1.2 Atomic Theory

Question 1

What is a atom?

Answer 1

Everything is made of elements but if we could look very closely at an
element, we would see that an element is made of very tiny particles called
atoms.

Question 2

What is an atom made of?

Answer 2

Electrons - Negatively charged particles that spin around the positive
centre of the atom in circles called energy levels. (Imagine
how the Moon spins around the Earth.) Their mass is so small it is nearly 0.

Protons - Positively charged particles that are contained in the
nucleus of the atom (the centre). They have a relative
mass of 1.

Neutrons - Neutrons are also contained in the nucleus of the atom
but have no charge. They also have a relative mass of 1.

Question 3

Where are protons found in the atom?

Answer 3

Nucleus.

Question 4

Where is the neutron located?

Answer 4

Nucleus

Question 5

Where is the electron located?

Answer 5

Energy level

Question 6

How are electrons arranged?

Answer 6

Elements are arranged in the periodic table in order of increasing atomic
number. For example hydrogen has the atomic number of 1, helium 2,
lithium 3 and so on. The atomic number gives the number of protons in
an atom. From this, the number of electrons can be worked out.

As you learned previously, the electrons that surround nuclei are
contained in energy levels. These energy levels can only hold a certain
amount of electrons. The first energy level (the one nearest the nucleus)
can hold a maximum of two electrons with the others being able to hold
up to a maximum of eight electrons. (This is only true for the first twenty
elements.) This information allows us to draw a diagram of an atom
showing how the electrons are arranged. (The electron arrangements of
all atoms can be found in the data booklet.)

Elements in the same group of the periodic table react in very similar
ways. This is because they all have the same number of outer electrons.
All the alkali metals for example have one outer electron. This makes
them all very reactive.

Question 7

How do we calculate how much neutrons are in an atom?

Answer 7

We have seen how we use the atomic number to calculate the number
of protons and electrons in an atom. We must also be able to work out
how many neutrons there are in the nucleus of an atom.
To do this we need the mass number of the atom. The mass number is
given in the top left of the element’s symbol. To find out the number of
neutrons, the number of protons needs to be deducted from the mass
number. Remember that electrons have a mass of zero and therefore
don’t affect the mass of an atom.

Key points !
The mass number of an element is equal to the number of protons plus
the number of neutrons.

Question 8

What is an isotope?

Answer 8

The masses of the atoms of an element are not always the same. Just like
people, atoms all have slightly different weights.
For example 12C and 13C are both carbon atoms.
They are isotopes; they have the same number of protons but the
number of neutrons in each is different.

Key points !
Isotopes are atoms with the same atomic number but different mass
numbers.

Elements are made up of isotopes so the mass given in the data booklet is
called the relative atomic mass (RAM), which is an average mass of
all the isotopes of a single element.

The RAM can be used to give an indication of the relative abundance of
isotopes in a sample.

Question 9

What is an ion?

Answer 9

Atoms are neutral because they have an equal number of positive
protons and negative electrons, but what happens when the numbers of
positive protons and negative electrons in a particle are not equal?

When there is an imbalance of electrons to protons an ion is formed.

Key points !

• An ion is a charged particle and they are formed when an atom gains
  or loses electrons.
• Metal atoms lose electrons to form positive ions and non-metal atoms
  gain electrons to form negative ions.

The reason that atoms gain or lose electrons is because of the noble
gases. The noble gases are like the pop stars, movie stars or sport stars
of our world. All elements in the periodic table want to be like them in
every way! This is why noble gases won’t react because they don’t want to
be changed at all. In chemistry terms they are said to be stable.

Key points !
The noble gases have a stable electron arrangement.
If other elements are to be like their heroes, they must try to achieve the
same electron arrangement as a noble gas, which has a full outer energy
level. Neon, for example, has eight outer electrons, whereas oxygen only
has six outer electrons. To become stable like a noble gas, oxygen must
gain two electrons from another atom.