What atoms are made of and how their electrons are arranged
Atoms contain three subatomic particles. Electrons are negatively charged and very light, protons are positively charged, and neutrons have no charge. Thomson’s model pictured the atom like a watermelon with electrons as seeds. Rutherford’s gold foil experiment showed instead that the atom is mostly empty space with a tiny, dense, positive nucleus. Bohr then proposed that electrons move in fixed shells, called K, L, M and N, without losing energy.
The atomic number Z is the number of protons, and the mass number A is the total of protons and neutrons. Isotopes have the same atomic number but different mass numbers, like carbon-12 and carbon-14, while isobars have the same mass number but different atomic numbers. Electrons fill shells following the 2n² rule, so the K shell holds 2 and the L shell holds 8. Sodium, with 11 electrons, has the configuration 2, 8, 1. Valency is the combining capacity of an atom, decided by the electrons in its outermost shell.
Why it matters
Knowing atomic structure explains why elements react the way they do and why some form ions. Isotopes are used in medicine, such as iodine-131 for thyroid treatment and cobalt-60 for cancer therapy, and in nuclear power. Electronic configuration and valency are needed to write chemical formulae and to understand the periodic table and chemical bonding.
Key terms
- Atomic number
- The number of protons in the nucleus of an atom.
- Mass number
- The total number of protons and neutrons in an atom’s nucleus.
- Isotopes
- Atoms of the same element with the same atomic number but different mass numbers.
- Valency
- The combining capacity of an atom, based on the electrons in its outer shell.