Atoms and Molecules

Week 1 OHPs

All atoms of any element contain the same number of protons. The number of protons is equal to the Atomic Number of the element. Each proton has a charge of +1.

In a neutral (uncharged) atom, the number of surrounding electrons (each having a charge of -1) must equal the number of nuclear protons (each having a charge of +1). Therefore the number of electrons equals the Atomic Number.

The nucleus of an atom contains uncharged neutrons which help hold the protons together. The number of neutrons may vary, giving rise to isotopes of the element which have the same atomic number but different atomic weights. The atomic weight is given by the sum of the protons and neutrons. In this table only the most common isotopes are shown.

Electrons occupy shells (i.e. different volumes of space at increasing distances) around the nucleus. Electrons in inner shells help shield the outer electrons from the full nuclear charge.

The stronger the nuclear charge, the closer the electrons are held to the nucleus; therefore the atoms get smaller as they increase their unshielded nuclear charge (i.e. from left to right). They also get larger as the number of inner electron shells increase (i.e. from top to bottom; e.g. compare Li and Na), as the electrons are forced further out.

The ionisation potential expresses the difficulty in removing an electron from an uncharged atom. It can be seen that the difficulty increases as the unshielded nuclear charge increases (i.e. from left to right). This becomes less so when the atoms are larger when the outer electrons are not held so strongly due to being further from the nuclear charge. Note that there is are minor discrepencies to this behaviour between Be and B due to the completion of the s orbital, and between N and O due to single electrons on the 3 p orbitals being slightly more stable than pairs of electrons within these orbitals.

The electron affinity expresses the ease in adding an electron to an uncharged atom. It can be seen that the ease increases as the unshielded nuclear charge increases (i.e. from left to right). This becomes more so when the atoms are larger when the outer electrons are not held so close together. Note that there is are minor discrepencies to this behaviour with Be and Mg due to the completion of the s orbital, and with N and P due to single electrons on the 3 p orbitals being slightly more stable than pairs of electrons within these orbitals.

The electronegativity expresses the balance between adding and losing electrons. Elements with low electronegativities lose electrons to elements with higher electronegativities (e.g. Li will lose an electron to F if placed in contact).

The size of the ions depend on their nuclear charge; i.e. ions with the same number of electrons but higher nuclear charge will be smaller.

When atoms lose electrons they become smaller as the remaining electrons are held more tightly than the electron(s) lost.

When atoms gain electrons they are held less strongly than the other electrons and are therefore further from the nucleus,giving rise to larger ions,

Home | Back

This page was last updated by Martin Chaplin
on 11 February, 2005