1. i. All elements are composed of tiny indivisible particles called atoms. 1. ii. Atoms of the same element are identical. The atoms of any one element are different from those of any other element 1. iii. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds 1. iv. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in a different combination. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.
Sizing up the Atom Atoms are very small the radii of most atoms fall within the range of 5 x 10^-11 m to 2 x 10^-10m atoms can be seen in scanning electron microscopes atoms can be moved 4. 2 Structure of the Nuclear Atom Overview Lots of Dalton’s theory is accepted except that atoms can be divided Atoms can be divided into three subatomic particles Protons Neutrons Electrons Subatomic Particles Electrons Discovered by UK physicist J. J. Thomson (1856-1940) Are negatively charged subatomic particles. Thomson’s Experiment passed electric current through gasses at low pressure.
Sealed the gases in glass tubes fitted at both ends with metal disks called electrodes electrodes connected to source of electricity one electrode called the anode became positively charged other electrode, called cathode, became negatively charged resulted in glowing beam called “Cathode Ray” that traveled from cathode to anode. Findings cathode ray is deflected by electrically charged metal plates positively charged plate attracts the cathode ray negatively charged repels it Thomson knew that opposites attract hypothesized that a cathode ray is a stream of negatively charged particles moving at high speed called particles “corpuscles”.
Later called “electrons” Millikan’s Experiment US physicist Robert A. Millikan (1868-1953) carried out experiments to find the quantity of an electron’s charge. Oil drop experiment suspended negatively charged oil droplets between two charged plates. then changed voltage on the plates to see how it affected the droplets’ rate of falling he found that the charge on each oil droplet was a multiple of 1. 60 x 10^19 coulomb meaning it was the charge of an electron calculated electron’s mass Millikan’s values for electron charge and mass are similar to those accepted today.
An electron has one unit of negative charge and its mass is 1/1840 the mass of a hydrogen atom Protons and Neutrons Four simple ideas about matter and electric charges 1. Atoms have no net electric charge; neutral 2. Electric charges are carried by particles of matter 3. Electric charges always exist in whole-number multiples of a single basic unit; that is, there are no fractions of charges. 4. when given a number of negatively charged particles combines wit an equal number of positively charged particles, an electrically neutral particle is formed.
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