Atomic Structure
Atoms and Molecules
An atom is the smallest particle of an element that can exist. All substances are made of atoms.
A molecule is formed when two or more atoms join together by chemical bonds. Molecules can consist of atoms of the same element (e.g. O2) or different elements (e.g. H2O).
Structure of the Atom
Every atom is made up of three types of subatomic particles:
| Particle | Relative Mass | Relative Charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 (neutral) | Nucleus |
| Electron | 1/1840 (negligible) | -1 | Shells around the nucleus |
- The nucleus is at the centre of the atom and contains protons and neutrons. It is positively charged overall.
- Electrons move rapidly around the nucleus in shells (energy levels). Their mass is so small it is considered negligible.
- Atoms have no overall charge because the number of protons equals the number of electrons.
Exam Tip
MASS = MASSIVE. The mass number is always the bigger of the two numbers on the periodic table; the smaller number is the atomic number.
Key Terms
Atomic number (proton number): the number of protons in the nucleus of an atom. This is unique to each element.
Mass number (nucleon number): the total number of protons and neutrons in the nucleus.
Since atoms are electrically neutral, the number of electrons always equals the number of protons.
Example: Lithium has atomic number 3 and mass number 7.
- Protons = 3
- Electrons = 3
- Neutrons = 7 - 3 = 4
Isotopes and Relative Atomic Mass
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have identical chemical properties but different masses.
Relative atomic mass (Ar) is the average mass of one atom of an element, taking into account the abundance of all its isotopes. It is relative to 1/12 the mass of a carbon-12 atom.
Calculating Relative Atomic Mass
Example: Chlorine exists as 75% Cl-35 and 25% Cl-37.
The Periodic Table
Elements are arranged in the Periodic Table:
- In order of increasing atomic number
- In groups (vertical columns): elements in the same group have the same number of electrons in their outer shell and therefore similar chemical properties
- In periods (horizontal rows): elements in the same period have the same number of electron shells
The group number (for main group elements) gives the number of electrons in the outer shell. The period number gives the number of occupied electron shells.
Metals and Non-metals
The Periodic Table is divided by a diagonal “staircase” line:
- Metals are on the left and towards the bottom. They tend to have 1-3 outer shell electrons, form positive ions, conduct electricity, have basic oxides, and are typically solid, lustrous and malleable.
- Non-metals are on the right and towards the top. They tend to have 4-7 outer shell electrons, form negative ions or share electrons, have acidic oxides, and are typically non-conductive.
| Property | Metals | Non-metals |
|---|---|---|
| Outer shell electrons | 1-3 | 4-7 |
| Ion formation | Lose electrons (positive ions) | Gain or share electrons |
| Electrical conductivity | Good conductors | Poor conductors (exceptions: graphite, graphene) |
| Oxide character | Basic | Acidic (some neutral) |
| Physical state at room temperature | Mostly solid (except mercury) | Solid, liquid or gas |
Electronic Configurations
Electronic configuration describes how electrons are arranged in shells around the nucleus. Electrons fill shells starting with the one closest to the nucleus.
- Shell 1 holds a maximum of 2 electrons
- Shell 2 holds a maximum of 8 electrons
- Shell 3 holds a maximum of 8 electrons (simplified model for first 20 elements)
Electronic configurations of the first 20 elements:
| Element | Atomic No. | Electronic Configuration |
|---|---|---|
| Hydrogen | 1 | 1 |
| Helium | 2 | 2 |
| Lithium | 3 | 2,1 |
| Beryllium | 4 | 2,2 |
| Boron | 5 | 2,3 |
| Carbon | 6 | 2,4 |
| Nitrogen | 7 | 2,5 |
| Oxygen | 8 | 2,6 |
| Fluorine | 9 | 2,7 |
| Neon | 10 | 2,8 |
| Sodium | 11 | 2,8,1 |
| Magnesium | 12 | 2,8,2 |
| Aluminium | 13 | 2,8,3 |
| Silicon | 14 | 2,8,4 |
| Phosphorus | 15 | 2,8,5 |
| Sulfur | 16 | 2,8,6 |
| Chlorine | 17 | 2,8,7 |
| Argon | 18 | 2,8,8 |
| Potassium | 19 | 2,8,8,1 |
| Calcium | 20 | 2,8,8,2 |
Reading Electronic Configuration from the Periodic Table
- The number of notations (separated by commas) = number of electron shells = period number
- The last notation = number of outer shell electrons = group number
Example: Chlorine has electronic configuration 2,8,7.
- Three notations, so it is in Period 3
- The last notation is 7, so it is in Group 7
Chemical Properties and Groups
Elements in the same group have the same number of outer shell electrons. This gives them similar chemical properties because it is the outer electrons that determine how an element reacts.
Noble gases (Group 0): Helium has 2 outer electrons; all other noble gases have 8 outer electrons. This full outer shell is extremely stable, so noble gases do not readily react. They are:
- Monatomic (exist as single atoms)
- Colourless, non-flammable gases at room temperature
- Very unreactive (inert)
Exam Questions
Q: An atom has atomic number 17 and mass number 35. State the number of protons, electrons and neutrons.
Protons = 17, Electrons = 17, Neutrons = 35 - 17 = 18.
Q: Define the term isotope.
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
Q: A sample of rubidium contains 72% Rb-85 and 28% Rb-87. Calculate the relative atomic mass.
Ar = ((72 x 85) + (28 x 87)) / 100 = (6120 + 2436) / 100 = 8556 / 100 = 85.6
Q: Explain why elements in the same group of the Periodic Table have similar chemical properties.
Elements in the same group have the same number of electrons in their outer shell. It is the outer electrons that determine how an element reacts, so elements with the same number of outer electrons undergo similar reactions.
Q: Explain why noble gases do not readily react.
Noble gases have a full outer shell of electrons (8 electrons, or 2 for helium). This is a very stable arrangement, so they do not need to gain, lose or share electrons, and therefore do not readily form chemical bonds.
Exam Tip
The group number tells you the number of outer electrons; the period number tells you the number of electron shells. You should be able to write the electronic configuration of any of the first 20 elements from their position in the Periodic Table, and vice versa.