Chemistry Atoms

Matter occupies space and has weight.

It can exist as a solid, liquid, or gas.

It may be possible to break some kinds of matter down into other kinds of matter with different properties. For example, water (H2O) can be broken down into hydrogen and oxygen.

Hydrogen and oxygen in the above example cannot be broken down any further because they are elements.

Elements

Elements cannot be broken down into substances with different properties by chemical reactions. For example, water (H2O) is not an element because it can be broken down into hydrogen (H) and oxygen (O).

Substances that are composed of two or more different elements are called compounds. For example, water is a compound because it is composed of hydrogen and oxygen.

The smallest particles of an element that have the characteristics of that element are atoms.

Elements are substances made up of only one kind of atom.

There are 92 naturally occurring elements. Matter is therefore composed of 92 different kinds of elements.

The following elements make up 96% of the body weight of organisms: Oxygen, Carbon, Hydrogen, Nitrogen.

Atoms

An atom is composed of subatomic particles. Three important kinds of subatomic particles are protons, neutrons, and electrons. Some atoms (ex: hydrogen) do not have neutrons.

Protons and neutrons are located in a central area called the nucleus.

Electrons move about the nucleus. The number of electrons is equal to the number of protons.

It is more accurate to represent the space occupied by electrons as a cloud. The electrons are likely to be located somewhere within the cloud.

Characteristics of Subatomic Particles

The mass of subatomic particles is too small to be conveniently measured in grams so atomic mass units (amu) are used instead. Atomic mass units are also called daltons. One amu (or dalton) is approximately 1.7 X 10-24g.

Protons and neutrons have a mass of approximately 1 amu. The mass of an electron is much less. The total mass of an

atoms

is due mostly to the mass of protons and neutrons.

Mass

charge
protons

approx. 1 amu

+
neutrons

approx. 1 amu

0
electrons

1/1836 amu

-

Charge is a state in which particles are either attracted to each other or they repel each other. Two particles that are attracted to each other have opposite charges (positive and negative). Particles that repel each other have the same charge; they are both either positive or they are both negative.

Protons have a positive charge and electrons have a negative charge. Particles with positive charges are attracted to particles with negative charges. Two particles with the same charge (both positive or both negative) will repel each other.
Atoms are neutral. The number of electrons (negatively charged) is equal to the number of protons (positively charged), therefore the overall charge is zero.

Atomic Mass and Mass Number

The atomic mass is calculated as the sum of the mass of protons, electrons, and neutrons.

Atoms The mass of electrons is small enough that we can generally disregard it in our calculations of atomic mass. The mass number is the number of protons and neutrons.

Most of an atom is empty space.