Atomic Structure & the Periodic Table

Protons, Neutrons & Electrons

( IGCSE Chemistry CIE Revision Notes )

Elements are made of tiny particles of matter called atoms

Each atom is made of subatomic particles called protons, neutrons and electrons

Their size is so tiny that we can’t really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass is used

One relative atomic mass unit is equal to the mass of a carbon-12 atom.

All other elements are measured relative to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units

Hydrogen for example has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon

The relative mass and charge of the subatomic particles are shown below:

Defining Proton Number

The atomic number (or proton number) is the number of protons in the nucleus of an atom. The symbol for this number is Z

It is also the number of electrons present in an atom and determines the position of the element on the Periodic Table

Defining Nucleon Number

Nucleon number (or mass number) is the total number of protons and neutrons in the nucleus of an atom. The symbol for this number is A

The nucleon number minus the proton number gives you the number of neutrons of an atom

Note that protons and neutrons can collectively be called nucleons.

The atomic number and mass number for every element is on the Periodic Table

Electrons (symbol e^–)

These subatomic particles move very fast around the nucleus

They move in orbital paths called shells

The mass of the electron is negligible, hence the mass of an atom is contained within the nucleus where the neutron and proton reside

The Basis of the Periodic Table

Elements are arranged on the Periodic table in order of increasing atomic number where each element has one proton more than the element preceding it

Hydrogen has 1 proton, helium has 2 protons, lithium has 3 etc.

The table is arranged in vertical columns called Groups numbered I – VIII and in rows called Periods

Elements in the same group have the same amount of electrons in their outer shell, which gives them similar chemical properties

Defining Isotopes

Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.

The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number.

So C-14 is the isotope of carbon which contains 6 protons, 6 electrons and 14 – 6 = 8 neutrons.

The atomic structure and symbols of the three isotopes of hydrogen

The three most stable isotopes of hydrogen: protium (A = 1), deuterium (A = 2), and tritium (A = 3). They each have one single proton (Z = 1), but differ in the number of their neutrons. The superscripts 1, 2 and 3 written before H are the atomic masses of the isotopes of hydrogen and the subscript 1 is the atomic number. Protium has no neutron, deuterium has one, and tritium has two neutrons as shown below

Types of Isotope

Isotopes can be divided into two categories: radioactive and non-radioactive

Radioactive isotopes (radioisotopes) are unstable due to the imbalance of neutrons and protons, which causes the nucleus to decay over time through nuclear fission and emit radiation. Examples of radioisotopes include tritium and carbon-14

Decay occurs at a different rate for each isotope, but the time taken for the radioactivity of an isotope to decrease by 50% is constant for that particular isotope and is known as the half-life

Radioactive isotopes have numerous medical and industrial uses

Non-radioactive isotopes are stable atoms which really only differ in their mass

Uses of Radioactive Isotopes

Medical uses

Radiation is extremely harmful and kills cells so isotopes are used to treat cancer. The isotope cobalt-60 is frequently used for this purpose

Medical tracers as certain parts of the body absorb isotopes and others do not. In this way an isotope can be injected into the blood and its path through the body traced with a radioactive detecting camera, revealing the flow of blood through bodily systems

Medical instruments and materials are routinely sterilized by exposure to radiation, which kills any bacteria present

Industrial uses

Radioactive dating uses the carbon-14 isotope to date carbon-containing materials such as organic matter, rocks and other artefacts. The half-life of C-14 is 5730 years and so this technique is often used to date very old historical objects

Similar to medical use, radioactive tracers are deployed to detect leaks in gas or oil pipes

The radioactive isotope uranium-235 is used as nuclear in power plants in controlled fission reactions

Why Isotopes Share 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 and this is what determines an atom’s chemistry

The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only

Electron Orbits

Electronic structure

We can represent the structure of the atom in two ways: using diagrams called electron shell diagrams or by writing out a special notation called the electronic configuration

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 then the more energy a shell has.

Electrons occupy the shell closest to the nucleus which can hold only 2 electrons and which go in separately

When a shell becomes full electrons then fill the next shell

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 can manage to completely fill this shell with electrons

Electronic configuration

The arrangement of electrons in shells can also be explained using numbers

There is a clear relationship between the outer shell electrons (called valence electrons) and how the Periodic Table is designed

The number of notations in the electronic configuration will show the number of shells of electrons the atom has, showing the Period in which that element is in

The last notation shows the number of outer electrons the atom has, showing the Group that element is in

Elements in the same Group have the same number of outer shell electrons

Groups and periods are two ways of categorizing elements in the periodic table. Periods are horizontal rows (across) the periodic table, while groups are vertical columns (down) the table. Atomic number increases as you move down a group

Period: is a horizontal row of the periodic table.  There are seven periods in the periodic table, with each one beginning at the far left.  A new period begins when a new principal energy level begins filling with electrons.  Period 1 has only two elements (hydrogen and helium), while periods 2 and 3 have 8 elements.  Periods 4 and 5 have 18 elements.  Periods 6 and 7 have 32 elements because the two bottom rows that are separated from the rest of the table belong to those periods.  They are pulled out in order to make the table itself fit more easily onto a single page

Group: is a vertical column of the periodic table, based on the organization of the outer shell electrons.  There are a total of 18 groups.  There are two different numbering systems that are commonly used to designate groups and you should be familiar with both.  The traditional system used in the United States involves the use of the letters A and B.  The first two groups are 1A and 2A, while the last six groups are 3A through 8A.  The middle groups use B in their titles.  Unfortunately, there was a slightly different system in place in Europe.  To eliminate confusion the International Union of Pure and Applied Chemistry (IUPAC) decided that the official system for numbering groups would be a simple 1 through 18 from left to right.  Many periodic tables show both systems simultaneously.

The Noble Gases

The atoms of the Group 8 (or 0) elements all have 8 electrons in their outer valence shells, with the exception of helium which has 2. But since helium has only 2 electrons in total and thus the first shell is full (which is the only shell), it is thus the outer shell so helium also has a full valency shell

All of the noble gases are unreactive as they have full outer shells and are thus very stable

All elements wish to fill their outer shells with electrons as this is a much more stable and desirable configuration

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