Chemistry- atoms and the periodic table

1 . An element is a pure substance made of one type of atom.

2. There are about 100 different elements. Elements are shown in the periodic table.

3. An atom is the smallest part of an element that can exist.

4. Atoms of each element are represented by a chemical symbol, e.g. O represents an atom of oxygen

5. All substances are made of atoms.

6. Atoms are very small, having a radius of about 0.1 nm (1 x 10-10 m).

7. The radius of a nucleus is less than 1/10,000 of that of the atom (about 1 x 10-14 m).

8. Almost all of the mass of an atom is in the nucleus.

9. Atoms contain three types of sub-atomic particle: protons, neutrons and electrons

• Atomic number = the number of protons in an atom of an element
•  All atoms of a particular element have the same number of protons.
• Different elements have different numbers of protons. The proton number identifies the element. 
• Mass number = the sum of the protons + neutrons in an atom [always the biggest number]
•  PROTONS = ELECTRONS = Atomic Number 19. NEUTRONS = Mass Number – Atomic Number 

Mass number(the biggger one)

• Atomic number  

Isotopes are different atomic forms of the same element

• Isotopes have the same number of electrons and protons but different number of neutrons and hence mass numbers
• The relative atomic mass of an element is an average value that takes account of the abundance(natural occurrence) of the isotopes of the element.

 (mass of isotope x % abundance of isotope)  

Rutherford fired alpha particles (positive helium nuclei) at gold foil and found that

• Most nuclei passed straight through
• A few nuclei were deflected or rebounded

 Rebound = Collison with dense ‘solid’ structure = NUCLEUS- concentration of mass in centre

• Deflection is related to the mass and charge of the particles.
•  Positive alpha particles would be deflected by positive nucleus due to repulsion of like charges.

This evidence led Rutherford to suggest a new model for the atom, called the nuclear model 

The electrons in an atom occupy the lowest available energy levels (innermost available shells). . Each energy level (shell/orbit) has a maximum number of electrons before becoming full 

Energy Level: 1 st -2            2nd- 8             3rd- 8

•  The electronic structure of an atom can be represented by numbers [2,8,1] or by a diagram. 
• The number of electrons in the outermost energy level is equal to the group number of the Periodic Table. 
• Elements in the same group in the periodic table have the same number of electrons in their outer shell (outer electrons) and this gives them similar chemical properties
• . Elements with FULL OUTER SHELLS are stable and are therefore INERT – they don’t react e.g. Noble Gases
• . Other elements react to form molecules and compounds in order to obtain full outer shells and increase their stability.
• . Group 1 elements readily lose their 1 outer electron which makes them very reactive. 
• Group 7 elements only need 1 more electron to become stable. They can gain or share electrons to fill their outer shell.

Newlands=

•  Atomic weight order
• Octet Rule was based on a repeating pattern of properties every eight element
• metals and non-metals in the same groups

Mendeleev (1869)=

• Atomic weight order BUT changed the order of atomic weights in some places to make similar trends and observations in each group match e.g. moving iodine to the same group as bromine. 
• left gaps for elements that had not been discovered. 
• Predicted the properties of the undiscovered elements 

The elements are arranged in order of atomic (proton) number

7. Properties include melting & boiling points, hard/soft, density, malleable/brittle, conductivity.

10. Elements that react to form positive ions are metals.

11. Elements that do not form positive ions are non-metals.

14. Elements with similar properties are in columns, known as groups.

• Group 1 = alkali metals,
• Group 7 = halogens,
• Group 0 = noble gases

 15. Elements in the same group in the periodic table have •

• the same number of electrons in their outer shell (outer electrons) 
• similar chemical properties