1. The Atom
Chapter 25

2. The Atom
The Atom is the smallest part of an element that still has the properties of that element.
An element is a substance that cannot be broken down into simpler substances by chemical means.

3. What are Atoms made of?
Atoms are made up of smaller particles called sub atomic particles.
There are 3 types of sub atomic particle:
Protons
Neutrons
Electrons

4. Structure of the Atom
Protons and Neutrons are found in the centre of the atom called the nucleus.
Electrons are found orbiting the nucleus in shells/orbits.

5. The Bohr Model

6. Structure of the Atom Part Mass Charge Location Proton 1 a.m.u. +1
In the nucleus
Neutron
Electron
1/1840 a.m.u. -1
Orbiting the nucleus

7. Chemical Symbols
Atomic Number
Symbol
Mass Number

8. Atomic Number
The atomic number of an atom is the number of protons balanced by an equal number of electrons.
(The small number)
Atomic Number

9. Mass Number
The mass number tells the number of protons and neutrons in the nucleus.
(The big number)
Mass Number = No of Protons + No of Neutrons
Mass Number

10. Arrangement of Electrons
n=1 1st shell holds 2e-
n=2 2nd shell holds 8e-
n=3 3rd shell holds 8e-

11. Periodic Table Horizontal lines are called periods.
The period tells us the number of shells/orbits in an atom.
Vertical lines are called groups.
Group number tells us how many electrons are in an atoms outermost shell.

12. Isotopes
Isotopes are atoms of the same element, which have different numbers of neutrons.
Same atomic number different mass number.
i.e. Carbon 12, carbon 13 and carbon 14.