Characteristics of atoms

Key features of atoms All atoms are electrically neutral All atoms of the same element contain the same number of protons and neutrons Number of protons = atomic number Most of the mass of an atom comes from Protons and Neutrons Hence, the relative atomic mass is #Protons + #Neutrons

Isotopes All atoms of an element have the same number of protons, however, the number of Neutrons can vary. Atoms with the same number of Protons, and different number of Neutrons are called isotopes Carbon 14 (draw examples on board)

Ions Atoms can gain or loose electrons, when they do they are called ions Atoms which gain an electron become negative i.e. Cl + e -  Cl - Atoms which loose an electron become positive i.e. Na  Na + + e - Draw examples on board

How are electrons arranged around the nucleus? Rutherford proposed that electrons moved in circular orbits, however physics says that electrons moving in circular orbits should emit electromagnetic radiation (such as light). As the radiation is emitted, the electrons should lose energy and spiral into the nucleus, therefore ‘killing’ the atom. Rutherford's model also didn’t explain why, when heated, elements only emitted light at certain wave lengths.

Emission spectrum tests

Niels Bohr Suggested that small particles such as atoms did not follow the laws physics displayed by large objects Therefore, electrons circled the nucleus without loosing energy Further, he proposed electrons could only move in fixed orbits of certain energy levels Electrons with low energy moved in a small orbit close to the nucleus, electrons with a larger energy moved in a larger orbit further from the nucleus

Jumping electrons Heating an element can cause an electron to ‘jump’ to a higher energy level (orbit), because the electron absorbs some of this energy. Shortly afterward the electron returns back to its initial state, releasing a fixed amount of energy. Electrons can return to their energy level in a number of different ways.

Ionisation energy So.. Electrons may be removed from their ‘shells’ by applying the atoms with energy. The further the negative electrons are from the positively charged nucleus, the easier they are to remove. The energy required to remove an electron is called ionisation energy

An example Sodium (Atomic number 11, mass number 23) has 11 protons, 12 neutrons and 11 electrons The electrons from Sodium can be removed, and the ionisation energy can be measured. Look at the diagram on the next page

Shells After looking at ionisation energies of many elements, scientists proposed that electrons are grouped into ‘energy levels’ called shells. Electrons in the same shell: - Are the same distance from the nucleus - Have the same energy The various shells can hold different numbers of electrons

Shells The first shell can hold 2 electrons The second, 8 The third, 18 The fourth 32 2n 2 where n = number of shells

Electronic configuration

Electron dot diagrams Notice anything about Potassium? Remember how many electrons the 3 rd shell can hold?

Limitations of the Bohr model Why do electrons move in a circular motion, and not an eliptical one? Why do the ‘shells’ have specific energies? Why does each shell have a maximum of 8 electrons, even though shells above 3 can hold more than 8?

**Electrons behave like waves In 1926 Erwin Schrodinger proposed electrons behave like waves around a nucleus Instead of circling in a specific orbit, they move in waves around the nucleus, acting like a cloud of negative charge. Schrodinger used his model (and the accompanying equation) to predict the energy levels of hydrogen, and larger atoms.

Quantum mechanics In this model, instead of occupying shells in an orbit, electrons occupy regions of space called orbital's. In this model we will call these orbital's (regions of space), ‘shells’, and will give them the shell numbers 1, 2, 3, 4 etc Within these shells, there are energy levels of similar energy called subshells, these are labelled s, p, d, f Energies are as follows s < p < d < f Subshells are made up or orbitals (regions of space where electrons move)

Electrons in subshells

Pauli Exclusion Principle Each orbital may hold a maximum of 2 electrons

Memorise This Chart!!!! Draw it in your sleep!!!!

Examples Lets figure out the electronic configurations of a few elements together Na, Fe, Ne Step 1: Find out how many electrons in the atom by looking at the atomic number Step 2: Draw the table from the previous page Step three: Fill the subshells until you run out of electrons

Ground and Excited states Ground states – The lowest possible energy state is called the ground state (these are what we have listed so far) Excited states – When energy is applied to an atom, an electron in the outer orbitals can move to a higher energy level. i.e. Helium: Ground state – 1s 2 Excited state - 1s 1, 2s 1

Timeline

Essential questions for this chapter! 1, 4, 5, 6, 7, 9, 10, 12, 13,14, 17, 19, 22, 24, 29, 30