Everything is made of atoms, which are far too small to see even with a light microscope. An atom has a tiny, dense nucleus in the middle, made of protons and neutrons. Electrons move around the nucleus in shells. Protons have a positive charge, electrons a negative charge, and neutrons no charge. An atom has the same number of protons and electrons, so overall it is neutral.
The number of protons is the atomic number, and it decides which element the atom is: every carbon atom has 6 protons. The mass number is protons plus neutrons. Most of an atom is empty space; if an atom were the size of a football stadium, the nucleus would be like a pea in the middle.
Worked example
An atom has 11 protons and 12 neutrons. What are its atomic number and mass number?
- Atomic number = number of protons = 11.
- Mass number = protons + neutrons = 11 + 12.
Answer:Atomic number 11, mass number 23 (it is sodium).
Key words
- proton
- positive particle in the nucleus
- neutron
- neutral particle in the nucleus
- electron
- negative particle in shells around the nucleus
- atomic number
- the number of protons
- mass number
- protons plus neutrons
New questions every time.
NGSS-referenced: MS-PS1-1