The Atom
GCSE and Alevel notes for OCR and Edexcel exam boards
The atomic number defines an element's position in the Periodic Table.
There are two particles contained in the nucleus these are the proton and the neutron.
GCSE and Alevel notes for OCR and Edexcel exam boards
The atomic number defines an element's position in the Periodic Table.
There are two particles contained in the nucleus these are the proton and the neutron.
Particles in the nucleus
The Proton
A proton carries a positive charge equal in magnitude and opposite in sign compared to the charge on the electron.
Thus an electronically neutral atom has the same number of protons inside the nucleus as electrons outside.
The Neutron
The mass of the atom is concentrated in the nucleus. The neutrons and protons contain nearly all of an atom's mass.
A neutron has the same mass as a proton but has no charge.
Atomic Number and Mass Number
Atomic Number (Z)
The most important difference between atoms is the number of protons in the nucleus.
This number determines the element to which the atom belongs to.
The Atomic Number Shows:
The number of protons
The number of electrons in a neutral atom
The position of the element in the periodic table
Mass Number (A)
The mass number is the total number of particles in the nucleus.
Thus the mass is the sum of the protons and the neutrons.
Atomic number (Z) = number of protons
Mas umber (A) = number of protons and neutrons
Summary Table
Particle Name | Relative Mass | Relative Charge |
Electron | 1/1846 | -1 |
Proton | 1 | +1 |
Neutron | 1 | 0 |
Isotopes
Discoved in 1913 by Frederick Soddy, isotopes are the same elements with different atomic masses. The word isotope means "equal place" referring to the fact that these atoms are positioned in the same place on the periodc table having the same atomic number but have different masses. In isotopes the number of protons must be he same but the number of neutrons varies.
For example, Hydrogen has three isotopes:
Discoved in 1913 by Frederick Soddy, isotopes are the same elements with different atomic masses. The word isotope means "equal place" referring to the fact that these atoms are positioned in the same place on the periodc table having the same atomic number but have different masses. In isotopes the number of protons must be he same but the number of neutrons varies.
For example, Hydrogen has three isotopes:
Protium | Deuterim | Tritium | |
H1 | H2 | H3 | |
Protons | 1 | 1 | 1 |
Neutrons | 0 | 1 | 2 |
Electrons
The electrons are located on the outer part of the atom. Electrons are the only components involved in chemical reactions. A model is used describes the electrons arranged in different shells. These shells have different energy levels which are occupied by the electrons. Electrons only possess energy in these fixed and stable quantised levels. When an electron would gain or lose energy it would have to move to a fixed level either higher or lower. The energy levels are commonly called shell. The shells are numbered 1,2,3,4,5 and so on. These numbers are called principle quantum numbers and have the symbol n. These numbers corresponds to the periods in the periodic table.
Shell 1 can contain a maximum of 2 electrons
Shell 2 can contain a maximum of 8 electrons
Shell 3 can contain a maximum of 18 electrons
Calculations
Number of protons = Z
Number of neutrons = A – Z
Number of electrons in a neutral atom = Z
Number of electrons in a positive ion = Z – charge on ion
Number of electrons in a negative ion = Z + charge on ion
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