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CHEMISTRY LECTURE SLIDESAn element is a substance consisting of one type of atom.
What is an atom?
Fundamental particle
Smallest particle which retains the chemical properties of an element. There are (currently) 118 different types of
atoms. Each of these corresponds to an element in the periodic table.
Single type of atom = chemical element
92 are found naturally on earth.
An atom is the smallest particle that has an independent existence.
An atom is the smallest unit that exhibits the chemical properties of the element. Smaller subatomic particles are not
able to exist on their own.
Atoms consist of a nucleus orbited by electrons. Three basic subatomic particles make up an atom:
▪ Protons (positive charge)
▪ Neutrons (neutral)
▪ Electrons (negative charge)
Charge is a fundamental property of matter, it can be either positive, negative or neutral. The basic rule is “like
charges repel, unlike charges attract”.
➢ Chemical elements are materials made of one sort of atom.
➢ An element is defined by the number of protons it has in the nucleus.
➢ The number of protons = the atomic number
➢ An element has equal number of protons and electrons.
➢ Neutrons add weight but do not change the element.
➢ Mass number = number of protons + number of neutrons
An element is defined by the number of protons in the nucleus.
Atomic mass is the number of protons plus the number of neutrons.
Isotopes are the same element (same number of protons) BUT a different number of electrons.
A molecule is made up of atoms which are chemically bonded together. The atoms can be of the same element or
different elements.
Hydrogen, H2, Oxygen, O2 Nitrogen, N2 Chlorine, Cl2 Sulphur, S8
A compound is a substance formed when two or more different chemical elements are chemically bonded together.
For example:
(Ethanol) (water) (carbon dioxide)
, • In chemistry, a mixture is a material system made up of two or more different substances which are mixed
but are not combined chemically.
A mixture can be separated by physical methods, a compound cannot.
Examples: solutions or suspensions
▪ Salt and sand, Vodka and water, Tea with sugar, Salad dressing, Paint
MOLE AND RMM:
One mole of a compound contains 6.02 x 1023 molecules of that compound. This number is known as Avogadro’s
number. One mole of a compound weighs the molecular mass in grams. For water, we know that the molecular mass
is 18. Therefore 1 mole of water weighs 18g
Electronic Structure Simplest model: electron shells: 2, 8, 8 rule.
1st shell – 2
2nd shell – 8
3rd shell – 8
In chemistry only the outer electrons are involved in forming chemical bonds. These outer electrons are called
valence electrons.
For example:
Lithium 3 = 2,1 i.e., 1 valence electron
The shells in fact consist of different shaped orbitals each containing two electrons. There are 4 different orbital
shapes – s, p, d and f.
The S orbital is spherical. Each s-orbital is spherical and can hold 2 electrons. The s-orbital in each band can hold 2
electrons. The orbital gets larger with each period.
The p orbital consists of three sets of two lobes. Each set of lobes sits on a different axis. Each can hold 2 electrons. So,
the p-orbitals in each band can hold 6 electrons.
There are five different d-orbitals. Each can hold 2 electrons. The d-orbital in each period can hold up to 10 electrons.
➢ Areas of space around the nucleus where there is a high probability of finding an electron. Each orbital can
hold two electrons. Each orbital has a name and a shape. Orbitals: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s…
Rules For Filling Orbitals:
Rule 1 – Orbitals must be filled in order, and lowest energy orbitals are filled first. Thus, the filling pattern is 1s, 2s, 2p,
3s, 3p, 4s, 3d, etc. (Aufbau or building up principle)
Rule 2 - Only two electrons are permitted per orbital. This is known as the Pauli Exclusion Principle.
Rule 3- Hund's Rule - Every orbital in a sublevel (e.g., 2p) is singly occupied before any orbital is doubly occupied.
Ionic bonds (swapping electrons):
, In ionic bonds, atoms loss or gain electrons, when this happens atoms form ions. They can be either positive (cations)
or negative (anions)
For example:
Ionic compounds are called ionic compounds because they contain IONS. Ions are atoms which have lost or gained
electrons and so have a charge. This charge can be positive or negative (+ve cations, -ve anions)
Properties of ionic compounds
❖ Normally formed between metals and non-metals
❖ In solid form exist as large crystalline structures (e.g., salt, calcium carbonate…)
❖ High melting and boiling points because of the strong bonds
❖ Dissolve in water to form ionic solutions.
❖ Ions transfer electricity – ionic compounds are good conductors of electricity and heat when in solution or
molten form.
Examples of anions
Sulphate (SO42-), Carbonate (CO32-), Nitrate (NO3+), Phosphate (PO43-), Hydroxide (OH-), Cyanide (CN-)
Note:
Ionic compounds are neutral, the overall charge must be zero.
Metallic bonding:
Metals have a different form of bonding to ionic compounds. Atoms of metals which are close together share their
electrons en masse… Transition metals tend to have particularly strong metallic bonds. Delocalisation is from 3d
electrons in the delocalisation as well as the 4s. The more electrons involved, the stronger the metallic bonds.
Covalent bonding – sharing electrons:
If an element needs to lose or gain more than 3 electrons to achieve a full shell, then they do not normally form ionic
compounds. Why? It is energetically unstable – a charge of 4+ or 4- is not.
Only up to 3 electrons can transfer to form ionic bonds. Elements in the centre of the periodic table form bonds by
sharing
Examples of covalent bonding:
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