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This is a block of GOLD

Being an ELEMENT, it is a PURE substance, so it contains particles of only ONE kind …

GOLD ATOMS

An atom is the smallest particle of an element that can exist.

Cut a diamond up with a “magic knife” until you have the smallest piece

possible, and you have a ….

CARBON ATOM

Let’s look at the atom in more detail…..

This is an atom of HELIUM.It’s made up of even smaller particles!

In the middle are larger particles in the NUCLEUS

…and there are some tiny particles spinning round it in ORBITS

Let’s “freeze” the action and take a closer look at these sub-atomic particles…

These are PROTONS

These are NEUTRONS

These are ELECTRONS

A PROTON is positively charged, and has the same mass as…

..a NEUTRON, which has no charge.

An ELECTRON is negatively charged, but has a mass less than 1/2000 of that of either of the other particles!

Looking at the Helium atom again, let’s see how many of each sub-atomic particle there are…

2 protons and 2 neutrons make up the nucleus. Together, let’s say they have a mass of 2 + 2 = 4

A helium atom is a typical atom, so we can say the rules are….

Number of protons

+

Number of neutrons

= “Mass number”

Number of protons

(is equal to)

Number of electrons

= “Atomic Number”

…so for Helium, that’s…

Mass number

Atomic number

He4

2

Two protons means 2 positive charges

…and two electrons means 2 negative charges.

…so that’s +2 -2 = 0 , so there’s no overall electric charge on the atom!

Three protons means 3 positive charges

…and 3 electrons means 3 negative charges.

This is Lithium

…so that’s +3 -3 = 0 , so there’s no overall electric charge on the atom!

But sometimes Li loses 1 of its electrons when it bonds…

…so NOW that’s +3 -2 = +1 , so there’s a positive 1 charge overall!

We call atoms with a charge IONS

Let’s look at carbon again…

There are 6 protons, so there must also be 6 electrons.

That makes its atomic number to be 6

6 protons and 6 neutrons means its mass number is 12

C12

6

…so we’ve got…

Time for you to get some practice in!

Fill in the gaps in this table:

Name Symbol Mass No.

Atomic No.

protons neutrons electrons

Hydrogen 1 1

Li 3 4

Potassium 19 20

Beryllium 5 4

Ca 40 20

Iron 26 30

Boron 6 5

Cl 35 17

Ne 10 10

Did you get it right?

Name Symbol Mass No.

Atomic No.

protons neutrons electrons

Hydrogen H 1 1 1 0 1

Lithium Li 7 3 3 4 3

Potassium K 39 19 19 20 19

Beryllium Be 9 4 4 5 4

Calcium Ca 40 20 20 20 20

Iron Fe 56 26 26 30 26

Boron B 11 5 5 6 5

Chlorine Cl 35 17 17 18 17

Neon Ne 20 10 10 10 10

Name Symbol Mass No.

Atomic No.

protons neutrons electrons

Hydrogen 1 1

Li 3 4

Potassium 19 20

Beryllium 5 4

Ca 40 20

Iron 26 30

Boron 6 5

Cl 35 17

Ne 10 10

endA presentation byR. G. SmithOctober 2005