Atoms, elements & compounds |

Atomic Structure and the Periodic Table

Atomic structure

Atoms are made up of sub-atomic – protons, neutrons and electrons
protons have a positive charge, neutrons has neutral charge (same mass as protons) and electrons are negatively charged (same as no. of protons

Particle	Relative charge	Mass (atomic mass)
Proton	+1	1
Neutron	0	1
Electron	-1	1/18371

Proton number: number of protons in an atom [atomic number]
Nucleon number: total number of protons and neutrons in the nucleus of an atom [mass number]

Definition of terms

Atom is the unit particle of an element
Element is a substance that consists of atoms all with the same proton number.
Proton no. is number of protons in an atom (and no. of electrons in an atom). in the periodic table, proton number increases with group number
Nucleon no. is number of protons + neutrons in an atom
Isotopes are atoms of same element with different number of neutrons eg chlorine 35 and 37

Electronic Configuration

Electrons are arranged in electron shells.
Electron shell structure: 2, 8, 18.
Atoms want to have full outer shells (full set of valency electrons), this is why they react.
Noble gases have full outer shells so they have no need to react.

Structure of the Periodic Table

The number of protons in each element increases by 1 across each row
Period number is the number of occupied shells in an element

Group number is the number of electrons in the outermost shell

Isotopes

atoms of the same element which have the same protons number, but a different nucleon number. They have the same chemical properties due to same number of outermost shell electrons.

E.g. Carbon 12 and Carbon 14.
Two types: Non-radioactive isotopes and radioactive-isotopes (unstable atoms that break down and produce radiation)
Medical use: cancer treatment (radiotherapy) – rays kill cancer cells using cobalt-6
Industrial use: to check for leaks – radioisotopes (tracers) added to oil/gas. At leaks radiation is detected using a Geiger counter.

Bonding

Terms to be familiar with

Element: Pure substance consisting of one type of atom
Mixture: two or more elements mixed together but not chemically combined
Compound: substance in which two or more different elements are chemically combined
Alloy: Mixture of two or more elements in which at least one is a metal, eg. brass (copper and zinc)

Elements, Compounds and Mixtures

An element is a substance that cannot be broken down into simpler substances through any chemical or physical means. Elements can exist as atoms or molecules. Each molecule of an element can consist of two or more atoms that are chemically combined.

A compound is a substance that contains two or more elements which are chemically combined in a fixed ratio. It can consist of either molecules or ions. The properties of a compound differ from its constituent elements.

A mixture consists of two or more substances that are mixed together. These substances can be elements or compounds. The ratio of these substances in a mixture is not fixed. The components in a mixture can easily be separated through physical methods.

Metals	Non-metals
Good conductors of heat & electricity	Poor conductors of heat & electricity (except graphite)
High m.p. and b.p.	Lower m.p. and b.p. than metals
High density	Low density
Forms basic oxides	Forms acidic oxides
Forms cations in reactions	Forms anions in reactions
Malleable and ductile	Not malleable or ductile

Ions and Ionic Bonds

Ionic bonding: Electrostatic force of attraction between a lattice of alternating positive and negative ions

Chemical bonds are formed by transfer of electrons from one atom to another
Metals lose electrons to form cations; non-metals gain electrons to form anions
Positive cations & negative anions attract to each other
Strong electrostatic force of attraction between positive cations and negative anions is called ionic bonding

Ionic bonding between Group I metal and Group VII non-metal

Property	Reason
Form giant lattice	Cations and anions attract
High m.p. and b.p.	Strong bonds between ions
Don’t conduct electricity when solid	Ions can’t move
Conduct electricity when molten/aqueous	Ions can move
Usually soluble in water	Not required

Molecules and Covalent Bonds

Covalent bonding: When atoms share electrons to obtain a full outer shell electron configuration; only between non-metals.

Single Bond	Double Bond	Triple Bond

2ē shared(1 from each atom)	4ēs shared(2 from each atom)	6ēs shared(3 from each atom)

Examples:


HCl	CO₂	N₂

Covalent bonding	Ionic bonding
Mostly volatile	Mostly non-volatile
Insoluble in water	Soluble in water
Poor electrical conductors	Good electrical conductors

Covalent bonds, due to the sharing of electrons between atoms, have weaker attractive forces than ionic bonds. Thus they have lower melting and boiling points.

Macromolecules

Diamond	Graphite	Silicon Dioxide

1 carbon atom bonded to 4 carbon atoms [tetrahedral structure]	1 carbon atom bonded to 3 carbon atoms [hexagonal layers]	Each silicon is bonded to 4 oxygen atoms, and each oxygen is bonded to 2 silicon atoms [tetrahedral structure]
High m.p. and b.p	High m.p. and b.p	High m.p. and b.p
no free electrons	Conducts electricity (free electrons)	no free electrons
Used for cutting as it is strongest known substance	Used in pencil lead and as a lubricant	Used in production of glass