Atoms and elements can combine in many ways to form useful compounds. An element is made of identical atoms, while a compound is formed from two or more elements chemically bonded together. Atoms are the simplest form of an element, with protons and neutrons in the nucleus and electrons orbiting around. The number of protons determines the element, while isotopes of an element differ in their number of neutrons. Electrons are usually contained within atoms but can give electric shocks. Atoms combine in fixed ratios to form molecules, with the molecular mass determined by adding the atomic masses. Large numbers of atoms are measured in moles, with one mole equaling 6.02x1023 units. Elements form ions by gaining or losing

1. ATOMS AND ELEMENTS
What is an element?
Simply put, an element is made of identical atoms
Every element is different from each other and they can combine together in so many ways to
produce many many useful compounds for our daily use!
An atom is the simplest form of an element
Compounds refers to two or more elements chemically combined together
You can form 10grams of water from 1.2grams of hydrogen and 8.8grams of oxygen
The basic unit of a compound is the repeating unit
Silicon is used to make computer clips!
As well as forming beautiful beaches

2. Do you know that silicon forms compounds of similar chemical structures as other elements
in its group!
For example, if silicon forms an oxide called SiO2, what do you think is the formula of an
oxide of Carbon and of Germanium?
CO2, SiO2, GeO2
Identify the element time! copper
Are they both of the same element?same
Which pile contains more atoms? Right side one
Can you imagine each little nugget to consist of many many tiny atoms?
Atoms are the simplest form of an element

3. Do you think that each element has its own unique atom?yes
Hmm have you gotten an electric shock before?
Since everything is made of atoms, is the thing that gave you an electric shock an atom?
It is an electron!
An electron usually contained within an atom! Like in this picture
Not every atom can give you an electric shock, because the electrons are held tightly and can
only travel around the centre of the atom because of the proton!
The protons being positively charged, attracts the electrons which are negatively charged
But doesn’t the protons repel each other?
What else is in the nucleus that can prevent the protons from repeling each other?
It is the Neutron!
Both the proton and the neutron are closely packed together in the nucleus
And looking at the size,

4. Clearly the protons and neutrons have more mass than electrons!
In fact, 2000 electrons have the same mass as one proton or neutron, which have roughly the
same mass!
Do you think scientist count the mass of electrons in measuring the mass of an atom?
No, the mass of electrons are too small, so scientist only care about the number of protons
and neutrons.
Together, the total number of protons and neutrons equal the atomic mass
Can you tell what is the atomic mass from the data in the periodic table?
How much heavier would you feel an atom of sulphur is, compared to an atom of oxygen?
___________2x_
Mass is measured in Kg right?
Yet do you think an atom can have a mass measured in Kg?
Do you think the mass of an atom can be measured?
But since you know how small they are, think about grams, if a packet of rice has a mass of
10 000 grams, it is easier if you group a number of smaller units together to form a bigger
unit, like a Kilogram
For atoms, a bigger unit other than one atom is the mole! 
One mole equals 6.02x1023. The number is 6 000 000 000…. (+ 14 more zeros)! It is a very
large number
A mole of sodium chloride just means 6x1023 of NaCl basic unit
One mole of sodium chloride contains 1 mole of sodium and 1 mole of chlorine
And the mole is a scientist version of “kilogram” or “metre” or “minute”! So we have
measure reactions in terms of how many moles of what react with how many moles of who.

5. And the mole is very exciting for us, because one mole of hydrogen atoms weigh 1 gram!
This means that 60000000000000000000000000 hydrogen atoms altogether weigh 1 gram!
And the best part is, since we know that hydrogen has an atomic mass
of 1, every other elements’ atoms have a mass that is just a multiple of
1. An atom of oxygen is 16 times more massive than an atom of
hydrogen… and an atom of magnesium?
_____________
And if one mole of hydrogen has a mass of 1 gram, what about one
mole of oxygen?
_____________
If you know the atomic mass of oxygen, and that of hydrogen, can you find out the molecular
mass of water? That means what is mass of one molecule of water.
______18 (because 1 oxygen atom has 16 protons and neutrons and 2 hydrogen atoms have 1
proton each)_______
Try it out for yourself 
Find out the molecular mass of calcium carbonate CaCO3 ____________
Do you remember from last lesson,
1) what are the three components of an atom?
2) what are the electrical charges of these three components?
3) what is atomic mass determined by?
4) what does molecular mass refer to?
4) scientists measure large numbers of atoms using the ___________
Back to electrons, remember that

6. Notice that electrons are all circling outside, won’t they repel each other because they are all
negatively charged?
In three dimensions, actually they all have their own path and will not be very close to
another electron! 
Do you notice that electrons are arranged in layers? And the inner layer appears to be shielded
by the outermost shell.
We call also this outershell the valence shell and it is very important because atoms all hope
to achieve a stable electronic configuration of anoctet valence shell.
Do you know how many electrons an atom have?
We know that the number of electrons equal the number of protons!
The atomic number at the bottom is the same as the
proton number.
Look at your periodic table, can atoms of any element
have different number of protons?

7. The answer is No! Because once the proton number changes, the element also change. An
atom of any element has a fixed number of protons, but the number of neutrons can change.
How many neutrons are there in 35Cl? ________
How many neutrons are there in 37Cl? _________
Isotopes refers to atoms with of the same element with different number of neutrons
What do you think is the main difference between isotopes?
________________________________________________________________
How many electrons does a35Cl atom have? __________
Electron arrangement

8. What is the atomic number of Helium? ______ = number of electrons
What is the atomic number of Neon? _______ = number of electrons
Notice that electrons are organised in shells, and the first shell can contain up to ____
electrons
The second shell can contain up to _____ electrons
The third shell can contain up to ______ electrons although in our O level syllabus, the
maximum number of electrons a third shell will have is 8 
You would usually write the electronic configuration of an atom by filling up the lower
number shells first!
If an atom has 15 electrons, it can fill up the first shell (2), the second one (8), and leave the
last 5 in the third shell
So its electronic configuration is 2.8.5
Try it out and see for yourself  just
- check the atomic number to find the number of electrons
- then see if the first shell can be filled
- see if the remaining electrons can fill the second shell
- leave the remaining electrons in the third shell
1) Sodium
2) Flourine
3) Phosphorus

9. You will notice that not all the atoms have fully filled outer valence shell from the below
diagram which only shows the number of electrons in the valence shell.
Is there a pattern you see here?
Also notice that the last column (He and Ne) are known as unreactive noble gases, why do
you think that is so?
For the pattern,The number of valence shell electrons remain the same down a group
For the last column, noble gases have octet electronic configuration, therefore they do not
need to react with other atoms to be stable
Helium does not have 8 electrons in its outer shell, it only has 2 so for helium we say it has
duplex electronic configuration
Whereas all the other noble gases have 8 electrons in their valence shells and hence, can be
described has having octet electronic configuration.
All atoms like to have fully filled valence shells, this is very important.

10. Notice that group 1-3 fall under metals
While group 4 to 8 fall under non-metals
Because metals have fewer valence electrons, they prefer to give away their valence
electrons and enjoy the stability provided by an octet electronic configuration.
What happens when an atom gives away electrons?
Notice that a sodium atom has 11 protons and that number of protons will never change. But
yet if it has given 1 electron away, how many electrons does it have left? _____
What happens is sodium has 1 more proton than electron, and we know that protons have a
positive charge of +1
And that atom changes from being neutral to
being an ion that has a charge of +1
As the diagram shows, a +1 ion produces an
electrostatic field that can attract or repel other
charged particles!

11. Solidify your understanding!
1) what is the ion formed by atoms of magnesium, boron, and lithium when they lose their
electrons?_________________________
For non-metals, they can get aoctet valence shell through 2 ways.
The first way is to gain electrons.
Nitrogen needs to gain ____ more electrons to reach octet configuration.
Oxygen needs to gain ____ more electrons to reach octet configuration.
Similar to the example on sodium above, when a non-metal atom such as fluorine atom gains
an electron, suddenly it has 10 electrons and only 9 protons and it has become an ______ with
a charge of -1
Solidify your understanding!
1) what is the ion formed by atoms of chlorine and sulphur when they gain some electrons?
____________________
Solidify your understanding!
1) what charge
Let me introduce a very important organic molecule called glucose.
It has a chemical composition, meaning it is made up of how many atoms and of which
element, of
6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms