A Mission Statement is not the same as an Objective Statement.
Apart from other reasons, the most glaring difference between the two is that an organisation can and should only have one mission in their mission statement. Whereas, having a single objective in an objective statement is a sign of laziness, tardiness, and total lack of vision. Why?
Essentially, a mission statement dictates the objectives that will need to be achieved by the organisation in order to fulfil its mission statement.
So, then, back to your question - what is a mission statement?
Simply put, a mission statement is a purpose statement. Its a statement that highlights three fundamental aspects:
1. its core activity, (what am I selling)
2. its core audience, (whom am I selling it to) and
3. its chief point/s of differentiation (why should they buy it from me)
I realise you are putting these statements together at a departmental level. For this, my suggestion is to have a over-arching organisational mission statement and follow it up with objective statements at departmental levels. However, your idea is also not wrong. In fact, the more I think as I write this reply, I am starting to like your idea of a departmental mission statement. I can already feel the energy a well crafted and truthful departmental mission statement can convey to its people and the organisation at large.
So, example of a Mission Statement for the Admin department could read something like this:
' To provide our internal and external clients with administration services that are professional, punctual, 'fit for purpose', and above industry standards'
In the above example, your three elements are:
1. What am I selling - Administration Services
2. Whom am I selling it to - Internal and External clients. This includes all departments within the organisation, as well as any vendors, partners, clients etc.. that admin may deal with
3. Why should they buy it from me - because it is professional, punctual, fit for purpose, and above industry standards.
Now, how do you translate this into a objective statement? Actually, you don't translate this into 1 objective statement. You translate this into 3 separate objective statements where each one addresses 1 mission statement element from above.
To write an objective statement, pick each mission statement element & start breaking down what activities and standards need to be done and achieved to fulfil that element. Repeat this for all 3 elements and voila - you've got your objective statements.
And, once you've got the 3 objective statements, combine them into 1 overarching objective statement.
Hope this helps you!