1. nucleon no.-> no. of protons& neutrons
-> relative atomic mass
2. atomic no.- no. of protons
- no. of electrons
3. ions- formed when electrons are lost or gained
In chemical bonding...
- the formation of ions
- the formation of ionic bonds and covalent bonds
- properties of ionic and covalent compounds
- structures of simple, molecular and giant molecular structures
Structure of noble gases(e.g. He, Ne, Ar, Kr, Xe):
- each atom of a noble gas has 8 valence electrons (except He->2)
- noble gases exist as single atoms
- do not form compounds-> described as unreactive
- *believed to be stable due to their fully filled outer shells
-> no tendency for a noble gas atom to combine with another atom to form molecules or compounds.
THE OCTET RULE: due to *, all atoms without a fully filled outer shell is said to be unstable.
An atom that is unstable would try to gain stability by surrounding itself with an 'octet'(8) of electrons. This tendency is known as the octet rule.
(*O*) Helium:
- only 2 outer electrons
- still considered a stable atom as the outer shell is fully filled.
- first shell can only hold 2 electrons. (The duplet rule)
Valency:
- number of electrons needed/ required to fill up the outer shell.
e.g. Fluorine has 7 outer/ valence electrons-> needs 1 more to achieve stable octet structure.
-> valency of Fluorine is 1.
- also refers to number of electrons stopping it from having a filled outer shell.
Formation of Ions
- an ion is a charged particle formed from an atom or a group of atoms by the loss or gain of electrons.
- metals form positively charged ions-> cations
-> positively charged ions are formed by the loss of electrons.
e.g. lithium ion is formed when a lithium atom loses 1 electron
By losing 1 electron, the lithium ion now has the same electronic
configuration as a helium atom, achieving a duplet structure.
Metals tend to form positive ions because most metal atoms have less than 4 electrons in the outer shell. Hence it is more likely to lose the few electrons than to gain many more to achieve octet structure.
- non-metals form negatively-charged ions-> anions(negatively charged ions are formed by the gain of electrons)
Non-metals tend to form negative ions because most non-metals have more than 4 electrons in the outer shell. Hence, it is more likely for the atoms to gain a few more electrons to complete the octet than to transfer their outer electrons to other atoms.
Some metals form more than one type of ion. e.g. iron-> Fe2+, Fe3+
- charge of ion is often shown in the chemical name. e.g. iron(II) sulphate contains Fe2+ ions
Some positive and negative ions can be formed by a group of atoms-> polyatomic ions.
Ionic bonds
- formed between metals and non-metals.
- involves the formation of both positively-charged ions(cations) and negatively- charged ions(anions).
- involves the transfer of electrons from one atom to another.
E.g. formation of calcium oxide:
calcium loses 2 electrons to Ca2+ (same configuration as Argon)
Then, Oxygen gains 2 electrons to form O2- (same configuration as Ar)
- different symbols used to show that the electrons are from different atoms.
-We can see that an ionic bond is formed when metal atoms transfer their outer electrons to non-metal atoms.
- oppositely charged ions are then formed
- the strong electrostatic force of attraction that holds oppositely charged ions is called ionic bonding.
Metal atoms-> electron donors
Non-metal atoms-> electron receivers
Electron transfer during the formation:
Ionic compounds
- made up of oppositely charged ions, not molecules
- no overall charge in an ionic compound as all the positive charges are balanced by all the negative charges.
Rules when writing the chemical formula of ionic compounds:
- subscripts are in the simplest terms. e.g. MgO not Mg2O2
- subscript '1' is not written
Polyatomic ions-> to be written in brackets if a subscript is added. e.g. Ca(OH)2
The chemical symbols of metals(cations) are to be written first, followed by non-metals (anions).
<-Ionic bond->
(electrostatic force of attraction)
Group 1-> to lose 1 electron
Group 2-> to lose 2 electrons
Group7-> to gain 1 electron
Covalent bonding
- non-metals tend to have 4,5,6 or 7 valence electrons. Therefore, they are unable to form oppositely-charged ions.
- When a covalent bond is formed, each of the bonding atoms contributes a valence electron to form a pair of electrons (one covalent bond).
Examples:
Chemical Bonding
