Chemistry Chapter 02

--- title: Chemistry Chapter 02 image: robots: noindex, nofollow lang: zh --- # Chemistry Chapter 02 ==**[Index]**== [TOC] ## Atom vs. Element **Atoms** are the smallest units of matter and are made from protons, neutrons and electrons. **Element** are substance that is made entirely from one type of atom. ## Dalton’s atomic theory Dalton assumed that elements are made of small particles called *"atoms"*.Furthermore, he stated the five essential postulates below: ### 1. **The indivisibility of an atom** > Matter is composed of exceedingly small particles called atoms. ++An atom is the smallest unit of an element that can participate in a chemical change.++ > (原子atom是最小的粒子，不可以再分割。)<figcaption>John Dalton</figcaption> :::warning **Drawbacks** This postulate was proved wrong by finding an atom can be further subdivided into protons, neutrons and electrons. (原子非最小粒子，其下更小的粒子包含電子、質子、中子，然而電子、質子、中子也非最小粒子。) ::: ### 2. **Atoms of same element has same size and mass.** > An element consists of only one type of atom, which has a mass that is characteristic of the element and is the same for all atoms of that element. > (只要是同一種元素，其質量、密度和性質都一樣。)<figcaption>John Dalton</figcaption> :::warning **Drawbacks** ISOTOPES!! Isotopes are variants of a element which differ in neutron number. That gives the stoms of same element having different masses. ALLOTROPES!! This postulate fails to explain the existence of allotropes, too. Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner. Also, Argon (Ar) and Calcium (Ca) atoms, each have an atomic mass of 40 amu. These atoms with similar atomic masses are isobars. ( 同種元素有同位素，和同素異形體。同位素(isotope)：指有相同質子數，但不同中子數的粒子。同素異形體(allotropes):由相同元素組成，但因為組成結構不同而有不同性質，比如說，鑽石和石墨，兩者都是由碳元素所組成，但是前者是以立體結構組成，後者以網狀結構組成，造就不同性質。最後，Argon和Calcium的原子量相同，但是卻不是同一元素。 ) ::: ### 3. **Different elements are made from different atoms.** > Atoms of one element differ in properties from atoms of all other elements. (所有元素都是以不同的原子組成。)<figcaption>John Dalton</figcaption> :::warning **Drawbacks** This postulate is fundamentally wrong. Because we now know atoms are made of only three smaller, more basic particles: the proton, the electron, and the neutron. Moreover, the quantity of each particle in the atom varies in different elements. (首先，元素不是由不同原子組成，而是由不同數量的電子、質子、中子組成。再者，每個元素中的單一電子和其他元素中的電子的性質是一樣的，質子、中子亦同。) ::: ### 4. **A compound consists of the fixed whole-number ratio of atoms.** >A compound consists of atoms of two or more elements combined in a small, whole-number ratio. In a given compound, the numbers of atoms of each of its elements are always present in the same ratio >(所有化合物都是以簡單整數比的元素組成，e.g. H~2~O，而不會有H~2.5~O。 >且相同的化合物組成相同。)<figcaption>John Dalton</figcaption> :::warning **Drawbacks** 這個假設後半是正確的，相同化合物的組成不會因為不同地方而不同。但是，現今對化學物質的研究，已發現並非所有化學物質都以簡單整數比形成，比如說超高溫導體：釔鋇銅氧( YBa~2~Cu~3~O~{7-x}~ )，而這些非整數比化合物的組成比例會在一定的範圍波動。 ::: ### 5. **Chemical Change = Rearrangement of Atoms + Conservation of Mass** >Atoms are neither created nor destroyed during a chemical change, but are instead rearranged to yield substances that are different from those present before the change.<figcaption>John Dalton</figcaption> :::warning 在化學變化中，原子不滅，新排列是正確的。 ::: ## Evolution of Atomic Theory ### 1. J.J. Thomson's Experiment(陰極射線實驗) <center> <img src="https://i.imgur.com/N5I4g1T.png"> </center> In the late 1800s, a number of scientists interested in questions like these investigated the electrical discharges that could be produced in low-pressure gases, with the most significant discovery made by English physicist J. J. Thomson using a cathode ray tube. The ray was simultaneously deflected by an appliedmagnetic field, and measurements of the extent of deflection and the magnetic field strength allowed Thomson to calculate the charge-to-mass ratio of the cathode ray particles. The results of these measurements indicated that these particles were much lighter than atoms. Thomson’s idea was gradually accepted, and his cathode ray particle is what we now call an electron, a negatively charged, subatomic particle with a mass more than one thousand-times less that of an atom. :::danger **TL;DR** **Thomson** 利用實驗中的電磁場(magnetic field)得到“**荷質比(charge-to-mass ratio)**”，並發現"**陰極射線(cathode ray)**"是由一種比原子(atom)還要輕的粒子組成，這種粒子輕小，又帶負電(negatively charge)，之後人稱“**電子(electron)**”。 >這個發現可是推翻Dalton’s atomic theory的[第一則理論呢](#1-The-indivisibility-of-an-atom) [color=tomato] ::: ### 2. Robert A. Millikan's "Oil Drop Experiments" <center> <img src="https://i.imgur.com/z42Ms6N.png"> </center> In 1909, more information about the electron was uncovered by American physicist Robert A. Millikan via his “oil drop” experiments. Millikan created microscopic oil droplets, which could be electrically charged by friction as they formed or by using X-rays. These droplets initially fell due to gravity, but their downward progress could be slowedor even reversed by an electric field lower in the apparatus. By adjusting the electric field strength and making careful measurements and appropriate calculations, Millikan was able to determine the charge on individual drops.Looking at the charge data that Millikan gathered, you may have recognized that the charge of an oil droplet is always a multiple of a specific charge, $1.6 × 10^{-19} C$. :::danger **TL;DR** **Millikan** 油滴實驗中，利用X-rays使油滴帶電，並使其在裝置電場中懸浮，由於每顆油滴都不一樣大(電量不同)，所以剛好可以懸浮的電場強度也不一樣，但這些電場強度都會是電荷$e$的整數倍，所以找到公因數**電子電量$e = 1.6 × 10^{-19} C$** 再利用Thomson的荷質比(charge-to-mass ratio, $${e \over \text{mass of electron} }= \text{charge to mass ratio}$$)，得到**電子質量 $=9.107 × 10^{−31} kg$**。 ::: ### 3. Ernest Rutherford's Experiment <center> <img src="https://i.imgur.com/VIyjtMq.jpg"> </center> Ernest Rutherford performed a series of experiments using a beam of high-speed, positively charged alpha particles (α particles) that were produced by the radioactive decay of radium; α particles consist of two protons and two neutrons. Rutherford and his colleagues Hans Geiger and Ernest Marsden aimed a beam of α particles, the source of which was embedded in a lead block to absorb most of the radiation, at a very thin piece of gold foil and examined the resultant scattering of the α particles using a luminescent screen that glowed briefly where hit by an α particle. Analyzing a series of such experiments in detail, Rutherford drew two conclusions: 1. The volume occupied by an atom must consist of a large amount of empty space. 2. A small, relatively heavy, positively charged body, the nucleus, must be at the center of each atom. :::danger **TL;DR** <center> <img src="https://i.imgur.com/tGwvr3x.jpg"> </center> Ernest Rutherford的**金箔實驗**，利用”**$a$粒子**“撞擊只有一層原子厚(理論上)的金箔，發現大部分的$a$粒子都穿過金箔，這和Thomson的原子模型理論是牴觸的， <center> <img src="https://i.imgur.com/mludZvx.png"> </center> 如果Thomson的原子模型理論是對的，$a$粒子應該串穿過才對，但是觀測結果卻出現偏折宇大角度反射，因此推測： 1. 原子分布不均，且集中在中心。 2. 中心之核帶正電，發現質子。 > 這項發現修正了原子模型！！ [color=tomato] ::: ## History of Atomic Model ![](https://i.imgur.com/itIBGWd.png) ![](https://i.imgur.com/BH3j6MW.jpg) ###### tags: `Chemistry` `Book`