6.1-6.6 - Atomic structure and isotopes

6.1-6.6 - Atomic structure and isotopes

Build a model of the atom, compare its particles and use nuclear notation to distinguish elements and isotopes. Charge and mass describe different properties: understanding that distinction explains why an atom is neutral.

Inside an atom

Imagine dividing a tiny grain of graphite again and again until the pieces are too small to see with any optical microscope. At the atomic scale, the useful model is no longer a solid miniature grain.

An atom has a very small, positively charged nucleus at its centre. The nucleus contains positively charged protons and uncharged neutrons. Negatively charged electrons surround the nucleus.

The nucleus is much smaller than the whole atom, yet it contains almost all the atom's mass. A typical atomic radius is of order 10−1010^{-10} m, while a typical nuclear radius is of order 10−1510^{-15} m. Their ratio is

10−1010^{-10} / 10−1510^{-15} = 10510^{5} = 100 000.

So the atom's radius is about one hundred thousand times the nuclear radius in this comparison. A diagram cannot show the nucleus and atom to the same scale while keeping both visible: any classroom atom drawing is a model, with sizes deliberately exaggerated.

Atoms and small molecules have a typical size of order 10−1010^{-10} m. This means their size is around a few tenths of a nanometre because

1 nm = 10−910^{-9} m, so 0.1 nm = 10−1010^{-10} m.

Particle mass and charge

The four particles required in this topic have these relative properties:

ParticleUsual location or role hereRelative electric chargeRelative mass
protonnucleus+11
neutronnucleus01
electronsurrounds nucleus-1about 1/1836, or 0.0005
positronelectron's antiparticle; not an ordinary part of an atom+1same as an electron: about 1/1836, or 0.0005

At GCSE scale the electron and positron masses are often described as negligible or almost zero relative to a proton. This does not mean they have no mass. It explains why almost all the mass of an ordinary atom is in its protons and neutrons inside the nucleus.

How charge balances in a neutral atom

An atom is neutral because it contains the same number of protons and electrons. Each proton contributes relative charge +1, each electron contributes -1, and each neutron contributes 0.

For an atom with 8 protons, 10 neutrons and 8 electrons:

  • total proton charge is 8 × (+1) = +8;
  • total electron charge is 8 × (-1) = -8;
  • neutron charge is 0;
  • total relative charge is +8 + (-8) = 0.

The positive and negative charges balance because their amounts are equal and their signs are opposite. Neutrons add mass but do not help to balance charge.

Continue this lesson.

You’re previewing NovaLearn without an account. Create a free account to read the rest.

Create free account