Periodic Table lists all known elements

Each box shows:

* the elements full name
* the elements one or two letter symbol
* atomic number
* atomic weight
* mass - things inside
* weight - gravitational pull

## Atoms make up all matter

Atom - smallest piece of an element that retains the characteristics of the element

* composed of protons, neutrons, and electrons
* electrons - surround the atomic nucleus
* protons and neutrons - in atomic nucleus
* protons - positively charged (+), mass 1
* neutrons - neutral (0) mass 1
* electrons - negatively charged (-), mass 0
* Atomic number - number of protons
* Atom mass number - number of protons and neutrons
* Ion - charged atom
* Atoms do not have charge

## Isotopes are different forms of the same element

The number of neutrons may vary which causes each isotope to have different masses.

The atomic weight on the periodic table is the average mass

## Chemical Bonds link atoms together

Atoms are organized into molecules

Chemical bonds are determined by electrons

* The number and distribution of electrons around an atom determines whether how the atom will react with other atoms

Energy shells, orbitals, contain the atoms electrons

* The shell farthest from the nucleus is the most important for bonding
* Electrons are arranged in pairs on the shells
* Unpaired electrons form bonds with other atoms
* Atoms are more stable when their outer shells have no vacancies
* The more vacancies, the more likely they are to bond with another atom
* Covalent Bonds - when atoms share electrons
* Double bonds - share 4 electrons between atoms
* Noble gases - full and do not bond

## Electronegativity measures an atom’s ability to attract electrons


* Differences determine chemical bonds
* The periodic table arranges atoms by electronegativity
* When atoms have similar electronegativity, neither will pull electrons more strongly than the other
* Nonpolar covalent bonds form during this

## Some atoms have a partial charge

Electrons spend more time - slightly negative charge (atom)

Electrons spend less time - slightly positive charge (atom)

This charge difference gives the bonds polarity

Different electronegativity - polar covalent bonds

## Polarity in water molecules creates hydrogen bonds

Slightly positive charge on the hydrogen atom attracts the negatively charge of a neighboring water molecule

Hydrogen bonds pull water molecules close to each other

* This gives water unique properties
* This is important to DNA structures and proteins

## Very high electronegativity forms ionic bonds

Some bonds have such different electronegativities that one atom completely pulls an electron away from each other

* electrons can completely transfer over to highly electronegative atoms
* atom that loses an electron - positively charged
* atom that gains - negatively charged
* charge difference attracts the atoms to each other, forming ionic bonds

In ionic bonds, both atoms get full outer shells, so both atoms become stable

| Type | Chemical Basis | Strength | Example |
|----|----|----|----|
| Ionic Bonds | Atoms with electronegative differences (large >1.7); one atom takes an electron from another. Opposingly charged ions attract each other | Strong but breaks in water | Sodium Chloride (NaCl) |
| Covalent Bond | Two atoms share a pair of electrons | Strong | |
| nonpolar | Electronegativity differences are small (