Atomic Structure and the Periodic Table
Atoms, elements, compounds, mixtures, atomic models, and the periodic table.
In this topic
Atoms, Elements, and Compounds
Everything is made of atoms. An atom is the smallest part of an element that can exist.
An element contains only one type of atom. There are about 100 different elements, listed in the periodic table.
A compound is formed when two or more elements are chemically bonded together. Compounds can only be separated by chemical reactions, not physical methods.
A mixture contains two or more substances that are NOT chemically bonded. They can be separated by physical methods: filtration, distillation, chromatography, evaporation.
Key Points
- Atoms have a nucleus (protons + neutrons) surrounded by electrons
- Proton: positive charge, relative mass 1
- Neutron: no charge, relative mass 1
- Electron: negative charge, relative mass ~0 (1/1836)
- Atomic number = number of protons; Mass number = protons + neutrons
Example Questions
3An atom of sodium has a mass number of 23 and an atomic number of 11. How many protons, neutrons, and electrons does it have?
Protons = 11 (atomic number). Neutrons = 23 - 11 = 12. Electrons = 11 (same as protons in a neutral atom).
[3 marks]
2Explain the difference between a compound and a mixture.
A compound is formed when two or more elements are chemically bonded in fixed proportions and can only be separated by chemical reactions. A mixture contains substances that are not chemically bonded, retain their own properties, and can be separated by physical methods.
[2 marks]
Development of Atomic Models
Our understanding of atoms has evolved over time:
1. Dalton (1803): atoms are tiny solid spheres that cannot be divided 2. Thomson (1897): 'plum pudding model' — positive sphere with negative electrons embedded (discovered electrons using cathode rays) 3. Rutherford (1911): nuclear model — most of the atom is empty space with a small, dense, positive nucleus (alpha particle scattering experiment) 4. Bohr (1913): electrons orbit the nucleus in fixed energy levels (shells) 5. Modern model: electron cloud model — electrons exist in orbitals/probability clouds
Key Points
- Each model was developed based on new experimental evidence
- Rutherford's gold foil experiment was key — most particles passed through
- The few particles that bounced back proved a small, dense nucleus
- Chadwick discovered the neutron in 1932
Example Questions
2In Rutherford's alpha particle scattering experiment, most particles passed straight through the gold foil. What does this tell us about the atom?
Most of the atom is empty space. The nucleus is very small compared to the overall size of the atom, so most particles pass through without hitting anything.
[2 marks]
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
For example, carbon has three isotopes: - Carbon-12: 6 protons, 6 neutrons (most common) - Carbon-13: 6 protons, 7 neutrons - Carbon-14: 6 protons, 8 neutrons (radioactive, used in carbon dating)
Isotopes have identical chemical properties (same number of electrons) but different physical properties (different mass).
Relative atomic mass (Ar) is the weighted average mass of all isotopes of an element.
Key Points
- Same element = same number of protons
- Different isotope = different number of neutrons
- Chemical properties depend on electrons, not neutrons
- Some isotopes are radioactive (unstable nucleus)
Example Questions
2Chlorine exists as two isotopes: Cl-35 (75%) and Cl-37 (25%). Calculate the relative atomic mass.
Ar = (35 × 75 + 37 × 25) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5
[2 marks]
The Periodic Table
The periodic table arranges elements in order of increasing atomic number. Elements with similar properties are in the same group (column).
Groups: - Group 1 (Alkali metals): Li, Na, K — very reactive metals, react with water - Group 7 (Halogens): F, Cl, Br, I — reactive non-metals, form salts with metals - Group 0 (Noble gases): He, Ne, Ar — unreactive, full outer shells
Periods are horizontal rows. As you go across a period, elements change from metals to non-metals.
Metals are on the left, non-metals on the right. The transition metals are in the middle block.
Key Points
- Group number = number of electrons in outer shell
- Period number = number of electron shells
- Group 1 reactivity INCREASES going down (easier to lose outer electron)
- Group 7 reactivity DECREASES going down (harder to gain an electron)
- Noble gases are stable — full outer electron shells
Example Questions
3Explain why sodium is more reactive than lithium.
Sodium has more electron shells than lithium, so its outer electron is further from the nucleus and more easily lost. There is also more shielding from inner electrons, reducing the attraction from the nucleus. This makes it easier for sodium to lose its outer electron and form an ion.
[3 marks]
3Explain why fluorine is more reactive than chlorine.
Fluorine has fewer electron shells, so incoming electrons are closer to the nucleus with less shielding. The attraction between the nucleus and the incoming electron is stronger, making it easier to gain an electron and form a negative ion.
[3 marks]
Electronic Structure
Electrons are arranged in energy levels (shells) around the nucleus.
The first shell holds up to 2 electrons. The second shell holds up to 8 electrons. The third shell holds up to 8 electrons.
Electrons fill the lowest energy level first, then the next, and so on.
Examples: - Sodium (11): 2, 8, 1 - Chlorine (17): 2, 8, 7 - Calcium (20): 2, 8, 8, 2
The electronic structure determines how an element reacts — elements want a full outer shell.
Key Points
- Atoms react to achieve a full outer shell (like noble gases)
- Metals LOSE electrons to form positive ions
- Non-metals GAIN electrons to form negative ions
- The number of outer electrons determines the group
Example Questions
2Write the electronic structure of an aluminium atom (atomic number 13) and an aluminium ion (Al³⁺).
Al atom: 2, 8, 3. Al³⁺ ion: 2, 8 (it has lost 3 electrons from its outer shell to achieve a full outer shell like neon).
[2 marks]