Atomic Structure: Atoms
Understanding Atoms
- An atom is the smallest part of an element that can exist, typically around a hundred millionth of a centimetre across.
- Every atom is made of three fundamental particles: protons, neutrons, and electrons.
- The nucleus at the centre of the atom contains the protons and neutrons.
- The electrons move in energy levels around the nucleus, also known as electron shells.
Atomic Number & Mass Number
- The atomic number (Z) of an atom is the number of protons in its nucleus. It gives the identity of the atom.
- The mass number (A) is the total number of protons and neutrons in the nucleus.
- An atom is neutral because it contains equal numbers of protons (positively charged) and electrons (negatively charged).
Isotopes & Ionisation
- Isotopes are atoms of the same element with different numbers of neutrons, and hence different mass numbers.
- Isotopes of an element have identical chemical reactions because they have the same electron arrangement.
- Ionisation occurs when an atom gains or loses electrons, becoming an ion. Positive ions (cations) form by losing electrons, negative ions (anions) form by gaining electrons.
Atomic Model Evolution
- The atomic model has changed as scientists have discovered new information.
- JJ Thomson’s 'Plum Pudding' Model discovered that atoms contain electrons. However, he believed these were embedded in a sphere of positive charge.
- Rutherford's Alpha Particle Scattering Experiment showed that most of the atom is empty space with a tiny, dense, positive nucleus.
- Niels Bohr improved Rutherford's model by placing electrons in specific orbits, or energy levels. The modern atomic model was based on Bohr's work.
Using Mass Spectrometry
- Mass spectrometry can be used to find relative isotopic abundance and identify elements.
- This method supports theories about atomic structure and isotopes.
- It helps to calculate the relative atomic mass of an element from the relative abundances of its isotopes.
Understanding Energy Levels
- Electrons fill the energy levels in a specific order – from lower to higher.
- Each energy level can hold only a certain number of electrons: 2 in the first level and generally 8 in the others.
- Electron configuration (distribution of electrons in energy levels) affects the chemical properties of atoms.