Chemistry

Basic introduction to atoms, ions and bonding

Atoms

• All matter is made up of tiny particles.

• Atoms are made up of even smaller particles.

• Diagram of an Atom

Electron (-ve charge)

Nucleus of protons (+ve) and neutrons

Atoms

• The number of PROTONS(+) = the number of ELECTRONS(-)

• Therefore, atoms are uncharged overall.

• (Atoms of different elements have different numbers of electrons, protons, and neutrons.)

Ions

• ‘Formed’ as an atom gains or loses an electron.

• Ions may have a positive charge or a negative charge

Ions

ATOM SYMBOL IONS

Sodium Na minus an electron Na+

Chlorine Cl plus an electron Cl-

Hydrogen H minus an electron H+

Carbon C does not form an ion

(Some symbols are from Roman names e.g. Iron = Ferrum = Fe)

Molecules

• An atom of Hydrogen cannot exist on its own

• But a molecule of Hydrogen can exist on its own

• Molecules consist of atoms linked together by chemical bonds caused by the interchange of electrons.

Hydrogen Water Sulphuric acid molecule molecule molecule H2 H2O H2SO4

Atomic Structure

• The nucleus contains Protons (+ve) and Neutrons (neutral)

• Electrons (-ve) occur in “energy levels”

Atomic Structure

Each level can take up to a certain number of electrons

Energy level 1 - max 2 electrons

Energy level 2 - max 8 electrons

Energy level 3 - max 8 electrons

When an energy level is full,

further electrons occupy the

next energy level until that is

filled and so on

Atom with 17 electrons

Atomic Structure Diagrams

• Diagrams are not to scale (If you draw a nucleus of 1 cm diameter, electrons in the first energy level would be 100 metres away from the nucleus)

• Electrons are not in concentric circles, but exist in regions, sometimes called orbitals or shells or clouds of electrons

• Diagrams are useful though to help explain BONDING

Atomic structure diagram for Chlorine

(Do not really need to show protons and neutrons in the nucleus as they do not change)

Atomic structure diagram for Chlorine

An atom of chlorine has 17 electrons altogether

The atomic number = the number of protons present in that atom

Number of PROTONS(+) = Number of ELECTRONS(-)

The mass number = the number of protons plus the number of neutrons

E.g. 3517

Atomic number is 17Mass number is 35

Bonding

• Why do Na and Cl join together?

• Why always one particle of Na to one particle of Cl?

• What holds the different particles together?

Bonding

• Noble gases are stable and unreactive - may be found as “free atoms”

• Noble gases are: Helium (Atomic number 2) or Neon (Atomic number 10).

• Reason for this is Fully Filled Outer Shells

• Atoms of other elements can become stable by Losing, Gaining or Sharing electrons until they also have Fully Filled Outer Shells

Chemical Reactions

• Involve sharing or exchanging electrons in order that atoms can obtain a more stable arrangement of electrons

• They obtain stability by bonding with other atoms.

• (Protons and neutrons play no part in chemical reactions)

Chemical Reactions

Basically, there are 2 main types of bonding:

Ionic or Covalent

Particles (i.e. molecules, compounds) form a chemical bond linking them together.

How bonds are made 1

Valence:

how easily an atom or molecule with and unpaired electron (a radical)  can combine with other chemicals.

Valance is based on the number of electrons that would be added, lost or share if it reacts with other atoms.

Valencies

• Elements and radicals can be given numbers which are called their valencies or combining powers e.g. Na = 1

• The valency of an element is usually the number of electrons an atom of the element must lose or gain (either completely or by sharing) in order to obtain a stable atomic structure.

Valencies

Symbol Ion Formed

Valency of 1

Hydrogen H H+

Chlorine Cl Cl -

Hydroxide OH OH-

Hydrogen HCO3HCO3-

Carbonate

Valencies

Symbol Ion Formed

Valency of 2

Oxygen O O- -

Calcium Ca Ca++

Iron Fe Fe++

Sulphate SO4 SO4 - -

Valency of 3

Phosphate PO4 PO4 - - -

Valency of 4

Carbon C No ion

How Bonds are made 2

• one atom losses an electron– -it can be attracted to another atom that may have excess

electrons

• This is electrostatic attraction between two oppositely charged ions.

• bonds between atoms that have lost an ion (cation)

• Bonds between atoms that have gained electrons (anion).

Ionic Bonding

The formation of Positive Ions

• Sodium atom (Na) Sodium ion (Na+)(2,8,1) (2,8,)

+

(plus an electron )

(Less energy is needed to lose one electron than gain seven)

Ionic Bonding

The formation of Negative ionsChlorine atom Chlorine ion(2,8,7) (2,8,8)

-

(An electron has been gained)

(Less energy is needed to gain one electron than lose 7)

Combining of Na and Cl

• One electron lost by Na can be gained by Cl

• The ions are held together in the compound because positively charged Na ions are attracted to oppositely charged Cl ions

• The force which holds them together is called Electrostatic Attraction

• Ionic compounds , when in aqueous solution, are called electrolytes

How Bonds are formed 3

• If the chemical bond involves the sharing of electron pairs between atoms it is called

– Covalent bonding. 

Covalent Bonding

• Sharing, rather than losing or gaining electrons, is called covalent bonding

• Therefore, no positive or negative ions are formed

Chlorine atoms x 2 Chlorine molecule Cl2

Covalent Bonding

• A pair of electrons (1 electron from each atom ) bonding 2 atoms together is called a single covalent bond

• Can also get double covalent bonds (and also triple covalent bonds)

• Examples of molecules bonded covalently: H2, O2, H2O, NH3 etc