Chemistry Structure 1.2 The nuclear atom

Elements

Elements

are substances which cannot be broken down into simpler componenets by chemical reactions.

Atom

is the smallest unit of an element.

Dalton’s proposition

• All matter is composed of tiny indivisible particles called atoms.

• Atoms cannot be created nor destroyed

• Atoms of the same element are alike in every way.

• Atoms of different elements are different.

• Atoms can combine together in small numbers to form molecules.

Thompsons plum pudding

Negatively charged electrons positioned in a positively charged sponge-like substances.

Ernest Rutherford

The atom is mainly empty space

Nucleons

Neutrons and protons.

Bohr’s model of the hydrogen atom

He pictured the hydrogen atom as a small ‘solar system’ with an electron moving in an orbit around the psoitively charged nucleus of ne proton.

Atomic number

The number each element is given that describes their relative position in the table.

The number of protons in the atom.

The number of electrons in an atom.

The mass number

The number of protonsplus the number of neutrons in an atom.

Ions

Atoms loses or gain electrons to form ions

Cation

Atom loses electron and forms positive ions

Anion

Atom gains electron to form negative ions

Isotopes

Atoms of the same element with different numbers of neutrons.

Isotopes shpw the same chemical properties, since the diffrence in th number of neutrons makes no difference as to how atoms react.

They lead to different physical properties such as boiling and melting points.

Average relative mass of isotopes

relative average mass = (total mass) / (number of atoms)

Radiosotopes

A nucleus which either contains too many or too few neutrons to be stable.

They emit radioactivity.