Mada za sehemu hiiDemonstrate mastery of concepts, theories and principles in ChemistryMada 9
- Describe the modern concept of atomic structure (Dalton's atomic structure and sub-atomic particles)
- Describe the concept of electronic arrangements
- Use the concept of atomic structure to determine the atomic and mass numbers of an element
- Explain the concept of chemical formulae and nomenclature (valence, oxidation state, radicals and naming of binary inorganic compounds using the IUPAC system)
- Determine empirical and molecular formulae of common compounds
- Describe the concept of chemical bonding (covalent and electrovalent bonding)
- Describe the concept of chemical reactions (chemical equations, balancing chemical equations, and types of chemical reactions)
- Relate the types of chemical reactions with common processes in daily life such as burning of fuel and digestion in living organisms
- Describe acids, bases and salts (reactions of acids and bases with various substances) and their applications in daily life
The Modern Concept of Atomic Structure
An atom is the smallest particle of an element that can take part in chemical reactions. Modern atomic theory builds on Dalton's ideas by recognizing that atoms are made of smaller sub-atomic particles.
John Dalton proposed the first scientific atomic theory in the early 1800s. His main ideas were:
- All matter is made of tiny particles called atoms
- Atoms of the same element are identical
- Atoms of different elements are different
- Atoms combine in simple whole-number ratios to form compounds
- Atoms cannot be created or destroyed in chemical reactions
Later discoveries modified these ideas. Scientists found that atoms can be split in nuclear reactions, and atoms of the same element can have different masses (isotopes).
Atoms are not solid spheres—they contain smaller particles called sub-atomic particles. There are three main ones:
Protons (p⁺) are positively charged particles found in the nucleus (center) of the atom. The number of protons defines the element.
Neutrons (n) are neutral particles with no charge, also found in the nucleus. They have almost the same mass as protons.
Electrons (e⁻) are negatively charged particles that move around the nucleus in shells or energy levels. Their mass is very small compared to protons.
Properties of Sub-atomic Particles
| Particle | Symbol | Location | Charge | Relative Mass |
|---|---|---|---|---|
| Proton | p⁺ | Nucleus | +1 | 1 |
| Neutron | n | Nucleus | 0 | 1 |
| Electron | e⁻ | Outside nucleus | -1 | 1/1840 |
Atomic number (Z) is the number of protons in an atom. It also equals the number of electrons in a neutral atom.
Mass number (A) is the total number of protons plus neutrons in the nucleus.
The relationship is:
Worked Example 1
An atom of magnesium has atomic number 12 and mass number 24.
(a) Number of protons = 12
(b) Number of electrons = 12 (neutral atom)
(c) Number of neutrons = 24 - 12 = 12
Worked Example 2
An atom Q has mass number 49 and atomic number 24.
(a) Neutron number = 49 - 24 = 25
(b) Number of electrons = 24
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.
Example: Carbon has three isotopes:
- Carbon-12: 6 protons, 6 neutrons
- Carbon-13: 6 protons, 7 neutrons
- Carbon-14: 6 protons, 8 neutrons
All have atomic number 6, but mass numbers 12, 13, and 14 respectively.
Electrons occupy shells (energy levels) around the nucleus. Each shell can hold a maximum of 2n² electrons, where n is the shell number:
- Shell 1 (K): maximum 2 electrons
- Shell 2 (L): maximum 8 electrons
- Shell 3 (M): maximum 18 electrons
Electrons fill the lowest shells first. For the first 20 elements:
| Element | Electrons | Configuration |
|---|---|---|
| Carbon | 6 | 2:4 |
| Oxygen | 8 | 2:6 |
| Sodium | 11 | 2:8:1 |
| Chlorine | 17 | 2:8:7 |
Understanding atomic structure helps explain how elements behave in chemical reactions. In Tanzania, knowledge of isotopes is used in medicine—radioactive iodine-131 is used at hospitals like Bugando Medical Centre to diagnose and treat thyroid diseases. The same principle applies in agriculture, where farmers benefit from research using phosphorus-32 tracers to study how crops absorb nutrients from fertilizers.
Swali
What is the relative mass of a neutron compared to a proton?
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