Atoms

• Everything in the universe (except energy) is made of matter, and matter is made of
atoms
• Everything - living as well as non-living

Paracetamol contains 20 atoms:
8 carbon atoms (grey)
9 hydrogen atoms (white)
1 nitrogen atom (blue)
2 oxygen atoms (red)
(Formula: C8H9NO2)
Atoms are tiny particles with a radius of 0.1 nm (i.e. 1 x 10-10 meters)
1 oxygen atom
2 hydrogen atoms
(Formula: H2O)
Water contains 3 atoms:

The average cell contains 100,000,000,000,000 atoms
(i.e.100 trillion atoms)
• Atoms are tiny particles with a radius of 0.1 nm (i.e. 1 x 10-10 meters)

• An atom itself is made up of subatomic particles: protons, neutrons, and electrons
• The protons and neutrons are located at the centre of the atom, which is called the
nucleus
• The nucleus is tiny compared to the rest of the atom, it is about 1/10,000 the size of the
atom
• Protons are positively charged, neutrons are neutral (i.e. no charge), around which orbit a
cloud of negatively charged particles called electrons
• The number of protons always equals the number of electrons in an atom
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electron
nucleus
proton
neutron
The structure of the carbon atom

• The electrons move very fast around the nucleus in orbital paths called shells; they are
tiny, but they cover a lot of space
• A shell is just an area where electrons are found (at GCSE level these are drawn as circles)
• Shells are sometimes called energy levels
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The structure of the carbon atom

• The mass of the electrons is negligible in comparison to protons and neutrons, so nearly all
of the mass of an atom is located in the nucleus
• The charge on the protons is +1
• The charge on the electrons is -1
Subatomic particle Proton Neutron Electron
Relative charge +1 0 -1
Relative mass 1 1 Very small
The relative charges and masses of protons, neutrons and electrons
# of
protons
# of
electrons
The number of positively charged protons
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the number of negatively charged electrons
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0 0r no charge

• Example-
• The carbon atom has 6 protons, 6 neutrons and 6 electrons
• Each proton has a charge of +1 and neutrons have no charge, so the total charge of
the nucleus is +6
• However, there are 6 electrons orbiting the nucleus and as you know each electron has
a charge of -1, so the charge of the nucleus is cancelled out.
• So, the atom doesn’t have a charge overall
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electron
nucleus
proton
neutron
The structure of the carbon atom

• The periodic table depicts atoms in their neutral state but if some electrons are added or
removed, the atom becomes electrically charged and is then called an ion
Example-
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Oxygen atom
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Oxygen ion
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The two gained electrons (purple dots) means that this
oxygen ion has 10 electrons (-10 charge) and only 8
protons (+8 charge), giving the ion a net charge of -2.

Elements

• An atom with one proton in its nucleus is hydrogen.
• An atom with two protons is helium.
• An atom with three protons is lithium.
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So, it’s the number of protons in the nucleus that decides what type of atom it is.
If a substance only contains one type of atom it’s called an element.

An element is a substance containing only one type of atom
Example-
• Lithium is an element. It is made up of lithium atoms only.
• So, lithium atom is the smallest part of the element lithium.
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Atoms of each element can be represented by a one or two letter symbol
• Some nuclear symbols make sense such as C= carbon, N= nitrogen and O= Oxygen.
• However, others are less obvious: Na= sodium, Fe=iron and Pb= lead

• The first 94 elements of the periodic table are found in nature, but the remaining
elements (numbers 95 to 118) have only been created in laboratories or nuclear reactors.
• Each element is represented by its own unique symbol as seen on the Periodic Table e.g.,
N stands for nitrogen and for nitrogen only.
• Where a symbol contains two letters, the first one is always written in uppercase
letters and the other in lowercase e.g., sodium is Na, not NA.
• The atomic number and mass number also shown on the periodic table
Example of a nuclear symbol:
chemical symbol for carbon
The atomic number (sometimes called proton number) is 6; that means there are 6
protons in an atom of Carbon
The mass number is 12; that means there are 6 protons and 6 neutrons in an atom
of Carbon
Note: practice calculating the number of protons, neutrons and electrons in an atom from its atomic number and mass number.

Isotopes

• Atoms of the same element always have the same number of protons, but the number of
neutrons can change. And that’s where isotopes come in.
What are isotopes?
Isotopes are different forms of the same element, which have the same number of protons
but a different number of neutrons.
This means they have the same atomic number but a different mass number.
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C
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C
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Note: Count number of protons, neutrons and electrons in both isotopes above