Atomic structure

There are 112 elements although elements 110-112 are as yet unnamed. These 112 elements are organized in the periodic table:

The modern chemical symbols were introduced by Berzelius. Rows of elements are called "periods" and columns of elements are called "groups" (1A, 2A 3B etc.). There are three general classes of elements distinguished by their physical properties: the metals (generally shiny and conduct electricity), the non metals (not shiny, sometimes gasses at STP and poor conductors of electricity) and the metalloids (properties in between those of metals and non metals.). Some groups have special names:

Many of the heavier elements are unstable - which means that the atoms of those elements break apart very quickly.

Elements within a group share similar chemical properties. Other chemical and physical properties of the elements can be deduced from their position in the periodic table. The structure of the periodic table and thus their chemical and physical properties is directly related to their atomic structure.

Atomic Weights

Most elements can be found on earth (with the exception of those elements that too unstable and thus must be synthesized in the laboratory). Since all elements have isotopes then we must consider how much of one isotope of an element exists versus another isotope of the same element. These are called the "natural" abundances on earth.

Natural Abundances:

Next, we can inquire what the mass of element X is? Since each isotope has a different mass (because each isotope has a different number of neutrons) the simplest answer is to give the "average" ...

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Atomic Weights

Natural Abundances:

Next, we can inquire what the mass of element X is? Since each isotope has a different mass (because each isotope has a different number of neutrons) the simplest answer is to give the "average" mass of element X - the atomic weight. After more analysis the mass of each isotope is determined to be the following:

Then the average mass (atomic weight) is given by:

The atomic weight of each element is included along with the element symbol in the periodic table. It is important to note that no one atom has a mass equal to that of the atomic weight. Remember: the atomic weight represents that average mass of the atoms.

Atomic Structure:

In chemistry and physics, an atom (is the smallest possible particle of a chemical element that retains its chemical properties. The word atom originally meant the smallest possible indivisible particle, but after the term came to have a specific meaning in science, atoms were found to be divisible and composed of smaller subatomic particles.

Most atoms are composed of three types of subatomic particles which govern their external properties:

electrons, which have a negative charge and are the least massive of the three;
protons, which have a positive charge and are about 1836 times more massive than electrons; and neutrons, which have no charge and are about 1839 times more massive than electrons.

Protons and neutrons make up a dense, massive atomic nucleus, and are collectively called nucleons. The electrons form the much larger electron cloud surrounding the nucleus.

Atoms can differ in the number of each of the subatomic particles they contain. Atoms of the same element have the same number of protons (called the atomic number). Within a single element, the number of neutrons may vary, determining the isotope of that element. The number of electrons associated with an atom is most easily changed, due to the lower energy of binding of electrons. The number of protons (and neutrons) in the atomic nucleus may also change, via nuclear fusion, nuclear fission or radioactive decay, in which case the atom is no longer the same element it was.

Atoms are electrically neutral if they have an equal number of protons and electrons. Atoms which have either a deficit or a surplus of electrons are called ions. Electrons that are furthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism atoms are able to bond into molecules and other types of chemical compounds like ionic and covalent network crystals.

Although the name "atom" was applied at a time when atoms were thought to be indivisible, it is now known that the atom can be broken down into a number of smaller components. The first of these to be discovered was the negatively charged electron, which is easily ejected from atoms during ionization. The electrons (or more specifically, electron clouds) orbit a small, dense body containing all of the positive charge in the atom, called the atomic nucleus. This nucleus is itself made up of nucleons: positively charged protons and charge less neutrons.

Peer Reviews

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skatealexia

21st of Aug 2012

Quality of writing

Punctuation, grammar and spelling is all to a good level. Format is good as the essay is split into clear sections.

Level of analysis

The candidate gives good definitions of rows and periods, as well as the general classifications of the different elements. They provide diagrams which aid their descriptions of the periodic table well as they directly rate to that section of the text. The candidate goes on to give accurate definitions of the structure of an atom in basic terms, but should be clearer with their definition of isotopes and mass. Diagrams again help them to explain a more minute atom structure that is knowledge beyond GCSE level.

Response to question

The focus on the question is done well. The candidate explains atomic structure in relation to the generic period table and generic properties of different groups, and then goes into more detail relating to the actual structure of an atom. To improve the candidate should explain trends between groups and rows and how these relate to their structure and reactivity

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