Electrons are present in spherical “shells” round the nucleus. Shell #1 is the tiniest and can accommodate approximately 2 electrons. Shell #2 is bigger and can accommodate approximately 8 electrons. Shell #3 is bigger still and can accommodate around 18 electrons, and so on.
In the shells you can find atomic orbitals. Every orbital can accommodate as much as 2 electrons. S-Orbitals are spheres focused at the nucleus. P-Orbitals are dumbbell shaped having the nucleus in the center. D-Orbitals are more complicated.
Shell 1: one 1s orbital. Shell 2: one 2s orbital, three 2p orbitals – 2p x, 2p y, and 2p z. [Imagine that all 4 orbitals have the nucleus in common because they do. In other words, move the p orbitals so their axes overlap the corresponding axes passing through the s orbital.
Shell 3: one 3s orbital, three 3p orbitals, five 3d orbitals. Electrons will tend to fill orbitals in order of increasing energy:
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s
Describe the structure of an atom, including number of neutrons, protons, electrons, atomic weight and atomic number.
Number of Protons
Number of Neutrons
Number of Electrons
Atomic structure is the basis of Materials Science. This material will form the foundation for knowing the interatomic and intermolecular forces to be covered within the next section.
Atoms consist of nuclei and electrons. Nuclei are composed of protons and neutrons. Protons carry a positive charge of 1.69x10 -19 coulombs and have a mass at rest of 1.67x10 -24 g.
Neutrons have no charge and have a similar rest mass like protons. Overall the nucleus is therefore positively charged. This charge is well balanced by an equal charge because of a number of electrons equivalent to the number of protons (for neutral atoms). Every electron has a charge of –1.69x10 -19 coulombs.
The number of protons in the nucleus is called the atomic number (this defines the elemental identity).
The number of neutrons in a nucleus is larger than or equal to the number of protons, with the larger excess of neutrons occurring for the larger atomic numbers.
Many elements have isotopes, i.e. their nuclei contain an equal number of protons but have different numbers of neutrons; some of these isotopes are stable while others decompose via radioactive decay.
To understand why pure substances have particular compositions and properties we need to know about the "electronic structure" of the atoms (i.e. the way the electrons are arranged about the nucleus of the atoms of different elements). Then we can rationalize the ratio in which atoms combine and whether they are molecular, polymeric or ionic, [e.g. why gaseous nitrogen consists of discrete N 2 molecules; methane, ammonia, water and hydrogen fluoride consist of discrete molecules of CH 4 , NH 3 , H 2 O, HF respectively; why sodium chloride consists of Na + and Cl – ions; why metals exist in nature mainly as cations, M x + ; why free-radicals are reactive.
Electrons and the Structure of Atoms:
The arrangement of electrons around the nucleus of the atom.
The properties of atoms can be understood in terms of Quantum Theory, which involves the Heisenberg Uncertainty Principle and the Schrödinger Wave Equation.
Quantum Theory: A theory that says that the energy of an item can only modify by discrete steps. A change involves a packet of energy known as a quantum.Heisenberg Uncertainty Principle: The momentum and position of any particle cannot both be known at the same time. This indicates that in an atom the momentum and position of an electron cannot both be known at the same time. (Thus a model of an atom that contains electrons in fixed orbits round the nucleus is untenable.)