CHAPTER 1 – ATOMIC STRUCTURE
CHAPTER 1 – ATOMIC
STRUCTURE
AS Chemistry
SUMMARY OF THE CHAPTER:
AS Chemistry
Syllabus topic: 1.1-1.2
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CHAPTER 1 – ATOMIC STRUCTURE
The Structure of the Atom
Key Words
Element – a substance containing only one type of atom. All the atoms in an element have the same
proton number
Atom – the smallest part of an element that can take part in a chemical change. Every atom contains
protons in its nucleus and electrons outside the nucleus. Most atoms have neutrons in the nucleus. The
exception is the isotope of hydrogen 11H.
Proton – positively charged particle in the nucleus of an atom.
Neutron – uncharged particle in the nucleus of an atom, with the same relative mass as a proton
Electron – negatively charged particle found in orbitals outside the nucleus of the atom. It has negligible
mass compared with a proton
Energy levels – the specific distances from the nucleus corresponding to the energy of the electrons.
Electrons in energy levels further form the nucleus have more energy than those closer to the nucleus.
Energy levels are split up into sub-levels which are given the names s,p,d, etc.

Atoms are mostly made up of empty space around a very small, dense nucleus that
contains protons and neutrons

The nucleus has an overall positive charge
o

The protons have a positive charge and the neutrons have a neutral charge
Negatively charged electrons are found in orbitals in the empty space around the nucleus
The structure of an atom:
Atoms contain protons, neutrons and electrons
AS Chemistry
Syllabus topic: 1.1-1.2
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CHAPTER 1 – ATOMIC STRUCTURE
Relative mass & charge of subatomic particles table
Mass & Charge Distribution

The mass of an atom is concentrated in the nucleus because the nucleus contains the heaviest
subatomic particles (the neutrons and protons)

The nucleus is also positively charged due to the protons

Electrons orbit the nucleus of the atom, contributing very little to its overall mass, but creating
a ‘cloud’ of negative charge

The electrostatic attraction between the positive nucleus and negatively charged
electrons orbiting around it is what holds an atom together
The mass of the atom is concentrated in the positively charged nucleus which is attracted to the
negatively charged electrons orbiting around it
AS Chemistry
Syllabus topic: 1.1-1.2
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CHAPTER 1 – ATOMIC STRUCTURE
Behaviour of Subatomic Particles in an Electric Field

Protons, neutrons and electrons behave differently when they move at the same velocity in
an electric field

When a beam of electrons is fired past the electrically charged plates, the electrons are
deflected very easily away from the negative plate towards the positive plate


o
This proves that the electrons are negatively charged; like charges repel each other
o
It also shows that electrons have a very small mass, as they are easily deflected
A beam of protons is deflected away from the positive plate and towards the negative plate
o
This proves that the proton is positively charged
o
As protons are deflected less than electrons, this also shows that protons are heavier than
electrons
A beam of neutrons is not deflected at all
o
Which proves that the particle is neutral in character; it is not attracted to, or repelled by
the negative or positive plate
The lighter electrons undergo much more deflection than the protons
Isotopes

The number of neutrons can be calculated by:
AS Chemistry
Syllabus topic: 1.1-1.2
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CHAPTER 1 – ATOMIC STRUCTURE
Key equation
Number of neutrons = mass number - atomic number
The mass (nucleon) and atomic (proton) number are given for each element in the Periodic Table:
Key Definition
Isotopes – atoms of the same element with different mass numbers. Note that the word ‘atom’ is
essential in this definition
Isotopes: Chemical & Physical Properties

Isotopes have similar chemical properties but different physical properties
Chemical properties

Isotopes of the same element display the same chemical characteristics

This is because they have the same number of electrons in their outer shells

Electrons take part in chemical reactions and therefore determine the chemistry of an atom
Physical properties

The only difference between isotopes is the number of neutrons

Since these are neutral subatomic particles, they only add mass to the atom

As a result of this, isotopes have different physical properties such as small differences in
their mass and density
AS Chemistry
Syllabus topic: 1.1-1.2
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