1. Atoms and their Structure

2. Elements, Compounds & Mixtures
We are surrounded by millions of different substances, some of which are naturally occurring like sand and water, and some of which are man-made like plastic and glass.
The three main types are
elements
compounds
mixtures

3. Elements Elements cannot be decomposed into smaller substances.
They must contain one type of species of particle only.
There are 105 elements at present of which eighty-four occur naturally
They are classified within a table known as the Periodic Table

4. Compounds
Elements can chemically combine with each other and form a compound.
Compounds contain more than one type of particle.
Compounds can only be broken down into smaller substances by chemical means.
The elements present are present in fixed proportions.
Eg water - H2O
always 2H to 1O

5. Mixtures These substances are only physically mixed.
No chemical combination has occurred.
The substances can be separated the by physical means.
The various substances in the mixture could be present in any proportion.

6. The Atom
Greek philosophers – Democritus and Leucippus proposed original idea of the atom from particles of sand (~530 B.C) .
Sand cut into smaller particles until –
Atomos – an indivisible part – cannot be cut

7. John Dalton Scientist and English school teacher
Early 1800s proposed modern day theory.
Combined ideas of elements and atoms from experimental evidence.
His theory is known as Dalton’s Atomic Theory.

8. Dalton’s Atomic Theory
All matter is formed from tiny indivisible particles (atoms).
All elements are made of atoms.
All atoms from one element are identical but are different from atoms of a different element.
Atoms from different elements can combine to form compounds but only in whole number ratios
No new atoms are created or destroyed during these chemical reactions.

9. What makes up the Atom?
Atoms are composed of small sub-atomic particles:
protons (p) which are positively charged,
neutrons (n) which are neutral and
electrons (e) which are negatively charged.
Nucleus – found in the centre of the atom
Contains protons and neutrons
Make up the dense core of the atom

10. Electrons Are extremely light, fast moving particles
Travel around the nucleus.
Impossible to specify the exact position of an electron relative to the nucleus
The probability of finding the electrons is greatest within a certain volume around the nucleus (orbital).
This volume is represented as an electron cloud.

11. Electrons
Nucleus

12. Mass of Particles

13. Mass of Particles
The neutron is slightly heavier than the proton which is much heavier than the electron.
The mass of the atom is concentrated in the central core, the nucleus, which is small and dense.
Hence the atom is mostly empty space.

14. Notations for the Elements
Symbols are used to represent the element
eg H for hydrogen, He for helium, S for sulphur and,
Fe for iron (Latin is ferrum).
Nucleur/nuclide notation may be used
where m is the mass number and
z the atomic number.

15. Atomic Number (z) Is the number of protons (p) in the nucleus and
Every element will have its own unique atomic number
e.g.
Has 2 protons

16. No of Electrons Number of electrons is equal to the number of protons
Hence the atomic number (z) is also the number of electrons.
e.g.
Has 2 protons
Therefore has 2 electrons

17. Mass Number Is the mass of the element
Given by the sum of the protons and the neutrons only
An electron mass is negligible. eg
Has a mass number of 4

18. No. of Neutrons
To calculate the number of neutrons in a nucleus we subtract the number of protons from the mass number.
e.g.
Mass Number = 4
No of Protons = 2
No of Neutrons = Mass no. – No of Protons
= 4 – 2
= 2

19. Isotopes Dalton was not quite accurate
Isotopes are atoms of the same element which have different mass numbers.
The different mass numbers result from different number of neutrons in the nucleus.
Chlorine has two

20. Relative Atomic Mass
Relative atomic mass is the average mass of one atom.
It is not a whole number
Hence the reason for decimal numbers on the periodic table
Chlorine contains two isotopes 3517CI, 3717CI comprising 75% of the 3517CI version and 25% of the 3717CI isotope.

21. e.g. Chlorine contains two isotopes comprising
Relative atomic mass
(i) 75  100 x 35 = 26.25
(ii) 25  100 x 37 = 9.25
Average mass of one atom (Relative Atomic Mass) =
= 35.5 amu

22. Try These Find the Number of protons Number of neutrons
Number of electrons
Atomic Number
Mass Number
Name the element

23. Find the Number of protons Number of neutrons Number of electrons
Atomic Number
Mass Number
Name the element

24. If an element has a atomic number of 26 and a mass number of 59, what is the
Number of protons
Number of neutrons
Number of electrons
Complete the symbol

25. If an element has 56 electrons and 81 neutrons what is the
Atomic number
Mass number
Number of protons
Complete the symbol

26. Calculating Relative Atomic Mass
Calculate the relative atomic mass of copper if copper has two isotopes as follows:
69.1% has a mass of 62.9 amu and the remaining isotopes have a mass of amu

27. The Periodic Table Vertical columns known as Groups.
Horizontal rows of elements are known as Periods.
Groups 1 – 7 are known as the ‘typical’ or representative elements.
Group 8 are known as the ‘noble’ gases.
Middle group referred to as the transition elements.
The number of the group refers to the number of the electrons in the outermost filled or filling electronic shell.

28. Group 1 – The Alkali Metals
Element
State
Type
Reactivity
Outer Electrons for Reaction*
Li (3)
solid
metal
very reactive
2, 1*
Na (11)
2, 8, 1*
K (19)
2, 8, 8, 1*
Rb (37)
2, 8, 18, 8, 1*
Cs (55)
violently reactive
2, 8, 18, 18, 8, 1*

29. Outer Electrons for Reaction*
Group 7 – The Halogens
Element
State
Type
Reactivity
Outer Electrons for Reaction*
F (9)
yellow gas
non metal
violently reactive
2, 7*
CI (17)
green gas
very reactive
2, 8, 7*
Br (35)
red liquid
reactive
2, 8, 18, 7*
I (53)
violet solid
2, 8, 18, 18, 7*

30. Electronic Configuration
Group 8 – The Noble Gases
Noble Gas
Electronic Configuration
Helium He (2) 2
Neon Ne (10)
2, 8*
Argon Ar (18)
2, 8, 8*
Krypton Kr (36)
2, 8, 18, 8*
Xenon Xe (54)
2, 8, 18, 18, 8*