atomic structure

What is an atom

An atom is the smallest component of an element having the chemical properties of the element. 

Basic Structure of an Atom

Basic Structure of an Atom 

• An atom has a nucleus at the centre containing neutrons and protons with one or more electrons  circulating around the nucleus in electron shells or at different energy levels.
• Protons are positively charged. 
• Neutrons have no charge. 
• Electrons are negatively charged.
• The electrons are held by an electrostatic force of attraction by its negative charge and positive charge of protons.
• An atom is electrically neutral because the same proton number (positive charge) in the nucleus is  balanced by the electron number (negative charge) in the shells.
  
  
  

How to identify atoms of different elements

• Proton number also called atomic number.
• The atomic number (proton number) determines the identity of an element. 
• Each element has different proton number and no two different elements have the same proton number.
• For examples, hydrogen has one proton number, carbon has six proton number and chlorine has seventeen proton number (see image below).

Hydrogen  has one (proton number) atomic number

Carbon has six (proton number) atomic number

Chlorine has 17 (proton number) atomic number

• In the Periodic Table, the elements are arranged in order of increasing atomic number.

Periodic Table

Relative charges and approximate relative masses of protons, neutrons and electrons

• Since an atom is too light to be measured with a weighing scale, relative atomic mass (amu, symbol Ar ) is used. 
• The relative atomic mass is the average mass of the atom of an element, relative to the mass of an atom of carbon-12. 
• 1 amu = 1/12 the mass of a carbon-12 atom

      The relative mass of a carbon-12 atom = 12 ( 6 protons + 6 neutrons )

      The relative mass of a hydrogen atom = 1 ( 1 proton + 0 neutron )

            Meaning, the average mass of a hydrogen atom is 1/12 of the mass of a carbon-12 atom.

      The relative mass of a magnesium atom = 24 (12 protons + 12 neutrons)

            Meaning, the average mass of a magnesium atom is twice the mass of a carbon-12 atom.

• Infact, the relative mass of an atom are the total number of protons and neutrons in the nucleus. 
• Electrons are omitted because their mass is so tiny. 
• A proton and a neutron weighted almost the same with a relative mass of 1 but an electron weights only 1/1837 of a proton.

Sub-atomic  Particle	Relative Mass	Relative Charge	Symbol
Proton	1 amu	+1	p
Neutron	1 amu	0	n
Electron	1/1837  amu	-1	e-

• Protons and neutrons are in the nucleus of the atom, so they are called nucleons.                                           Nucleon number (mass number/ atomic mass) =  protons + neutrons

• The proton number and nucleon number of an element are often expressed in a short way as the following:

                              

            Example 1:  A hydrogen element

An atomic structure of hydrogen element                  Hydrogen element written on a short way


     Example 2:  A helium element

List of the atoms of the first 20 elements in the Periodic Table, arranged in order of increasing atomic number.

Element	Symbol	Atomic number    (Protons)	Electrons	Neutrons	Nucleon number (protons+neutrons)
hydrogen	H	1	1	0	1
helium	He	2	2	2	4
lithium	Li	3	3	4	7
beryllium	Be	4	4	5	9
boron	B	5	5	6	11
carbon	C	6	6	6	12
nitrogen	N	7	7	7	14
oxygen	O	8	8	8	16
fluorine	F	9	9	10	19
neon	Ne	10	10	10	20
sodium	Na	11	11	12	23
magnesium	Mg	12	12	12	24
aluminium	Al	13	13	14	27
silicon	Si	14	14	14	28
phosphorus	P	15	15	16	31
sulphur	S	16	16	16	32
chlorine	Cl	17	17	18	35
argon	Ar	18	18	22	40
potassium	K	19	19	20	39
calcium	Ca	20	20	20	40

Electron Configuration/ Electron Structure 

Electrons move very fast circulating the nucleus. Electrons can only occupy certain energy levels (shells) and cannot exist between different levels (shells) at the same time. Each energy levels (shells) can hold only up to a certain number of electrons.

Chlorine atom, electron configuration: 2,8,7

First shell, the closest to the nucleus, has the lowest energy level, holds up to 2 electrons.

Second shell, has higher energy level, holds up to 8 electrons.

Third shell can also holds up to 8 electrons.

The further a shell is from the nucleus, the higher the energy level.

Electron configuration means the arrangement of electrons in an atoms.

For any atom, the electrons fill the energy levels starting from the lowest energy (innermost shell). When the first shell is full with two electrons, the remaining electrons start filling the second energy level (second shell) up to a maximum of 8 electrons. Then, electrons continue filling the third energy level and so-on. As the picture shown above, chlorine atom contains 17 electrons. 2 of its electrons fill the first shell, then the next 8 electrons fill the second shell and the remainder seven electrons fill the third shell. The electron configuration for chlorine can be written  as 2,8,7

Electron Configuration/ Electronic Structure (first 20 atoms)

Electron Configuration and The Periodic Table

The horizontal raw defined as Period. Atoms of the same Period have the same number of shells.

• All atoms of Period 1 have one shell
• All atoms of Period 2 has two shells and so-on. 

The vertical column defined as Group. Atoms of the same Group have the same number of valency electrons (the same number of electrons in the outermost shell or the last shell).

• All atoms of Group 1 have one valency electron (one electron in the outermost shell or the last shell).
• All atoms of Group 2 have two valency electrons (two electrons in the outermost shell or the last shell).

Define Isotopes

• Isotopes are atoms of the same element with the same proton number but different neutron number 

         or

• The atoms of the same elements with different nucleon numbers or mass numbers due to different neutron number.

     Isotopes of Hydrogen:

Hydrogen

Atomic Mass 1

1 proton, 0 neutron, 1 electron

Tritium, an isotope of Hydrogen

Atomic Mass 2

1 proton, 1 neutron, 1 electron

Deuterium, an isotope of Hydrogen

Atomic Mass 3

1 proton, 2 neutrons, 1 electron

     Isotopes of Carbon:

Isotopes: Identical Chemical Properties

Isotopes of the same element have identical chemical properties. This is because isotopes of the same element have the same electron distribution. In other words, they have the same proton number and same number of electrons.

There are two types of isotopes.

1. Stable isotopes
2. Unstable isotopes which is called radioisotopes 

• unstable isotopes due to extra neutrons in the nuclei which are radioactive.
• examples: cobalt-60 used in radiotherapy treatment and uranium-235 used as a source of nuclear power.

Use of Radioisotopes in Industry  (Note: Not carbon dating)

1. Detection of leaks 
2. Monitor of thickness of paper production
3. Nuclear fuel for generating electricity
4. Radiographs of welds
5. Measuring wear
6. Sterilising  food (food preservation)

Use of Radioisotopes in Medical Practice

1. Medical treatment of cancer 
2. Radiotherapy
3. Treatment of thyroid gland
4. X rays
5. Tracer studies in body
6. Sterilising equipment
7. Locating tumours

For revision: Atomic Structure - IGCSE past year questions, paper 1