我唔係好明chem...

first you should know how to write chemical symbol. eg, Na for sodium , O2 for oxygen..

you should recite these.

for balancing equation, you don't need to recite. your teacher 's saying is not right.

sodium reacts with oxygen to form sodium oxide

for a chemical equation, we should balance it.

if you just write, Na + O2 -----> Na2O

it is wrong.

we should balance it

in the left hand side, two oxygen atoms are present.

but in the right hand side, only one oxygen atom is present, so add 2 in the right hand

side.

and in the right hand side, 4 sodium atoms are present. so we should add 4 in the

left hand side so that the equation is balanced.

4Na + O2 -------> 2Na2O

Chemical equation 同數學既eqaution 係一樣，兩邊個數量係要balance.
以sodium + oxygen為例，sodium (Na) 的份子結構係Na，而oxygen 的份子結構係O2。Na的離子係single valent，一個正電荷，即係Na+，而O的離子係divalent ，兩個負電荷，即係O2-，所以sodium oxide 的份子結構係 Na2O。
當寫equation時，先寫上所有reagent既名：
Na + O2 -> Na2O
然後就佢左右兩邊element既數目balance，依家左邊有1個Na同2個O，右邊有2個Na同1個O，由於左邊已有2個O，而右邊只得一個，所以先將右邊倍大2倍，變成有2個Na2O，即係依家右邊共有4個Na同2個O，然後再做左邊，O已經balance咗，要睇Na，由於右邊倍大之後有4個Na，所以左邊的Na都要倍大4倍，變成4個Na，咁樣兩邊就可以Balance。
4Na + O2 -> 2 Na2O

2XNA2 2XO=4NA+O2