Of atoms and elements
Over 100 elements Atomic numbers Atomic mass The Periodic Table

As stated in Atoms, every thing is made of atoms - rocks, living things, the Earth, the stars, all that we can see. Now we will investigate the types of atoms that exist and how we describe them.

Over 100 elements
There are over 100 different known atoms and each of these atoms represents an element. For example, hydrogen atoms, with their 1 proton and 1 electron, compose the element hydrogen. The element uranium is made up of atoms with 92 protons and 92 electrons and an average of 146 neutons.

The elements are the macroscopic manifestations of their atoms. When scientists talk about the element sodium, they say it is a soft, reactive metal. When they talk about sodium atoms, they say they have 11 protons, 11 electrons and generally 12 neutrons.Click for larger image

Click for larger imageThe two terms, "elements" and "atoms" are quite often used in place of each other. Remember, "element" is the correct word for the substance and "atom" for the individual particles making up the element.

Atomic numbers
The number of protons in an atom determines its chemical properties and hence which element the atom belongs to. The atomic number of an atom is the number of protons it has and is the best way of cataloguing atoms.

For example: all helium atoms have 2 protons and atomic number 2 and
all iron atoms have 26 protons and atomic number 26

Atomic mass
The second important number to know about is an atom's atomic mass. This is the average mass of an atom of an element. The relative atomic mass of an element is determined by comparing the average mass of the atoms of an element on a scale where hydrogen equals approximately 1 (the real definition is some what more complex as it is now based on Carbon -12 = 12, but the result is the same). On this scale, the average iron atom has a mass of 55.8; that is, the average iron atom is 55.8 times heavier than the average hydrogen atom.

The Periodic Table
Click for larger imageAll the known elements can be listed on a special table called the Periodic Table. The Periodic Table lists all the known elements in order of increasing atomic number and arranges them into groups (vertical columns) and periods (horizontal rows) according to how the electrons are arranged in each atom. It turns out that atoms in the same group have similar arrangements of electrons and similar chemical and physical properties. This can be very handy in understanding the properties of the elements.

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.

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  Matter and light
The atomic and kinetic theories of matter
Atomic theory - early models
Atomic theory - modern models
From atoms to molecules
Making molecules - the rules of covalency
  Question 1
Question 2
Question 3
Question 4