Structure of the Atoms Different elements have different numbers of protons, neutrons and electrons. Each element has
Different Sorts of Atoms
Different elements have different numbers of protons, neutrons and electrons.
Each element has its own atomic number and mass number.
Atomic Number
Atomic number = number of protons in an atom.
In a neutral atom: Number of protons = Number of electrons
Mass Number
Mass number = protons + neutrons
Formula
Neutrons = Mass Number − Atomic Number
Example: Lithium
Atomic number = 3
Protons = 3
Electrons = 3
Mass number = 7
Neutrons = 7 − 3 = 4
Arranging Electrons
Electrons move around the nucleus in shells.
The shell closest to the nucleus fills first.
A shell must be filled before electrons move to the next shell.
Electron Shell Capacity
1st shell = 2 electrons
2nd shell = 8 electrons
3rd shell = 8 electrons (simple model)
Examples
Beryllium
4 electrons
Arrangement: 2,2
Neon
10 electrons
Arrangement: 2,8
Carbon
6 electrons
Arrangement: 2,4
Bohr Model of the Atom
Proposed by Niels Bohr.
Electrons move around the nucleus in shells.
The nucleus contains protons and neutrons.
Electrons occupy different energy levels around the nucleus.
Key Terms
Proton (+): Positive particle in the nucleus.
Neutron (0): Neutral particle in the nucleus.
Electron (-): Negative particle found in shells around the nucleus.
Nucleus: Dense centre of the atom containing protons and neutrons.