The atomic structure and the periodic table
- It is the study of the structure, composition and reactions/ behaviour of matter.
- Matter is anything that occupies space and has mass - The basic unit of matter
is the atom.
An Atom
- It is the smallest electrically neutral, indivisible particle of an element which can take part in
chemical reactions (smallest particle of element which takes part in chemical reaction) - The atom
comprises of three fundamental particles which are:
1. Proton
2. Neutron
3. Electron
Particle
Relative mass
Relative charge
Location
Proton
1
+1
Nucleus
Neutron
1
0
Nucleus
-1
Shells/ Orbitals
Electron
Masses and charges of an atom are compared to each other using ‘relative atomic masses’ and
‘relative atomic charges’
• These are not actual charges and masses but they are charges and masses of particles relative to
each other
o Protons and neutrons have a very similar mass so each is assigned a relative mass of 1
whereas electrons are 1836 times smaller than a proton and neutron
o Protons are positively charged, electrons negatively charged and neutrons are neutral
•
Exam Tip
The relative mass of an electron is almost negligible.
The charge of a single electron is -1.602 x 10-19 coulombs whereas the charge of a proton is -1.602 x 10-19 coulombs,
however, relative to each other, their charges are -1 and +1 respectively.
Important terms
1. Atomic number (or proton number) is the number of protons in the nucleus of an atom and
has symbol Z.
**The atomic number is equal to the number of electrons present in a neutral atom of an
element e.g. the atomic number of lithium is 3 which suggests that the neutral lithium atom has 3
protons and 3 electrons
CHIS – Year 10 Chemistry Notes. (2021-22) ** Atomic structure
2. Mass number (or nucleon number) is the total number of protons and neutrons in the
nucleus of an atom and has symbol A
Note: The number of neutrons can be calculated by:
Number of neutrons = mass number – atomic number
** Protons and neutrons combined are also called nucleons
Nuclide notation
A nuclide is a species whose nucleon/ mass number is specified (sometimes proton number also)
Note:
The mass (nucleon) number and atomic (proton) number are given for each element in the periodic
table
Determining the number of protons/ electrons or neutrons in atoms and ions of elements
•
•
An atom is neutral and has no overall charge
Ions on the other hand are formed when atoms either gain or lose electrons, causing them to
become charged
• The number of subatomic particles in atoms and ions can be determined given their atomic
(proton) number, mass (nucleon) number and charge
Protons – enables chemists to identify an atom or ion
All atoms and ions of the same element have the same number of protons (atomic number)
e.g. Lithium has an atomic number of 3 (three protons) whereas beryllium has atomic number of
4 (four protons)
** The number of protons of an unknown element can be calculated by using its mass number
and number of neutrons as follows:
Mass number = number of protons + number of neutrons
Number of protons = mass number – number of neutrons
CHIS – Year 10 Chemistry Notes. (2021-22) ** Atomic structure
Worked example: Determine the number of protons
 Answer 1: The atomic number of a magnesium atom is 12 suggesting that the number of protons in the
magnesium element is 12
 Therefore the number of protons in an Mg2+ ion is also 12
 Answer 2: The atomic number of a carbon atom is 6 suggesting that a carbon atom has 6 protons in its nucleus
 Answer 3: Use the formula to calculate the number of protons
Number of protons = mass number – number of neutrons
Number of protons = 63 – 34
Number of protons = 29
 Element X is therefore copper
Electrons
** An atom is neutral, therefore the number of protons and electrons is the same
 Ions have a different number of electrons to their atomic number depending on their charge o
A positively charged ion has lost electrons and therefore has fewer electrons than protons
o A negatively charged ion has gained electrons and therefore has more electrons than protons
Answer 1
 The atomic number of a magnesium atom is 12 suggesting that the number of protons in the neutral magnesium
atom is 12
 However, the 2+ charge in Mg2+ ion suggests it has lost two electrons, thus has only 10 electrons left Answer 2
 The atomic number of a carbon atom is 6 suggesting that the neutral carbon atom has 6 electrons orbiting
around the nucleus
Answer 3
 The number of protons of element X can be calculated by:
Number of protons = mass number – number of neutrons
Number of protons = 63 – 34
Number of protons = 29
** The neutral atom of element X therefore also has 29 electrons
Neutrons
 The mass and atomic numbers can be used to find the number of neutrons in ions and atoms:
Number of neutrons = mass number (A) – number of protons (Z)
Answer 1: The atomic number of a magnesium atom is 12 and its mass number is 24
Number of neutrons = mass number (A) – number of protons (Z)
Number of neutrons = 24 – 12
Number of neutrons = 12 (The Mg2+ ion therefore has 12 neutrons in its nucleus)
CHIS – Year 10 Chemistry Notes. (2021-22) ** Atomic structure
Answer 2: The atomic number of a carbon atom is 6 and its mass number is 12
Number of neutrons = mass number (A) – number of protons (Z)
Number of neutrons = 12 – 6
Number of neutrons = 6 (The carbon atom has 6 neutrons in its nucleus)
Answer 3: The atomic number of an atom of element X is 29 and its mass number is 63
Number of neutrons = mass number (A) – number of protons (Z)
Number of neutrons = 63 – 29
Number of neutrons = 34 (The neutral atom of element X has 34 neutrons in its nucleus)
Isotopes
•
Isotopes are atoms of the same element that contain the same number of protons and electrons
but different number of neutrons and hence have different mass numbers
• The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the
mass number e.g. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8
neutrons respectively
Other isotopes: Chlorine-35 and Chlorine-37; Carbon-12 and C-14
•
•
Isotopes can be divided into two categories: radioactive and non-radioactive
Radioactive isotopes (radioisotopes) are unstable due to the imbalance of neutrons and protons
Electronic structure
• The structure of the atom can be represented in two ways i.e. using diagrams called electron shell
diagrams or by writing out a special notation called the electronic configuration
• Electronic configuration is the arrangement of electrons in an atom
Electron shell diagrams
• Electrons orbit the nucleus in shells (or energy levels) and each shell has a different amount of
energy associated with it
• The further away from the nucleus a shell is, the more energy the shell has.
• The innermost shell holds a maximum of 2 electrons and these go in separately
• The second shell can hold 8 electrons and the third shell can also hold 8 electrons and the electrons
organise themselves in pairs in these shells
• The outermost shell of an atom is called the valence shell and an atom is much more stable if it
CHIS – Year 10 Chemistry Notes. (2021-22) ** Atomic structure
can manage to completely fill this shell with electrons
Electronic configuration
•
•
•
•
The arrangement of electrons in shells can also be explained using numbers
The position of an element on the Periodic Table can be used to deduce electronic configurations
A periodic table has horizontal rows called Periods and vertical columns called Groups
Period number, tells number of shells an atom has and group number tells number of electrons in
valence or outer shell
• Elements in the same group have the same number of outer shell electrons and those in the same
period have the same number of shells
Consider an atom of Chlorine
Chlorine is in period 3 of the periodic table and in group 7, so Cl has;
 Three shells
= Period number  Seven
valence electrons
= Group number
 Chlorine’s electronic structure can therefore be
written as follows:
Calculating relative atomic mass
Relative mass is the mass of an atom or molecule
compared to that of 1/12 of a carbon-12 atom
Relative atomic mass is the weighted average of
the masses of an element’s isotopes in
comparison to the mass of a carbon-12 atom
CHIS – Year 10 Chemistry Notes. (2021-22) ** Atomic structure
Example
A sample of chlorine gas is a mixture of 2
isotopes, chlorine-35 and chlorine-37. These
isotopes occur in specific proportions in the
sample i.e. 75% chlorine-35 and 25% chlorine-37.
Calculate the relative atomic mass of chlorine in
the sample.
Relative atomic mass= Isotope mass * isotope abundance
100
CHIS – Year 10 Chemistry Notes. (2021-22) ** Atomic structure
Example 2
CHIS – Year 10 Chemistry Notes. (2021-22) ** Atomic structure
Exercise
1. Calculate the relative atomic mass of the
following
2.
CHIS – Year 10 Chemistry Notes. (2021-22) ** Atomic structure
Home work
1.
2.
CHIS – Year 10 Chemistry Notes. (2021-22) ** Atomic structure
CHIS – Year 10 Chemistry Notes. (2021-22) ** Atomic structure