DRAWING ATOMS. This year, I ve been basing my introduction to basic chemistry for my middle

Transcription

1 DRAWING ATOMS This year, I ve been basing my intrductin t basic chemistry fr my middle schl students arund the peridic table f the elements. The first step, hwever, is t teach them hw t draw basic mdels f atms. Prep: Memrizatin ver the Winter Break I started it ff by having the students memrize the first 20 elements (H thrugh Ca), in their crrect rder by atmic number ver their winter break. A diagram f an xygen atm. S that they d have a bit f cntext, I went ver the basic parts f an atm (prtns, neutrns, and electrns) and made it clear that the name f the element is determined slely by the number f prtns. I even had them draw a few atms with the prtns and neutrns in the center and the electrns in shells.

2 Since I d dumped all f this n them in a single class perid, it prbably was a bit much, but since it was just t give them sme cntext I did nt expect the 7th graders, wh had nt seen this befre, t remember it all; fr the 8th graders it shuld have been just a review. Mst students did a gd jb at the memrizatin. Sme fund sngs n the the internet that helped, while thers just pushed thrugh. Having the tw weeks f winter break t wrk n it prbably helped t. The Parts f an Atm When we gt back t schl, the first thing I did was give them an utline f the upper part f the peridic table and asked them t fill it in with the element names. Template fr the first 20 elements f the peridic table. (pdf)

3 After they d filled ut their peridic table template, I went int the parts f the atms in mre detail, and had them practice. The key pints I wanted them t remember were: The atmic number is written as a subscript t the left f the element symbl. The atmic number is the number f prtns. Since they memrized the elements in rder, they shuld be able t figure this ut n their wn but they culd als lk it up quickly n the peridic table, r lk at the element symbl where the atmic number is smetimes written n the lwer left. The atms have the same number f electrns as prtns. Prtns are psitively charged, and electrns are negatively charged, s an atm needs t have the same number f bth fr its charge t be balanced. We dn t talk abut ins where there are mre r less electrns until later. The atmic mass (4) is written as a superscript t the left f the element symbl. The atmic mass is the sum f the number f prtns (2) and the number f neutrns (2). The small atms that we re lking at tend t have the same number f neutrns as prtns, but that s nt necessarily the case.

4 S hw d yu knw hw many neutrns? Yu have t ask, r lk at the atmic mass number, which is usually written t the upper left f the atm. Since the atmic mass is the sum f the number f prtns and neutrns, if yu knw the atmic mass and the number f prtns, yu can easily figure ut the number f neutrns. (Nte that electrns dn t cntribute t the mass f the atm because their masses are s much smaller than the masses f neutrns and prtns. This xygen atm has 8 electrns in tw shells. Electrn Shells: Electrns rbit arund the nucleus in a series f shells. Each shell can hld a certain maximum number f electrns (2 fr the first shell; 8 fr the secnd shell; and 8 fr the third). And t draw the atms yu fill up the inner shells first then mve n t the uter shells. S, if I wrte just the element symbl and its atmic mass n the bard that students shuld be able t figure ut the number f particles.

5 Example: Carbn-12 Fr example, the mst cmmn frm (istpe) f carbn-12 is written as: Prtns = 6: Since we knw the atmic number is 6 (because we memrized it), the atm has 6 prtns. Neutrns = 6 : Since the atmic mass is 12 (upper left f the element symbl), t find the number f neutrns we subtract the number f prtns (12 6 = 6). Electrns = 6: This atm is balanced in charge s it needs six electrns with their negative charges t ffset the six psitive charges f the six prtns. (Nte: we haven t talked abut unbalanced, charged atms yet, but the charge will shw up as a superscript t the right f the symbl.) Electrn shells (2-4): We have six electrns, s the first tw g int filling up the first electrn shell, and the rest can g int the secnd shell, which can hld up t 8 electrns. This gives an electrn cnfiguratin f 2-4. Diagram f a carbn-12 atm.

6 Example: Carbn-14 Carbn-14 is the radiactive istpe f carbn that is ften used in carbn dating f histrical artifacts. It is written as: Prtns = 6: As lng as it s carbn it has six prtns. Electrns = 6: This atm is als balanced in charge s it als needs six electrns. Neutrns = 8 : With an atmic mass f 14, when we subtract the six prtns, the number f neutrns must be 8 (14 6 = 8). The nly difference between carbn-12 and carbn-14 is that the latter has tw mre neutrns. These are therefre tw istpes f carbn. Diagram f a carbn-14 atm.

7 Example: Helium-4 Diagram f helium-4 atm. Example: Sdium-23 Diagram f sdium-23 atm. Surce: