Atoms

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Space-Filling Model of Atoms

An atom is the smallest part of an element.

Scientists use models to describe the internal structure of atoms because they cannot look inside atoms with existing microscopes. Only recently have we been able to see images of whole atoms, such as the gold atoms sitting on a bed of carbon atoms shown in the image to the right.

When scientists originally discovered that matter was made of atoms, they thought atoms were solid indivisible spheres, rather like billiard balls.

Although this model of the atom was found to be incorrect, the idea is useful for understanding how atoms are arranged in molecules (groups of bonded atoms) and in lattices of crystals.

In the space-filling model, different coloured spheres are used to represent individual atoms.

The spheres give a good idea of the relative size of the atoms, and building a model of a molecule gives you a picture of how the atoms are arranged.

There is an international colour code that is used when making or drawing space-filling models of atoms or molecules (see below).

Mini Solar System Model of Atoms

As scientists investigated atoms more closely, they discovered that atoms were actually made of smaller sub-atomic particles. The three important sub-atomic particles that you need to know about are protons, neutrons and electrons. The protons and neutrons are packed into the nucleus (central core) of the atom. The electrons travel at high speed around the nucleus. Atoms are largely empty space except for the dense nucleus.

The New Zealand scientist Ernest Rutherford thought that the electrons orbited the nucelus rather like the planets orbited the Sun. In his model, atoms were considered to be like miniature solar systems. But electrons are not found in simple orbits like planets.

Sub-Atomic Particles

Different elements have different numbers of the three subatomic particles in their atoms, i.e. protons, neutrons and electrons.

Scientists have also compared the mass and electrical charge of the three sub-atomic particles.

Electrons are very light, negatively charged (-) particles travelling at high speeds around the nucleus.
Protons and neturons are heavier particles that are closely packed in the nucleus.
Protons have a positive charge (+) but neutrons have no charge (0).

Atoms are largely empty space except for the dense nucleus. The attraction of the positively charged protons have for the negatively charged electrons keeps those electrons flying around the nucleus.

The electrons have a low mass and move very rapidly around the nucleus, taking up a large amount of space. They do not fly away, as their negative charge is strongly attracted to the positive charge of the nucleus.

The area where electrons move around the nucleus is called a shell, orbital or electron cloud.

Neutral Atoms

Each proton has a positive electrical charge and each electron has an equal size but negative electrical charge. If an atom has equal numbers of protons and electrons, then it is said to be neutral. The overall positive charge of the protons is balanced by the overall negative charge of the electrons.

A carbon atom has 6 protons and 6 electrons. The 6 positive charges balance the 6 negative charges, giving a neutral atom.

Atoms can gain or lose electrons. This makes them charged atoms, or ions. The ions will be either positively or negatively charged.

Atomic Number and Mass Number

On the Periodic Table of elements, each element is represented by a symbol, a chemical chorthand that is either one or two letters. There are two numbers beside the symbol that give information about the eatoms that make up the element.

The atomic number tells you the number of protons in the nucleus of an atom. Each type of atom has a distinct atomic number.
The mass number is the number of protons PLUS the number of neutrons in the nucleus of an atom.

In an atom, there is always the same number of electrons moving around the nucleus as there are protons in the nucleus.

This example shows that carbon (C) has:

6 protons (Atomic number = 6. There are 6 positive charges around the nucleus.)
6 electrons (6 negative charges in the shells around the nucleus, because atoms have the same number of electrons as protons.)
6 neutrons (6 neutral charges in the nucleus. Mass number (protons + neutrons) minus the Atomic number (protons only) = number of neutrons.)

Because the number of positive protons is the same as the number of negative electrons, the charges cancel each other out and the overall charge of the carbon atom is zero. This mean the carbon atom has no charge; it is electrically neutral.

Electron Shells

Compared to the size of the atom, there is a large amount of empty space between the electrons and the nucleus. If all that space were removed, your body would take up millions of times less room than this full stop '.'

Electrons are thought to be confined to energy shells, as they vibrate around the nucleus as waves. Each shell can only hold a limited number of electrons:

The first shell holds up to TWO electrons
The second shell holds up to EIGHT electrons
The third shell holds up to EIGHT electrons
The fourth shell holds up to TWO electrons

Electrons fill the first or lowest energy level shell first. Then they start on the second shell until it is full and so on. The way electrons are arranged in shells around the nucleus is called the electron arrangement or electron configuration. It can be drawn as a diagram or written as a series of numbers separated by commas.

Whether an atom has a filled or partly filled, outer energy shell (also known as the valence shell) affects how that particular atom reacts with other atoms.

Isotopes

Sometimes atoms of the same element can have different numbers of neutrons in their nuclei. They are called isotopes of an element. Most elements in nature consist of a mixture of isotopoes. For example, there are three isotopes of hydrogen:

hydrogen (or protium) has 1 proton and no neutrons in its nucleus (the lightest and most plentiful isotope of hydrogen)
deuterium has 1 proton and 1 neutron in its nucleus
tritium has 1 proton and 2 neutrons in its nucleus.

The different isotopes of an element behave in the same way in chemical reactions because it is the electrons, not the protons and neutrons, that are involved in the reaction.

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