Elements

Chemistry in Biology

An element is a pure substance that cannot

be broken down into other substances by

physical or chemical means.

There are over 100 known elements, 92 of

which occur naturally.

Each element has a unique name and symbol.

These are found on the periodic table.

6.1 Atoms, Elements, and Compounds

Chapter 6

IsotopesChemistry in Biology

Atoms of the same element that have the

same number of protons and electrons but

have a different number of neutrons

The atomic mass on the periodic table is an

average of all the isotopes.

Compounds

Chemistry in Biology

A pure substance formed when two or more

different elements combine

Compounds are always formed from a specific

combination of elements in a fixed ratio.

For example, water is always made from 2

hydrogen atoms and one oxygen atom

6.1 Atoms, Elements, and Compounds

Chapter 6

Valence Electrons

A valence electron is an electron in the outermost

shell/ring.

All atoms want to have a full outer shell.

They will either take, give, or share electrons to get a full

outer shell.

Chemical Bonds

Covalent bonds

A chemical bond that

forms when electrons

are shared

These bonds form

between two non-

metals

A molecule is a compound in which the

atoms are held together by covalent

bonds.

Ionic Bonds

One atom gives electrons to another atom.

This makes one atom positive (more protons)

and one negative (more electrons) so they

attract to each other.

Sometimes in covalent molecules, the

sharing of electrons is not even. One

atom may have the electrons more

often. This creates slight positive and

negative charges within the molecule.

This is called a dipole.

Water is a prime

example. The oxygen is

more electronegative,

meaning it pulls the

electrons towards itself

Hydrogen (polar) bonds

These dipoles allow

molecules to be

attracted to each

other (the positive

attracts to the

negative charge on

another molecule)

These kind of

molecules are called

polar.

Chemistry in Biology

6.3 Water and Solutions

Chapter 6

Van der Waals forces

The electrons in

all molecules are

moving around

So sometimes

when molecules

come close to

each other, a

dipole will be

formed for a brief

period

These temporary

dipoles hold

molecules

together