All matter consists of atoms, which cannot be divided up.
All atoms of the same elemtnt are identical, but different from atoms of every other element.
Mass Number and Isotopes
Not all atoms of elements have the same number of neutrons. Atoms of the same element with different masses are called isotopes.
The proportion of each isotope is fixed, and it is the same in both the pure element and in the compounds containing that element.
Because isotopes have the same number of electrons they all react in the same way.
Compounds and mixtures
To work out the relative formula mass (Mr) of a compound you find the atomic number of each element multiplied by the number of atoms of that element present in the formula.
Calsium Sulfate CaSO4
Atoms and A Ca=40 S=32 O=16
(40x1) + (32x1) + (16x4) = 136
Atoms and molecules are far too small to be counted. To make it easier chemicals call the atomic or formula mass measured in grams a mole.
The electrons occupy the lowest available energy levels.
They fill up each shell in turn. 2 go in the first shell, then 8 in the second and the third and the remainder go in the fourth.
For elements beyond calcium, the number of outer electrons is the same as the group number.
The number of outer electrons determines the reactivity of the element.
When a metal reacts with a non metal the metal atoms lose electrons and become posotive ions, the non metal atoms gain electrons and become negative ions.
A sodium atom (Na) loses an electon to become a sodium ion (Na+) with a 1+ charge. It has become a posotive ion.
The oppositly charged ions are electrostatically attracted, this is ionic bonding.
Group 1 metals react in similar ways because their atoms all hve just 1 electron in their outer shell.
Lithium justs floats on water and fizzes gently. Potassium bursts into flame and zooms across the water. Caesium explodes on contact with water.
The reactivity increases down the group.
When halogen elements form compounds, flourine becomes flouride, bromine becomes bromide and the ion always has a charge of 1-.
Flourine is the most reactive group 7 element, then chlorine,bromine and finally iodine which is the least reactive.
Solid sodium chloride forms an ionic lattice that is very strong. The oppositly charged ions attract each other in all directions and as a result, each sodium ion is attracted equally to six chloride ions.
Melting and boiling points are high as it is hard to separate the ions from each other.
ONly when the ions are free to move by melting or disolving in water can electricity be conducted.
When ionic compounds conduct electricity, they decompose back to their elements.
When atoms of two non metals combine, they share electrons to acheive noble gas electronic structures, which are stable.
The outer electron shells of atoms overlap. This sharing forms a covalent bond which hold the two atoms together.
Covalent bonds are very strong.