Atomic theory and bonding Flashcards
(31 cards)
What is a nanometre?
one billionth of a metre (10^-9) m
Bottom up method
Using specially designed molecules or atoms in chemical reactions to gradually build up the new nanoparticles from the smaller atoms or molecular units.
Top down method
Uses the larger bulk material as the starting material/ The bulk material is broken down into nanoparticles by mechanical or chemical means.
What is the size range for nanoparticles?
1 to 100 nm
What did Dalton’s atomic theory propose?
- All matter is composed of tiny indivisible particles called atoms
- All atoms of the same element are identical in mass and properties
- Compounds containing atoms of different elements combine in whole number ratios
- Atoms are neither created nor destroyed in chemical reactions
- Most stable compounds of two elements contains atoms in 1:1 ratio
What has know been known about atoms compared to Dalton’s theory?
Scientists now know that atoms are not indivisible or indestructible and are made up of even smaller subatomic particles.
What is an element?
A substance composed of one type of atom only, with the same atomic number.
What is a compound?
Two or more different types of atoms chemically bonded together in a fixed ratio.
Why are atoms neutral?
Number of positive charges is equal to number of negative charges.
What is an isotope?
Atoms with the same atomic number (number of protons) but different mass numbers (number of neutrons).
What did J J Thompson propose?
He identified the electron showing atoms were not indivisible. He proposed atoms consisted of a soft positively charged sphere into which negatively charged electrons were embedded. plum pudding model
What did Rutherford propose?
He fired alpha particles, heavy positive at very thin gold foil indicating a very dense positively charged core, the nuclei, with most of the atom being empty space. He then proposed the mass and positive change is concentrated in the centre, the nucleus. Negatively charged electrons occupy space around it, taking up most of the volume of the atom.
What did the Bohr model propose?
- Electrons revolve around the nucleus in fixed, circular orbits.
- The electron’s orbits correspond to specific energy levels of higher energy.
- Electrons can only occupy fixed energy levels and cannot sexist between two energy levels.
- Orbits of larger radii correspond to energy levels of higher energy.
How did Bohr come to this conclusion?
He put an electric current through a sample of hydrogen. Some electrons in their ground state, electrons in their lowest energy level, absorbed some of this energy and moved to an excited state, higher energy level. In the excited state the electrons are unstable and quickly returned to their ground state, emitting the energy absorbed in the form of electromagnetic radiation. If this energy is equivalent to the energy of light within the visible region of the spectrum we see this as coloured light. If this light is passed through a prism it is split into into component wavelengths, seen as coloured lines on a black background or emission spectrum.
What is the ground state?
When the electron is in the lowest possible energy shell, lowest energy state of the atom.
What are the two exceptions to the normal way sub shells are filled?
Chromium and Copper
What is core charge?
The measure of the electrostatic attraction felt by valence electrons towards the nucleus, taking into account any inner shell electrons.
What is core charge equal to?
Atomic number minus number of full inner shell electrons.
What is atomic radius?
Distance from nucleus to valence shell
What happens to atomic radium down a group?
Down a group core charge stays constant as number of energy levels (shells increases). Valence electrons are less strongly attracted to the nucleus.
What happens to atomic radium across a period?
Decreases. As you move across a period he number of occupied shells in the atoms remains constant but the core charge increases. the valence electrons become more strong attached to the nucleus so atomic radii decreases across a period.
What is the key feature that changes across a period bringing about these trends?
the number of protons in the nucleus and core charge.
What is the changing factor that explains trends down a group?
The increase in the number of energy levels is what causes the trends down a group. As you go down a group you add a shell, valence electrons get further from the nucleus, more easily lost and reactivity increases.
What is electronegativity?
The ability of an atom to attract electrons towards itself.
