# Summary of session 1

1. All matter is made up of tiny particles called atoms which are composed of a central positively charged nucleus surrounded by negatively charged electrons.
2. The nucleus comprises positively charged protons and neutrons carrying no charge. The masses of protons and neutrons are the same, but the mass of electrons is negligible in comparison. The number of protons must equal the number of electrons, where there is an imbalance the species are referred to as ions.
3. The number of protons in the nucleus is the atomic number of an atom, and a substance made up of atoms of the same atomic number is an element.
4. Atoms of the same element that differ only in the number of neutrons they contain are known as isotopes. They have different atomic masses to other isotopes of the same element.
5. Electrons are not located in a precise orbits around the nucleus, rather in regions of space where there is a high probability of finding them – these are atomic orbitals.
6. Electrons in atoms are labelled by a unique set of quantum numbers.
7. The arrangement of electrons in the atomic orbitals of an atom is known as its electronic configuration. The outermost electrons which are involved in chemical bonding are known as valence electrons. The remainder are the core electrons.
8. In the periodic table, elements are arranged into horizontal rows called periods and vertical columns called groups. Patterns in the chemical and physical properties of the elements is revealed by the appearance of similar elements in the same Group.
9. Different solid forms of the same element are known as allotropes.

OpenLearn - Discovering chemistry
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