C3 - Atomic structure

Number of neutrons

number of neutrons = mass number - atomic number

The NUCLEUS of an atom

- It's in the middle of the atom. It contains protons neutrons - It has a positive charge because of the protons - Almost the whole mass of the atom is concentrated in the nucleus. - Models of atoms help us to understand their structures, but most models don't really give a correct impression of the scale. The overall diameter of an atom can be 100 000 times the diameter of its nucleus.

Isotopic abundance

1) Different isotopes of an element occur in different quantities, or isotopic abundances. 2) You need to know how to calculate the relative atomic mass of an element from its isotopic abundances. 3) To work out the relative atomic mass of an element, you need to find the average mass of all its atoms. - MULTIPLY each relative isotopic mass by its isotopic abundances, and ADD up the results. - DIVIDE by the sum of the abundances. (If the abundances are given as percentages, this will be 100).

How has the model of the atom changed over the last 200 years?

1805: Dalton's billiard ball model of an atom - solid sphere. 1904: Thomson's Plum pudding model - negative particles and solid positive particles 1911: Rutherford's nuclear atomic model.

The nuclear atom

1909, Ernest Rutherford was working with others to investigate the structure of atoms. In one experiment, tiny positive particles were fired at a thin gold foil. The particles passed straight through the gold foil, with a few being deflected and very small number bouncing back. Rutherford explained this by suggesting that atoms are mostly empty space, with a small positive central nucleus that contains most of the mass.

What does the atomic number tell you about an element?

Atom number = number of protons; this defines the element

How can you calculate the numbers of protons, neutrons and electrons in atoms?

Atomic number = number of protons Mass number = protons + neutrons, so neutrons = mass number - atomic number In an atom, electrons = protons.

Relative masses/charges of subatomic particles

Atoms always have the same number of protons and neutrons so have no overall charge, as the charges cancel out.

Bohr's atomic theory

Bohr's electron orbits Niels Bohr further developed Rutherford's nuclear atom model. He used experimental evidence to support the idea that electrons occupy particular orbits or shells around the nucleus of an atom. The development of the theory of atomic structure is an example of: How a theory may change as new evidence is found How a scientific explanation is provisional but may become

Dalton's atomic theory

Dalton's atomic theory John Dalton (1766-1844) John Dalton developed an atomic theory in the 1800s. He did experiments, worked out some atomic weights and invented symbols for atoms and molecules. His most important conclusions are summarized below: All matter is made of atoms Atoms cannot be broken down into anything simpler All the atoms of a particular element are identical to each other and different from the atoms of other elements Atoms are rearranged in a chemical reaction Compounds are formed when two or more different kinds of atoms join together Dalton's theory was developed and changed as new evidence was discovered.

How do the parts of atoms compare with each other?

Electrons: small mass (1/2000 of protons or neutrons0, charge -1 Protons: mass = 1, charge +1 Neutrons: mass = 1, charge = 0 (neutral)

Why do atoms have no overall charge?

In an atom, there are equal numbers of protons and electrons. In an ion, electrons have been lost or gained, so protons and neutrons don't balance.

How can you describe and identify isotopes of elements?

Isotopes have same number of protons, but different numbers of neutrons. This means isotopes have the same atomic number but different mass numbers.

Isotopes

Isotopes: atoms of an element with the same number of protons but different mass numbers, due to different number of neutrons.

Thomson's atomic theory

JJ Thomson's discovery of the electron JJ Thompson discovered the electron in 1897. This showed that the atom contained smaller pieces, whereas Dalton had thought that atoms could not be broken down into anything simpler.

Why is most of the mass of an atom found in its nucleus?

Protons and neutrons are found in the nucleus, and these are the heavy particles. Electrons are lighter, and are outside the nucleus.

Rutherford's atomic theory

Rutherford's nuclear atom In 1911 Ernest Rutherford used experimental evidence to show that an atom must contain a central nucleus. This was further evidence that an atom contained smaller pieces.

Why are the relative atomic masses for some elements not whole numbers?

The R.A.M is the average for all the atoms of an element. For example, chlorine is a mixture of Cl - 35 (17 protons, 18 neutrons) and Cl - 37 (17 protons, 20 neutrons). there is about 3 times as much Cl - 35 as Cl - 37, so the average chlorine atom is Cl - 35.5.

Atoms

The atom is made up of three subatomic particles: 1) Protons - are heavy and positively charged 2) Neutrons - are heavy and neutral 3) Electrons - have hardly any mass and are negatively charged.

Atomic number

The atomic number tells you how many protons an atom has. Every atom of an element has the same numbers of protons. The modern periodic table places the elements in order of the number of protons in their atoms. this is the atomic number and it is this that defines and element - all the atoms of a particular element have the same unique atomic number.

Mass number

The mass number tells you the total number of protons and neutrons in the atom. - The mass number of an electron is describe as 'negligible' - it is so small that it can be ignored. This explains why the nucleus of any atom contains nearly all its mass. - For that reason the total number of protons and neutrons in an atom is called its mass number.

Relative atomic mass

The mass of an atom is really small, so we measure their masses relative to (compared to) an atom of carbon -12. This isotope is used as a standard and given a mass of exactly 12. The masses of all other atoms are compared to this. For example, the mass of helium -4 is one - third of that of carbon -12 and so its relative mass is 4. Relative atomic mass: (RAM) - Ar

How do you calculate the relative atomic mass of an element?

You will be given the percentage abundance of each isotope and its mass. To work out the average R.A.M, multiply the mass by the abundance for each isotope. Add these multiplication together; the answer should be similar to the masses of the different isotopes.