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Atomic Structure

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Introduction • The Greek philosopher Democritus first

suggested that all matter is made up of tiny particles, he called atoms.

• His ideas were rejected and for 2000 years scientists believed that every substance was made up of earth, fire, air and water.

• Modern atomic theory was first put forward by John Dalton in 1803.

• He imagined atoms to be tiny, indivisible particles.

• In this lesson you will learn:1. About the structure of the atom.

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Size of an atomLook at the diagram of a crystalline solidAtoms are placed in a regular arrangement, called a crystal latticeFirst images of crystal lattices were obtained by X-ray diffraction, now electron microscopes are used.

Now look at any one atom in this crystal

D

D

D

Whole lattice can be divided into identical cubes, each containing one atom.We can use the Avagadro constant, NA to estimate the size of the atom

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Volume of each atom is about D3

Lets find the size of a copper atom, molar mass of copper is 0.064 kg and will contain , NA = 6.02 x 1023 atoms in 0.064 kg.Density of copper is 8900 kgm-3, so the volume of one atom of copper can be calculated:density = mass / volume, so volume = mass / density

Volume of 0.064 kg of copper = 0.064 kg / 8900 kgm-3

= 7.19 x 10-6 m3

Volume of copper atom, D3 = 7.19 x 10-6 m3 / 6.02 x 1023

= 1.19 x 10-29 m3Diameter of atom = 3√(1.19 x 10-29 m3) = 2.28 x 10-10 mValue agrees with diameters measured by X-ray diffraction. Most atomic diameters range from 1 x 10-10 to 5 x 10-10 m

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Alpha particle scatteringJohn Dalton said that atoms were tiny indivisible spheres

But JJ Thompson (1897)discovered all matter contains tiny negatively charged particles (electrons) – he discovered the first sub-atomic particle.He thought the atom was a positive sphere of matter in which electrons were embedded – plum pudding model

Ernest Rutherford (1909) decided to probe the atom using alpha particles.

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In 1909, Hans Geiger and Ernest Marsden were working at Manchester university under the supervision of Ernest Rutherford, professor of Physics.

Investigating the scattering of alpha particles by a thin gold foil. Found that about 1 in 8000 alpha particles were ‘reflected’, scattered back through an angle of more than 90o.

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Nuclear model of the atomA comparatively massive alpha particle travelling at 10,000 kms-1 should not have bounced off.Rutherford said ‘It was almost as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you’.

In 1911 Rutherford described the nuclear model of the atom:1. All the atom’s positive charge and most of the

mass is concentrated in a tiny core – called the nucleus

2. Electrons surround the nucleus at relatively large distances from it – atom is mainly empty space.

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Structure of the atomWe now know that the nucleus is made up of two particles – protons and neutrons (referred to as nucleons)

All atoms (except hydrogen) are made up of three particles. Table below shows their properties

particle location mass charge

proton nucleus 1.00728u +e

neutron nucleus 1.00867u 0

electron orbiting nucleus

0.00055u -e

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As masses are very small. It’s useful to measure them in a small unit – atomic mass unit (u)

1 u = 1.6605 x 10-27 kg

Notice that:1. Mass of proton and neutron are approximately 1 u2. Electron has a much smaller mass – about 1/1800

u3. Charge on a proton is equal and opposite to that of

an electron +1.6 x 10-19CNeutrons and protons make up the tiny positive nucleus.Electrons spin round the nucleus giving the effect of a thin cloud of negative charge.An atom is electrically neutral – unless it loses an electron to become a positive ion or gains an electron to become a negative ion.

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Proton and nucleon numbers

There are over 100 elements in the Periodic Table – 92 occur naturally.

Each element has atoms with a specific number of protons – lithium always has 3 protons, carbon always has 6 protons

Number of protons in an atom is its proton number, Z and sometimes is called the atomic numberNumber of protons + neutrons in the nucleus is called the nucleon number, A and sometimes called the mass numberNucleon number for lithium is 7 (= 3 protons + 4 neutrons)Word nuclide is used for a particular combination of protons and neutrons in a nucleus

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All nuclides can be described using the format:

Nucleon number A

Proton number Z

Chemical symbol of element

Isotopes Isotopes are atoms of an element with the same number of protons, but different nucleon number – i.e. number of neutrons changes

Number of neutrons = A - Z

Number of electrons remains the same – so have same chemical properties.Isotopes have different masses – so different physical properties.

All elements exist as a mixture of isotopes.

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Nuclear diametersPeriodic table lists elements in order of proton number.Hydrogen is the first element in the table, uranium is the last of the naturally occurring ones.Z Element + symbol Commonest

isotope

1 Hydrogen H 11H

2 Helium He 42He

3 Lithium Li 73Li

4 Beryllium Be 94Be

5 Boron B 115B

6 Carbon C 126C

7 Nitrogen N 147N

8 Oxygen O 168O

79 Gold Au 19779Au

92 Uranium U 23892U

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Suppose each nucleon is a sphere with a constant volume, vThen v = 4πro

3/3, where ro is the radius of the nucleonVolume V of the nucleus with A nucleons will be roughly AvWhat value does this give for the radius of the nucleus, r ?Assuming the nucleus is a sphere, then V = 4 πr3/3

Since V = Av, then 4 πr3/3 = A x 4πro3/3

Cancelling and taking cube root gives r = roA1/3

ro is the radius of the nucleon and found by experiment to be 1.2 x 10-15 m

Equation gives nuclear radii in the range 2 x 10-15 m to 8 x 10-15 m. Values have been experimentally confirmed.Calculate the radius of a Pb nucleus of nucleon number 206Answer is 7.1 x 10-15 m