Atoms

Atoms contain protons, neutrons and electrons

Atoms have a radius of about 10 nanometres(1x10(-9)). There are a few different modern models of the atom.

The nucleus

• It’s in the middle of the atom
• It contains protons and neutrons
• The nucleus has a radius of around 1x10(-14) which is around 1/10,000 the radius of an atom
• It has a positive charge because of the protons
• Almost the whole mass of the atom is concentrated in the nucleus

The electrons

• Move around the nucleus in electron shells
• They’re negatively charges and tiny, but they cover a lot of space
• Electrons have virtually no mass

Particles, relative mass and charge

Proton:1 and +1

Neutron:1 and 0

Electron:Very small and -1

Number of protons equals number of electrons

• Atoms are neutral-they have no charge overall(unlike ions).
• This is because they have the same number of protons as electrons
• The charge on the electrons is the same size as the charge on the protons, but opposite-so the charges cancel out
• In an ion, the number of protons doesn’t equal the number of electrons. This means it has an overall charge. For example, an ion with a 2- charge, has two more electrons than protons

Atomic Numbers and Mass Number describe an Atom

• The nuclear symbol of an atom tells you its atomic(proton) number and mass number
• The atomic number tells you how many protons there are
• The mass number tells you the total number of protons and neutrons in the atom
• To get the number of neutrons, just subtract the atomic number from the mass number