Building+Blocks-+Atomic+Stucture


 * BUILDING BLOCKS - ATOMIC STRUCTURE **

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All matter is made up of tiny particles called atoms. Each atom is made up of a central nucleus of protons (which are positively charged) and neutrons (which are neutral or uncharged). Surrounding the nucleus are negatively charged electrons which move in high speed orbits in the space around the nucleus.
 * Atoms **

The following table shows the relative mass and charge of the sub-atomic particles.


 * **Sub-atomic particle ** ||  **Charge **  ||  **Mass (compared to a proton) **  ||  **Where found **  ||
 * proton || +1  || 1  || In the nucleus  ||
 * neutron || 0  || 1  || <span style="display: block; font-family: Arial,sans-serif; font-size: 11pt; text-align: center;">In the nucleus  ||
 * <span style="display: block; font-family: Arial,sans-serif; font-size: 11pt; text-align: center;">electron || <span style="display: block; font-family: Arial,sans-serif; font-size: 11pt; text-align: center;">-1  ||   || <span style="display: block; font-family: Arial,sans-serif; font-size: 11pt; text-align: center;">Moving about the nucleus  ||

<span style="font-family: Arial,sans-serif; font-size: 11pt;">All atoms are electrically neutral as they contain the same number of protons as electrons.


 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Atomic number **<span style="font-family: Arial,sans-serif; font-size: 11pt;"> **(Z)** of an atom is the number of protons in the nucleus. For a neutral atom it is also equal to the number of electrons. Since it is the number of electrons that gives an atom its chemical properties, all atoms with a given atomic number will react in the same manner. Therefore the atomic number characterises the atoms of a particular element, e.g. all atoms with atomic number 8 are atoms of the element oxygen. The O2- ion will also have the atomic number 8 since the ion is formed by gaining 2 electrons, without any change to the number of protons in the nucleus.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Mass Number (A) **<span style="font-family: Arial,sans-serif; font-size: 11pt;">is the number of protons plus neutrons in the nucleus. Sodium, Na, has 11 protons and 12 neutrons so has mass number 23 and atomic number 11. This is written as

<span style="font-family: Arial,sans-serif; font-size: 11pt;">For any atom the number of neutrons is **(A - Z).**

<span style="font-family: Arial,sans-serif; font-size: 11pt;">All the atoms of an element have the same number of protons (atomic number) but may have different numbers of neutrons resulting in different mass numbers e.g. Carbon 12 and Carbon 14. Such atoms are called isotopes. They have the same number of electrons so their chemical properties are identical. <span style="font-family: Arial,sans-serif; font-size: 11pt;">Some isotopes are unstable and are radioactive. This means they decay spontaneously by emitting sub-atomic particles and radiation from the nucleus. Radioactive isotopes have many uses including the treatment of cancer, in the dating of fossils, in industry to determine the thickness of paper and plastic.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Isotopes **

<span style="font-family: Arial,sans-serif; font-size: 11pt;">The electrons are always moving rapidly around the nucleus. It is found that groups of electrons have similar amounts of energy and can therefore be located in energy levels. These energy levels correspond to volumes of space where the electrons are most likely to be found. The electrons always fill the lowest energy levels (nearest the nucleus) first.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Electron Arrangement. **

<span style="font-family: Arial,sans-serif; font-size: 11pt;">The maximum number of electrons in each level varies - <span style="font-family: Arial,sans-serif; font-size: 11pt;"> 2 electrons in the first level <span style="font-family: Arial,sans-serif; font-size: 11pt;"> 8 electrons in the second level <span style="font-family: Arial,sans-serif; font-size: 11pt;">18 electrons in the third level – (although only 8 electrons enter this level before electrons begin filling the 4th level).

<span style="font-family: Arial,sans-serif; font-size: 11pt;">The electron arrangement (or configuration) of an atom is a description of which energy levels the <span style="font-family: Arial,sans-serif; font-size: 11pt;">electrons occupy e.g. Na has 11 electrons with electron arrangement 2.8.1. //<span style="font-family: Arial,sans-serif; font-size: 11pt;">Exercise: Give the electron arrangement of //

//<span style="font-family: Arial,sans-serif; font-size: 11pt;">P Ar Ca Cl //

<span style="font-family: Arial,sans-serif; font-size: 11pt;">The **valence electrons** are those in the highest energy level (furthest from the nucleus). These are the electrons involved in chemical reactions where atoms generally react to achieve a filled valence shell. This may be done by gaining or losing electrons (to form ions and hence ionic compounds) or by sharing electrons in covalent bonds (and forming molecules or covalent networks).

<span style="font-family: Arial,sans-serif; font-size: 11pt;">When an atom gains or loses electrons it forms an ion, which as completely different properties and behaviour to the original atom.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">In formation **
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">A **cation,** which is positively charged, forms when electrons are lost e.g. when a Na atom loses its one valence electron it forms Na+. Metal atoms form cations.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">An **anion,** which is negatively charged, forms when electrons are gained e.g.O2ˉ is formed when an O atom, having 6 electrons in its valence shell, gains 2 electrons. Non-metal atoms form anions.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">If two different ions have the same number of electrons they are said to be **isoelectronic.**