The Structure of the Atom

All matter is made up of atoms

Everything in the world is made up of different combinations of atoms from the elements on
the periodic

The periodic table displays the elements in increasing atomic number and shows how
periodicity of the physical and chemical properties of the elements relates to atomic structure.
Atomic mass and diameter
Atomic mass




Atoms are extremely small – it cannot be measured in grams and kilograms
A different unit is needed. This unit of measurement is called the atomic mass unit,
abbreviated amu or u.
This unit is based on the mass of an atom of carbon-12 which has a mass of exactly 12 amu
It was then determined that one atomic mass unit (1 u) is equal to 1,66 x 10-24 g.
Worked Example 1
Calculate the mass (in grams) of an atom of iron with an atomic mass of 56 u.
Answer:
Mass = 56 x (1,66 x 10-24 g) = 9,3 x 10-23 g
Relative atomic mass

It is used to compare the average mass of atoms of each element with each other.

Relative atomic mass (Ar) is the ratio of the average mass per atom of an element to 12 of

the mass of a carbon-12 atom.
Relative atomic mass is a ratio and so it is has no units.
1
Example:
Hydrogen atoms have relative atomic mass of 1 which means that they have
1
the same mass as 12 of a carbon-12 atom.
Atomic diameter


The volume or size of the atom is large compared to its mass.
The atom is mostly made up of empty space, with its mass concentrated in the centre of the
atom. (Rutherford)
Electrons, Protons and Neutrons

An atom contains three types of sub-atomic particles: protons (p+), neutrons (n°) and
electrons (e-).

The nucleus of the atom is in the centre and contains the protons and the neutrons that are
called nucleons

The electrons are found in the space around the nucleus.

Protons and neutrons have got approximately the same mass of 1,67 x 10 -27 kg.

The mass of the electron is much less (9,1 x 10-31 kg)

A proton has a positive charge and an electron has an equal but opposite negative charge.

The neutrons have no charge

Atoms are generally neutral (have no charge) because the total number of protons are equal
to the total number of electrons.
Label the accompanying
diagram of the atom fully
THE NUCLEUS OF THE ATOM
1.





Atomic number
The number of protons in the nucleus is called the atomic number (Z)
The atomic number Z corresponds with the position of the element on the Periodic Table.
All the atoms of a certain element have the same atomic number.
For neutral atoms, the atomic number will also indicate the number of electrons.
For example: A CI-atom (chlorine atom; element 17) has an atomic number of 17 which
means that there are 17 protons in the nucleus of the atom and there are 17 electrons
around the nucleus of a neutral CI atom.
2.
Mass number (A)


The mass number is the total number of protons plus neutrons (thus the number of
nucleons) in the nucleus of the atom
We indicate the atomic mass (Z) and the mass number (A) by writing it next the
symbol of the element:
𝐴
𝑍𝑋
where:
X is the symbol of the element
Z is the atomic number showing the number of protons in the nucleus and the number of electrons in
the atom if the atom is neutral
A is the number of protons + neutrons in the nucleus
The number of neutrons is given by A-Z
Number of protons = Z
Consider the following:
35 −
17𝐶𝑙
Atomic number = ……………….
Number of protons = …………..
Mass number = ………………...
Number of neutrons = …………
Number of electrons = …………
IONS OF ATOMS - change in the number of electrons




When an atom gains electrons , it contains more electrons than protons and thus gains a
negative charge.
If the atom loses electrons, it contains more protons than electrons and thus gains a positive
charge.
An atom with a charge is called an ion.
Negative ions are also sometimes called anions and positive ions are also called cations.
Consider the following:
Consider the following:
27 3+
13𝐴𝑙
35 −
17𝐶𝑙
Mass number = ………………...
Mass number = ………………...
Atomic number = ……………….
Atomic number = ……………….
Number of protons = …………..
Number of protons = …………..
Number of electrons = …………
Number of electrons = …………
Number of neutrons = …………
Number of neutrons = …………
Isotopes
Isotopes are atoms of the same element with the same atomic number (and thus the same
number of protons) but with different mass numbers. ,
Example: Hydrogen has 3 isotopes
protium


dueterium
tritium
All three hydrogen isotopes have the same atomic number and thus the same number of
protons (and electrons) but have different mass numbers because their neutrons in the
nucleus differ.
Chlorine has two isotopes that are called chlorine-35 and chlorine-37.
Question
Which of the following symbols represent isotopes?
19
9𝑋
Exercise
19
10𝑋
20
9𝑋
21
11𝑋
19
8𝑋
ELECTRONIC CONFIGURATION
ENERGY LEVELS IN AN ATOM









Although electrons all have the same mass and charge, they have different amounts of
energy.
Electrons with less energy appear closer to the nucleus than electrons with more energy.
Electrons are arranged in fixed areas around the nucleus, called energy levels, that depends
on the amount of energy that they have.
Main energy level are indicated with the following numbers n = 1, 2, 3 . . . etc. The greater "n"
is the higher is the energy of the electron.
Each main energy level consists of one or more sub—energy levels. These sub-levels are
indicated with the letters s, p, d and f.
Main energy level n=1 only has one energy level called the 1s energy level.
Main energy level n=2 consists of two sub- energy levels: the 2s and 2p sub-energy levels.
Main energy level n=3 consists of three sub- energy levels: the 3s, 3p and 3d sub-energy
levels.
In a given main energy level, the s sub-energy level is the level with the lowest energy.
(s < p < d)
Orbitals:




An orbital is a region of space around the nucleus where there is a 90% chance
of finding an electron
Each sub-energy level consists of one or more orbitals.
A s-sub-level consists of one orbital, the s-orbital.
A p-sub-energy level consists of three p-orbitals, the px, py and pz-orbitals.
A d-sub—energy level consists of five d—orbitals.
Energy level
Sub-Energy levels
Orbitals
Max. amount of
electrons (2n2)
n=1
s,p and d
3s, 3px, py, pz and 5 dorbitals
2 +6 +10 = 18
n=2
s and p
2s, 2px, py, pz
2+6=8
n=3
s
1s
2
ORBITALS
1.
s-orbitals
 these orbitals are spherical in shape
2.
p-orbitals
 these orbitals are double tear-drop shaped
 there are three types, arranged perpendicularly to each other as the px, py and pz orbitals.
Electron distribution in orbitals





The distribution of the electrons of an atom in it’s orbitals occur according to a certain
principle, that is called the Aufbau
An energy level diagram shows the main energy Ievels and orbitals of an atom.
Blocks or circles are used as the orbitals.
Electrons in the orbitals are represented by arrows (↑↓)
The lowest energy orbitals are filled up first.
The Aufbau Principle includes the following procedures:
(1)
Pauli's exclusion principle states that only two electrons can occupy an orbital and
these must spin in opposite directions.
(2)
Hund’s rule states that, when filling sublevels of p-orbitals, electrons are placed in
individual orbitals before they are paired up.
An electron that is alone in an orbital, is called an unpaired electron.
If an orbital is filled with two electrons, the electrons are referred to paired electrons.
The Aufbau diagram for the calcium atom (atomic number = 20) is as follows:
n=4
4s
↑↓
4p
n=3
3s
↑↓
↑↓
↑↓
↑↓
3p
n=2
2s
↑↓
↑↓
↑↓
↑↓
2p
n=1
1s
↑↓
Electronic configuration notation: either …..
condensed notation:
1s2 2s2 2p6 3s2 3p6 4s2
expanded notation:
1s2 2s2 2𝑝𝑥2 2𝑝𝑦2 2p2z 3s2 3p2x 3p2y 3p2z 4s2