0.2 The atom  (Page 4/14)

The actual value of 1 atomic mass unit is $1,67\times {10}^{-24}\mathrm{g}$ or $1,67\times {10}^{-27}\mathrm{kg}$ . This is a very tiny mass!

How big is an atom?

Atomic radius also varies depending on the element. On average, the radius of an atom ranges from $32\mathrm{pm}$ (Helium) to $225\mathrm{pm}$ (Caesium). Using different units, $100\mathrm{pm}=1\mathrm{Angstrom}$ , and $1\mathrm{Angstrom}={10}^{-10}\mathrm{m}$ . That is the same as saying that $1\mathrm{Angstrom}=0,0000000010\mathrm{m}$ or that $100\mathrm{pm}=0,0000000010\mathrm{m}$ ! In other words, the diameter of an atom ranges from $0,0000000010m$ to $0,0000000067m$ . This is very small indeed.

The atomic radii given above are for the whole atom (nucleus and electrons). The nucleus itself is even smaller than this by a factor of about 23 000 in uranium and 145 000 in hydrogen. If the nucleus were the size of a golf ball, then the nearest electrons would be about one kilometer away! This should give help you realise that the atom is mostly made up of empty space.

Atomic structure

As a result of the work done by previous scientists on atomic models (that we discussed in "Models of the Atom" ), scientists now have a good idea of what an atom looks like. This knowledge is important because it helps us to understand why materials have different properties and why some materials bond with others. Let us now take a closer look at the microscopic structure of the atom.

So far, we have discussed that atoms are made up of a positively charged nucleus surrounded by one or more negatively charged electrons . These electrons orbit the nucleus.

The electron

The electron is a very light particle. It has a mass of $9,11\times {10}^{-31}\mathrm{kg}$ . Scientists believe that the electron can be treated as a point particle or elementary particle meaning that it can't be broken down into anything smaller. The electron also carries one unit of negative electric charge which is the same as $1,6\times {10}^{-19}\mathrm{C}$ (Coulombs).

The electrons determine the charge on an atom. If the number of electrons is the same as the number of protons then the atom will be neutral. If the number of electrons is greater than the number of protons then the atom will be negatively charged. If the number of electrons is less than the number of protons then the atom will be positively charged. Atoms that are not neutral are called ions. Ions will be covered in more detail in a later chapter. For now all you need to know is that for each electron you remove from an atom you loose $-1$ of charge and for each electron that you add to an atom you gain $+1$ of charge. For example, the charge on an atom of sodium after removing one electron is $-1$ .

The nucleus

Unlike the electron, the nucleus can be broken up into smaller building blocks called protons and neutrons . Together, the protons and neutrons are called nucleons .

The proton

Each proton carries one unit of positive electric charge. Since we know that atoms are electrically neutral , i.e. do not carry any extra charge, then the number of protons in an atom has to be the same as the number of electrons to balanceout the positive and negative charge to zero. The total positive charge of a nucleus is equal to the number of protons in the nucleus. The proton is much heavierthan the electron (10 000 times heavier!) and has a mass of $1,6726\times {10}^{-27}\mathrm{kg}$ . When we talk about the atomic mass of an atom, we are mostly referring to the combined mass of the protons and neutrons, i.e. the nucleons.