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tags: [physics]
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# Matter and atoms

## Matter, elements, compounds and mixtures

- Matter is anything that occupies space and has weight. It may be found in any
  one of three states: liquid, solid, or gas.
- Matter can be classed in terms of **elements**, **compounds** and
  **mixtures**.
  - An element is a substance that cannot be reduced to a simpler substance by
    chemical means.
  - The chemical combination of two or more elements is a compound. Compounds
    _can_ be separated or reduced by chemical means but they cannot be
    physically separated.
  - A _physical_ (note not _chemical_) combination of elements and compounds is
    a mixture.
  - For example "air" is a mixture which is made up of oxygen, nitrogen, carbon
    dioxide, and other gases.

## Atoms and molecules

- Elements are made up of **atoms**.
- A **molecule** is a type of compound where the atoms of two types of elements
  have joined together. For example sodium and chlorine are two elements that
  can join to form sodium chloride. We call this **chemical bonding**.

## Atomic particles
![](static/atom-diagram.svg)



- There are three particles comprising atoms: **protons**, **neutrons** and
  **electrons**.

- Protons and neutrons comprise the nucleus of the atom. A proton is positively
  charged. A neutron has no charge.

- Electrons orbit around the nucleus in concentric rings. Electrons are
  negatively charged. Because of this protons and electrons attract each other.

### Atomic number

- The number of protons in the nucleus is called the element's **atomic**
  number.
- We use the atomic number of elements to distinguish one element from another.

### Atomic weight

- The atomic weight of an atom is its mass.
- The mass is determined by the number of protons and neutrons in the nucleus.
- An electron $\frac{1}{1845}$ the mass of a proton. Its mass is so neglible
  that it makes no significant difference to the atomic weight.
